Grounding two (mains and generator) power feeds

I'm posting this here instead of sci.engr.electrical.compliance because
I'm looking more for technical answers, rather than code/legal answers.
This pertains to the US power system, either split single phase or wye
three phase.
The mains power comes into the building at one point where a panel that
splits that power into N different subfeeds. There is a main breaker
and branch breakers for each subfeed. The generator power comes into
the building at a different point where it has its own panel that splits
that power into M (where M
Reply to
phil-news-nospam
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I know this will go on for weeks but the short answer is you only want one main bonding jumper in the system. If you do bond the neutral at the genset and at the service the transfer equipment will have to switch the neutral too.
Reply to
Greg
| I know this will go on for weeks but the short answer is you only want one main | bonding jumper in the system. If you do bond the neutral at the genset and at | the service the transfer equipment will have to switch the neutral too.
I was afraid of something like that. What I would expect is to ground the genset (outside), but bring in only the hots and neutral (not the ground wire ... e.g. just like the power company does with grounding the transformer and feeding hots and neutral). But having separate points of entrance complicates this. If mains and gen power can be brought in at the same point, then that one point can be the master bonding point for all ground electrodes, all neutrals, and all grounding wires.
The reason for asking this is to figure out how much of a hassle it would be if there were separate entrance points. I was guessing, and you are confirming, that it would be a lot. I'll try to design things around having a common entrance, even if it means the power company might be required to run its service drop around back to the garage, where the generator actually is, rather than the front of the house, which they tend to prefer.
Reply to
phil-news-nospam
Right on
With remote transfer switches you will need to carry a code gauge ground through out the system. You will also have to ground the generator and the electrical service to the same point. That could be some really big conductor a long way. If you have 3 pole transfer switches make sure that the neutral for the generator and the service are at equal potential. Good idea even if you have 4 pole switches.
Take the maroon that designed this out to the wood shed. Won't help your situation but it will make you feel better. Good luck
Reply to
SQLit
Just don't bond the neutral in the generator. If you buy one of the homeowner types the neutral is usually not bonded anyway. Then your main bonding jumper will still be the "master grounding point". You will be running a 4 wire feeder to the genset, just like you would to a sub panel. If you feel you want a ground rod out there it will only connect to the green wire ground. YMMV on the 'legal" need depending on the details of the generator installation.
Reply to
Greg
| Just don't bond the neutral in the generator. If you buy one of the homeowner | types the neutral is usually not bonded anyway. Then your main | bonding jumper will still be the "master grounding point". You will be running | a 4 wire feeder to the genset, just like you would to a sub panel. If you feel | you want a ground rod out there it will only connect to the green wire ground. | YMMV on the 'legal" need depending on the details of the generator | installation.
It will be a larger one, and probably three phase even though I don't need three phase (I'll just try to evenly split my loads three ways). I'm already going to be dividing the house power into some subpanels. I'll just try to make the division reasonably balanced.
And that's not my only option. I'm just weighing the options now, and getting valuable input on those weights.
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phil-news-nospam
|> I know this will go on for weeks but the short answer is you only want one | main |> bonding jumper in the system. If you do bond the neutral at the genset and | at |> the service the transfer equipment will have to switch the neutral too. | | Right on | | With remote transfer switches you will need to carry a code gauge ground | through out the system. You will also have to ground the generator and the | electrical service to the same point. That could be some really big | conductor a long way. If you have 3 pole transfer switches make sure that | the neutral for the generator and the service are at equal potential. Good | idea even if you have 4 pole switches. | | Take the maroon that designed this out to the wood shed. Won't help your | situation but it will make you feel better. Good luck
Nothing is designed, yet. I'm just looking at options ... and what ones I should disregard (though I'm more wanting to get "why to disregard it" than "I would disregard it").
Maybe a better option is to bring the utility power in at the garage where the generator is, and bond everything together there. Then run things to the house (underground). Either that will be separate runs for generator and utility (switched at the house), or separate runs for switched and utility (for separate circuits at the house). Can that be done with one grounding wire (grounded again at the house) and one neutral (not bonded to ground at the house) even though there are a variety of different hot wires?
