I use a generator since I have no utility power where I live. I bought a second, back-up generator and want to wire it such that I can switch between one or the other. I bought a 240-volt two-pole transfer switch. The generators are both in a shed about 200 feet away from the house. There is a grounding rod at the house, and the neutral is bonded to the ground are the house service panel. I also put a grounding rod in at the shed (do I need it?). I'm confused how to wire the transfer switch.
The first generator has four wires - two hot, a neutral, and ground. The second generator has three wires - two hot and a neutral. The transfer switch only has terminals for two hot wires. There are three wires running from the shed to the house (two hots and the neutral).
What do I do with the neutral wires? Can I connect all three neutrals together at the shed? What do I do with the ground wire from the first generator? Do I need to ground the second generator to the grounding rod at the shed?
Trying to keep this simple. Not knowing where you are I am assuming (ass out of you and me) that your in the US.
I assume that only one generator will run at a time. I would first see if you can exchange the 2 pole double pole double throw switch with an 3 pole double throw switch. Ya I know the price difference. Problem is that the neutral really does carry current and voltage. You really want to isolate each gen set from the other, electrically and mechanically. Assuming that you will do this then you can land the neutrals on the appropriate places. If not then you land them all together. UG You could run a risk of destroying the standby genset if something happens to the primary set even if the second gen is off line. Complete electrical/mechanical isolation is preferred. I work at a place where all new transfer switches must be 4 pole. I proved the point and helped write the spec. In the long run there are less problems.
Number One (4 wire) gen establishes the neutral through the operation of the gen set. The number 2 ( 3 wire) establishes the neutral by the ground locally, at the gen set. So the grounding of the gen sets is critical. I would at the least drive 3-8 foot ground rods at least 10 feet apart and connect with an minimum conductor of the maximum of the 2 gen sets. If 40 amps is the out-put of the largest genset then #8 wire. Of course testing the ground system to less than 10 ohms is acceptable. Where I live, Phoenix, more is better. Testing is always better than guessing. I have driven, in Phoenix, 3 ground rods and never achieved 25 ohms or less, per the NEC. You will be running sensitive electronic equipment in your home so >10 ohms is recommended. It is called performance grounding. I have seen equipment wanting 3 ohms or less. 10 ohms is not unreasonable today. Test if possible test grounds, at least once a year in the driest time, IF applicable. If your not in the SW then probably the first test will be enough.
You will have to change the wiring to your home. Add conductors of sufficient size that your not having a voltage drop. 200 feet is a far piece.
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You need 2 hots, a neutral and a ground. The ground must be at least the largest of the 2 gensets. DO not scrimp. Ya I know what cost of what your doing. Do it right in the beginning and you will save money in the long run. REMOVE ground at the house. You really want to trip the gen breaker so the ground at the house presents a problem. If you do not remove the ground at the house it is an NEC CODE VIOLATION..... Section 230 of the NEC states that the ground must be in-common with the source. Many folks get this wrong.
If you connect the ground wires all together at the gensets then you can not go wrong. Make sure that the largest gen is the smallest conductor used. DO NOT de rate for smaller genset, if applicable.
I admire your situation. I too thought I could go off grid recently and then my situation changed.
If you have specific questions please contact me directly. IF you want to hear what I have to say.
Yes, only one generator runs at a time, but I want to make things foolproof if possible. What about dumping the transfer switch all together and putting a 30-amp plug on each generator. Then an outlet that goes to the house. I would plug the appropriate generator in and the other would remain unplugged?
I'm out of my league here, I think. What is the point of all this grounding. I have one 10-foot rod; is that not enough? I've been using a single generator with that rod for many years; have I just been lucky?
I have 2 guage and that seems to work.
What do you mean by "largest"? Do you mean it must be able to handle the greatest amperage the biggest generator can put out?
Again, I would like to know how important this is since things are working now. I am lucky?
If you do not remove the ground at
Code is somewhat irrelevant up here, however, I want to be safe. Do I risk fire or damage to equipment (generator, inverters, etc?)
Just to recap so I understand: isolate the neutrals but the grounds can all be together?
good idea would work well. I recommend twist locks. Less of a problem with vibration.
The point of grounding is to establish the neutral. It is also a return path so that the breaker will trip when neeeded. Using two ground rods spaced far apart you create a ground loop. ie one at the generators and one at the house. This could cause the current to travel through the ground from rod to rod. If this happened and you were walking between the rods you could be electruted. This is also know as step potential. Cows die in lightning storms because their feet are more than 1 meter apart. Example : you have a electrical fault at your house, it travels to the ground rod at the house, then through the ground covering the distance to the ground rod at the generator. Then and only then the breaker trips. As long as you NEVER have a fault oky doky.
You must carry the circuits, the neutral and the ground you established at the generators to the house. That is 4 wires if you using 220v and 3 wires if 120v
YES
Possibly, electricity can be a funny animal. A short circuit sometimes is not the shortest path in distance. It is the least resistance back to the ground path.
Irrelevant? Ok it is your life and your familys. There is a reason that all of the building codes get bigger each year. I remember that one code update included a specific reference to not putting electrical panels inside the bath tub. Even if they were water proofed. Some loony in Florida actually did install the panels in an apartment complex and fought with the building officials in court. Because the code did not state you could not do it.
It only takes 20 mili-amps to kill.
I used to tell people that I was a wizard. What I deal with is invisible, try to touch it and it will kill. My only intention is to keep ya safe, and happy.
I really appreciate your help; things are starting to make a little more sense to me. I'm leaning toward the plug-and-recepticle idea now if I can make it work safely. I have a couple more questions concerning grounding. I do not currently have a ground wire running from the generator to the house; only three wires for the 240-volt system. The lines are buried and the ground is already freezing up here in Fairbanks; I cannot easily access it again until next June. I have a vested interest in trying to use the house ground if at all possible; but safety is obviously my ultimate concern.
Also, we have a battery bank and inverter at the house that we use when the generator is not running, which is about 90% of the time in the winter. The inverters are currently grounded to the house ground.
Is it possible to not ground anything at the generators and only use the house ground? I looked at
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which discusses when grounding rods are required at portable generators. The second figure shows what is considered not a "separately derived system" and the generator does not need to be grounded. Could this apply in my situation? The generator would not be grounded and I would only run three lines (2 hot, 1 neutral) to the house. The house would be grounded and the neutral established there?
However, then I wonder if there is a danger with the generator "shorting out"? I'm obviously naive about engineering aspects such as these.
I appreciate that. Thanks in advance for all your help.
The lines are buried and the ground is already freezing up
Your having freezing weather and it was a balmy 108 today. Our nights are finally below 80.
In the interest of you not digging up frozen ground. Been there done that, you could consider laying a solid number 10 gauge wire staked to the ground every 5 or so feet so it does not become a tripping hazard. Until you can install the ground wire correctly.
The battery bank at the house with the inverter is another seperately derived system. You really do not use the generator power to run the house. You use the generator to charge the batteries. Is this correct? absolute isolation here? That would make a huge differance. I thought you were running the house on the gen sets. ya still need a ground wire between the gens and inverter set up but I would not be in that much of a hurry if I understand correctly... The inverter and battey bank is grounded, keeping the house safe. The issue is when the gen is running and your charging the batteries. Much smaller chance that something will go wrong.
I apoligize if I caused you and yours any stress. Text sometimes is not the best method of communication.
Your situation is closer to the third picture changing the service on left to your second gen set
notice that there are 4 wires through out the system. change the panel board to be your inverter set up. and add a ground rod. The ground rods need to be connected.
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