The catch will be how readily the utility will go along with putting the service in at the garage (which will be further back from the house) and not at the house. They tend to like putting service entrances at the front (for things like making it easier to access their meter, presumably).
Reply to
phil-news-nospam
Snipped
OR you could put the generator in the garage or behind it and locate the service and transfer switch at the service location on the house. Just run a feeder to the transfer switch from the gen set. Check with you serving utility, here, Phoenix area the utilities will NOT put a service on a remote building. Unless the remote building has the load for another meter. A friend has a 3 phase 240 v on his shop and a single phase 240v on the house. He had to show something like 100kw of machines, easy for him to do. Pick up a copy of the Soars book on Grounding. Explains a lot of this in simpler verbiage. Even has pictures. Just a WAG, trying to help not hinder. Sorry about the wood shed quip, it WAS meant in a humorous way
Reply to
SQLit
| OR you could put the generator in the garage or behind it and locate the | service and transfer switch at the service location on the house. Just run a | feeder to the transfer switch from the gen set. Check with you serving | utility, here, Phoenix area the utilities will NOT put a service on a remote
How do they define what "remote" means? I consider all the buildings to be peers. For all the builings being powered by one service drop, why would they care which? Are they merely wanting to drop power into the closest building?
OK, so how then is the garage powered? I've already found that some jurisdictions do not allow "double feeding" (e.g. two ways between the same two buildings).
Here's a thought. I build the garage first. It will have a small living area in the upstairs part of it. I then have power brought out to it first and put in the generator. Then the main house is built, and power is "extended" to it. Then, if necessary, the existing service is upgraded to the capacity needed (I doubt I would need over 225 amps).
There will be a total of 4 buildings.
| building. Unless the remote building has the load for another meter. A
What do you (or they) mean by "has the load for another meter"?
| friend has a 3 phase 240 v on his shop and a single phase 240v on the house. | He had to show something like 100kw of machines, easy for him to do. Pick | up a copy of the Soars book on Grounding. Explains a lot of this in simpler | verbiage. Even has pictures.
Utilities have varied basis for requirements for three phase. Some just plain say no, period. Others say you can have it if you ask for it and the lines are already there. Some require specific size motors. Some require a minimum demand. And some even require you to take three phase over certain demand levels (167 kVA on the last one I saw). All charge extra, but some charge a lot extra (even if they didn't have to extend primaries).
I considered three phase a while back, but ultimately decided it would be a big hassle, particularly running 240 volt stuff on 208 volts. So I don't want it. But with a 12-lead three phase generator, I can get a pair of 120 volt outputs on each phase angle, so I can just use it as single phase. But utility power doesn't come that way, requiring transformers to derive a 240/120 system.
Reply to
phil-news-nospam
They make the rules, they are the utility...
Might work. 225 puts you to a 400 amp service... This is a red flag around here. Yes I have installed 400, 600, and 800 amp services on residences in the past. Most were single phase. A few were 3 phase 208v my choice because of the equipment we were installing. Heck one house had 40 tons of a/c on the gargage. You would not want to get into a hot car would you?
His out building was a shop he has 20-30 pieces of wood working equipment. Smallest has a 3 hp motor.
You really need to contact your serving utility and see what their requirements are. What I am telling you may be a mute point with your utility.
> | friend has a 3 phase 240 v on his shop and a single phase 240v on the house. > | He had to show something like 100kw of machines, easy for him to do. Pick > | up a copy of the Soars book on Grounding. Explains a lot of this in simpler > | verbiage. Even has pictures. > > Utilities have varied basis for requirements for three phase. Some just > plain say no, period. Others say you can have it if you ask for it and the > lines are already there. Some require specific size motors. Some require > a minimum demand. And some even require you to take three phase over certain > demand levels (167 kVA on the last one I saw). All charge extra, but some > charge a lot extra (even if they didn't have to extend primaries). > > I considered three phase a while back, but ultimately decided it would be a > big hassle, particularly running 240 volt stuff on 208 volts. So I don't > want it. But with a 12-lead three phase generator, I can get a pair of > 120 volt outputs on each phase angle, so I can just use it as single phase. > But utility power doesn't come that way, requiring transformers to derive > a 240/120 system. > > -- > --------------------------------------------------------------------------
Reply to
SQLit
> I'm posting this here instead of sci.engr.electrical.compliance because > I'm looking more for technical answers, rather than code/legal answers. > This pertains to the US power system, either split single phase or wye > three phase. > > The mains power comes into the building at one point where a panel that > splits that power into N different subfeeds. There is a main breaker > and branch breakers for each subfeed. The generator power comes into > the building at a different point where it has its own panel that splits > that power into M (where M breaker and branch breakers for each subfeed. Each of the subfeeds goes > to a different area of the building where a transfer switch is located > specific to that area of the build (except for areas fed by mains power > only). Each area then has its own breaker panel to branch out to > individual circuits. > > The big question is how to properly ground this. Normally the panel > at the service entrance would be grounded. But in this case, there are > two service entrances at different locations. > > Should both entrances be grounded? Should the grounds wires be run to > a common (set of) ground electrode(s) or should the be separate? > > Should the neutral be bonded to ground at both entrances or just one? > > I'm assuming the neutrals would all be connected together via each of > the area transfer switches. > > It would seem to me there are a lot of opportunities for ground loops > and other strange current paths. Keep in mind that some areas may be > switched to generator while others are switched to mains (for example > to wait until mains is considered stable before switching back). Also, > some areas may be wired as single phase under a three phase setup. And > just to make things even more complicated, consider the possibility that > the one source might be single phase and the other source three phase, > with most loads being single phase and roughly balanced. > > -- > --------------------------------------------------------------------------
Reply to
Gerald Newton
|> |> |> | OR you could put the generator in the garage or behind it and locate the |> | service and transfer switch at the service location on the house. Just | run a |> | feeder to the transfer switch from the gen set. Check with you serving |> | utility, here, Phoenix area the utilities will NOT put a service on a | remote |> |> How do they define what "remote" means? I consider all the buildings to |> be peers. For all the builings being powered by one service drop, why |> would they care which? Are they merely wanting to drop power into the |> closest building? | | They make the rules, they are the utility...
But what is the definition?
How would they (or the AHJ doing an inspection) define what appear to be separate buildings, but can be reached by undeground hallways?
|> OK, so how then is the garage powered? I've already found that some |> jurisdictions do not allow "double feeding" (e.g. two ways between the |> same two buildings). |> |> Here's a thought. I build the garage first. It will have a small living |> area in the upstairs part of it. I then have power brought out to it | first |> and put in the generator. Then the main house is built, and power is |> "extended" to it. Then, if necessary, the existing service is upgraded |> to the capacity needed (I doubt I would need over 225 amps). |> |> There will be a total of 4 buildings. | | Might work. 225 puts you to a 400 amp service... This is a red flag around | here. Yes I have installed 400, 600, and 800 amp services on residences in | the past. Most were single phase. A few were 3 phase 208v my choice because | of the equipment we were installing. Heck one house had 40 tons of a/c on | the gargage. You would not want to get into a hot car would you?
What makes it a red flag? Asking for 225 amps (as opposed to 200 amps)? What is it a flag for? Does it indicate someone doing something funny?
How easy was it to get higher amperage service for a residence from the power company?
How easy was it to get three phase for a residence from the power company? Did they want proof of need? How much more did they charge?
Did the homes where you installed 208Y/120 have any 240 volt appliances?
40 tons of A/C? Sounds like an ice factory. A car in an uncooled garage does not get as hot as a car out in a parking space, depending on windows in the garage, blacktop on the parking lot, whether it is Texas, etc. But 40 tons for a garage? What's it holding? An antique car collection?
|> | building. Unless the remote building has the load for another meter. A |> |> What do you (or they) mean by "has the load for another meter"? | | His out building was a shop he has 20-30 pieces of wood working equipment. | Smallest has a 3 hp motor.
My grandfather had a small wood working shop and 3 phase power to it. But I don't think he had anything over 3 HP. He had maybe 6 major machines in about 10 foot by 20 foot space.
| You really need to contact your serving utility and see what their | requirements are. | What I am telling you may be a mute point with your utility.
I won't know what utility will be involved until after I find the land I will be building on, and that probably won't happen for a few more years. But by then I hope to have the choice of house design complete. Right now I have 5 different designs, only one of which looks like a "normal" house.
Reply to
phil-news-nospam
Why not get a single phase generator?
Is the generator for backup or some other use?
What are you planning that needs 225a?
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Reply to
Jim Michaels
Your first post describes a large parallel distribution system with multiple transfer switches such as you would see in a large commercial facility. However in your subsequent posts you are talking about a large house. For a 225 amp service residence why not do the following. Select the loads you want on emergency power and group them in a subpanel (lets call it panel PE1). PE1 might be a 100 amp panel fed from a breaker in the main service entrance panel (the 225 amp panel, call it MDP). However install the ATS between MDP and PE1 and feed from the generator. Locate the generator wherever you want. The wire and conduit between the generator and the ATS will be cheaper than the parallel distribution system you described. You should ground and bond the service entrance per code and also ground the generator and bond the neutral to create a separately derived system. This will require a transfer switch that will switch the neutral. (a.k.a. a 4 pole switch if you are 3 phase). There is a lot of debate as to whether to switch the neutral or not. My research has shown that it is better to switch the neutral. This of course assumes your generator can be grounded. Larger ones can; the small portable from Home Depot may not be able to be bonded. However, if your emergency load is 100 amps you will not be using a small portable.
Why would you need the parallel distribution system with multiple transfer switches. Is there a need to have the garage on generator power if the house is on utility? If your house is so large that you are concerned about voltage drop you could locate subpanels fed from PE1 and MDP at the other end of the house or in the garage but you would still only need one transfer switch. A fully loaded 16Amp 120V circuit will see a 3% voltage drop at about 75 feet. Since circuits are seldom loaded to 16 amps you can round up to 100 feet. That means a panel centered in a space can cover 100 feet in all directions. How big is your house?
As for the other questions you brought up about 240 V appliance running on 208V. Nearly all will. many times an appliance will carry 240V rating because that is the normal US residential voltage and that is the highest the manufacture wants the appliance to see(don't hook it up to 277V). 208 is a common commercial voltage and most residential appliance will work with only a slight derating.
> I'm posting this here instead of sci.engr.electrical.compliance because > I'm looking more for technical answers, rather than code/legal answers. > This pertains to the US power system, either split single phase or wye > three phase. > > The mains power comes into the building at one point where a panel that > splits that power into N different subfeeds. There is a main breaker > and branch breakers for each subfeed. The generator power comes into > the building at a different point where it has its own panel that splits > that power into M (where M breaker and branch breakers for each subfeed. Each of the subfeeds goes > to a different area of the building where a transfer switch is located > specific to that area of the build (except for areas fed by mains power > only). Each area then has its own breaker panel to branch out to > individual circuits. > > The big question is how to properly ground this. Normally the panel > at the service entrance would be grounded. But in this case, there are > two service entrances at different locations. > > Should both entrances be grounded? Should the grounds wires be run to > a common (set of) ground electrode(s) or should the be separate? > > Should the neutral be bonded to ground at both entrances or just one? > > I'm assuming the neutrals would all be connected together via each of > the area transfer switches. > > It would seem to me there are a lot of opportunities for ground loops > and other strange current paths. Keep in mind that some areas may be > switched to generator while others are switched to mains (for example > to wait until mains is considered stable before switching back). Also, > some areas may be wired as single phase under a three phase setup. And > just to make things even more complicated, consider the possibility that > the one source might be single phase and the other source three phase, > with most loads being single phase and roughly balanced. > > -- > --------------------------------------------------------------------------
Reply to
milo
Utilities "are" the AHJ up in NESCland. From the "service point", back to the electrical generation plant, is controlled by the utility and they don't operate under the same rules as we do. The electrical inspector may not like something he sees but this will probably end up in the public utilities commission before they do something about it. Utilities do conform to the NESC but that is a more basic code than the NEC and it does not micromanage their installations. As for what types of service they might choose to allow, that is their call with your only recourse being the public utilities commission if you don't like it. Generally, if you can get it past the local engineering office and sales at the electric company, you can have it tho.
Reply to
Greg
| Why not get a single phase generator?
Suggest a model.
| Is the generator for backup or some other use?
Basically for backup. I might end up locating where power is unreliable. Where I live right now, it seems the POCO has no such thing as tree crews. No granted, tree overgrowth is significant here (many places have primary lines entirely under solid canopy growth). One out of two rain storms will knock out power in ways I suspect is just broken off segment of a tree branch dropping onto the lines and forcing a recloser lockout if it doesn't get it cleared off.
| What are you planning that needs 225a?
The ability to continue with the cooking for a family gathering even if the power goes out. Actually, it's more a case of designing the house so that the design itself does not prevent that capability. So as I do the design, I "drop in" that capability to see how it plays out. I may never actually get a generator that big. But I want to have the option to do so without having to rip out the entire electrical system and rebuild things in some ugly way to work around limitations in the designs. For example, if all I ever wanted to do was run a 5000 watt generator to keep the refrigerator going, I could just stick an inlet on the back porch and set up a small transfer panel and put the refrigerator outlet on that. But doing so with a 60000 watt generator is not so simple. 60000 watts is a big generator you don't just lug up the steps. And running it for a few days requires a lot of fuel. This isn't the kind of thing you want to wait until the power actually goes out to decide how you want to do it. For example I need to decide between diesel and LP gas. The former stores badly, but then, it is likely to get used fairly soon if I stay around here. LP stores better, but requires better planning, like where to put the tank for refill access. A transfer switch for this would basically need to be switching the whole service. And I do not want to have a transfer switch be the main disconnect, so that means a separate main disconnect ahead of the transfer switch (one each for both mains and generator).
Reply to
phil-news-nospam
| Your first post describes a large parallel distribution system with multiple | transfer switches such as you would see in a large commercial facility.
I was trying to generalize the question enough to get generalized answers that address the technical theory, rather than having someone try to make assumptions about one particular case that ends up limiting the scope of flexibility. I want to understand the issues so I can choose the solution that matches changing (while I go through the design phase) situations.
| However in your subsequent posts you are talking about a large house. For a | 225 amp service residence why not do the following. Select the loads you | want on emergency power and group them in a subpanel (lets call it panel | PE1). PE1 might be a 100 amp panel fed from a breaker in the main service | entrance panel (the 225 amp panel, call it MDP). However install the ATS | between MDP and PE1 and feed from the generator. Locate the generator | wherever you want. The wire and conduit between the generator and the ATS | will be cheaper than the parallel distribution system you described. You | should ground and bond the service entrance per code and also ground the | generator and bond the neutral to create a separately derived system. This | will require a transfer switch that will switch the neutral. (a.k.a. a 4 | pole switch if you are 3 phase). There is a lot of debate as to whether to | switch the neutral or not. My research has shown that it is better to | switch the neutral. This of course assumes your generator can be grounded. | Larger ones can; the small portable from Home Depot may not be able to be | bonded. However, if your emergency load is 100 amps you will not be using a | small portable.
I'm not entirely convinced on the issue of switching the neutral. What I see as the result of not switching the neutral is still NOT a case of the neutral currents going over the ground wire. That is, no loops are formed that involve conductors expected to carry current (as long as you either do not bond at the generator, or do not carry the ground wire in with the feed from the generator). What you do get is a doubly grounded system if the generator comes in at a different location. OTOH, I'm not really opposed to switching the neutral; the grounding wire would not be switched. The biggest issue, perhaps, is that switching the neutral involves wear on switch contacts that may, at some point in the future, result in a contact not being made as well.
| Why would you need the parallel distribution system with multiple transfer | switches. Is there a need to have the garage on generator power if the | house is on utility? If your house is so large that you are concerned about | voltage drop you could locate subpanels fed from PE1 and MDP at the other | end of the house or in the garage but you would still only need one transfer | switch. A fully loaded 16Amp 120V circuit will see a 3% voltage drop at | about 75 feet. Since circuits are seldom loaded to 16 amps you can round up | to 100 feet. That means a panel centered in a space can cover 100 feet in | all directions. How big is your house?
One of the complications is that I want to be able to use a three phase generator. I won't necessarily go to this full power level, but if I do, I don't want to have designed in a limitation that prevents me from doing this. I believe that if I subdivide my circuits in three roughly balanced groups, then I can just parallel them from utility power, and run them as separate phases from three phase power.
The size of the house is still undecided. I've got 5 current designs I am still working on (and changing around). I'm not concerned about voltage drop. I plan to use some subpanels which would be fed with AWG 2/0. I'd rather have one big fat run than 30 small 12-2G runs.
| As for the other questions you brought up about 240 V appliance running on | 208V. Nearly all will. many times an appliance will carry 240V rating | because that is the normal US residential voltage and that is the highest | the manufacture wants the appliance to see(don't hook it up to 277V). 208 | is a common commercial voltage and most residential appliance will work with | only a slight derating.
I've experienced motor burnouts due to running a 230 volt single phase version on 208 volts. Many appliances like electric cooktops will drop to 75% performance at 208 volts (though I have also found some that do have alternate elements available for 208 volts ... but that's not what I want to be doing when the power goes out).
240 volts (not 230 volts) is the nominal level in the US. It's supposed to stay within 5% on a long term basis, and 10% on a short term basis. That means a voltage anywhere between 228 and 252 could be what you get all the time, and it could occaiasionally go out to 216 to 264. If my voltage is regularly too far from nominal, I will do what it takes to get it to be where I want, such as with a buck/boost transformer, either on the whole system if the problem is wider, or on specific appliances if only they need the correction.
Reply to
phil-news-nospam
See below
Keeping full (NO cutbacks) function in an all electric mansion during a utility failure is a BIG luxury.
My situation is somewhat similar to what you are seeking.
I have a 400amp service that has two 200a main breakers in the meter pedestal, these feed two 200amp transfer switches, which each feed two 200amp 40 space panels.
I can hook up either one or two generators to the transfer switches.
I have successfully used a portable 5kw generator during a winter power outage and Friday I am picking up a Generac 15kw natural gas unit from Home Depot to provide permanent automatic back-up.
I have electric cooking with natural gas for clothes drying and space and water heating . I anticipate that when using the 15kw I will only be able to run one central air conditioner at time and will have to shut down the Jacuzzi and generally be frugal with power. For the 30 hours per year that I experience outages this seems more than adequate.
If there is a long term outage ???, I could bring in a portable generator to supplement or one or two portable generators to replace the 15kw.
If you want massive functionality during an outage you could get a pair of :
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connect one to each transfer switch.
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Reply to
Jim Michaels
| If you want massive functionality during an outage you could get a | pair of :|
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| and connect one to each transfer switch.
They don't indicating the winding configuration on that. But I did find another generator of the same size from Kohler, and it does use a double delta type configuration, which is what I want to avoid.
Reply to
phil-news-nospam
It is a true single phase generator.
All the tech info is in the PDF specification sheet. The link is at the bottom of the main page.
Kohler also offers a true single phase unit in 45kw (model 4Q10)
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Why do you want to avoid the Zig-Zag (double delta) configuration?
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Reply to
Jim Michaels

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