single phase from 3-ph

This was mentioned a few days ago but would someone please clarify it for me. I am installing 3-ph to the shop. It is 200A delta wound, 2 transformers on pole, 3 hot wires 1 neutral from pole, ground rod driven into the dirt and hooked to the neutral buss bar. Drawing with the meter box shows the neutral wire coming from the middle of one side of the triangle that forms the delta. Is this what is called "center tapped"? I think I want to run 5 wires to each machine. 3 hots, 1 neutral, and one ground from the machine base all the way back to the neutral buss bar in the sub panel. Is this right? I also want to have a 120 outlet and 120 work light on each machine. AT THE MACHINE, I am going to pick one of the hot legs(NOT the power/stinger leg) and the neutral. Will connect the ground leg in the light to the base of the machine. Is this right or do I have to go all the way to the sub panel with the wires? Next, I have a 5-hp 220V single phase air compressor. For that I think I need only 4 wires. 2 hots(NOT the power leg) 1 neutral, and one ground from chassis all the way back to the neutral buss bar in the sub panel. Is this correct? I want this to be safe and legal. Thanks for your time, Chief.

Reply to
Chief McGee
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The only reason you need to bring the neutral to the machine is for your light or other 120v load. (but I bet you know that). Personally I would put all of my single phase loads on the center tapped transformer. They are probably only using 2 transformers so the red leg can drift around a bit if you don't.

Reply to
Greg

Chief,

What you have is an open delta. You can have a look at a wiring diagram of the bank of transformers at

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Take a look at the diagram that is open wye-open delta. Don't concern yourself with the primary or open wye. But the secondary open delta is just what you have.

I can't answer your question as to the number of wires to each machine as I don't know the inside code, my field is as a lineman/troubleshooter.

Hope this helps, Don

Reply to
Don Murray

That side of the triangle, with the center tap, is identical to standard

120/240 single-phase service. The two hots are 2 of your 3 phase line terminals, as well. The additional transformer provices the 3rd phase line terminal.

Yes, that sounds right, so you can have 120 at each machine location. The safety ground should actually have a separate ground bus bar in the panel, but some installations don't separate the neutral and the ground.

Mostly. It seems OK to me to use the same ground conductor for the machine AND the light/socket. But, you can't use ANY line terminal for the 120! You must use one of the two that are on the center-tapped side. Going from Neutral to the 3rd line terminal will give you roughly 208 V, as this is not a center-grounded (balanced) Wye system.

Maybe. If it is entirely 220, including the contactor/motor starter, you only need 3 wires. Any two hots (but, maybe best to run it off the extra transformer) to reduce load on the center-tapped one that is going to get a lot of single-phase loads. Just be sure to get it across one of the transformers. IE, if the tapped transformer is supplying phases A and B, and the extra transformer is wired from B to C, don't connect the compressor from A to C. This L-L circuit will have more variation from 240 than the other 2 possible connections. If you need 120 V there for the contactor, then use A-B, and run the neutral as well.

I believe the recent NEC has changed requirements, and grounding through the conduit alone is no longer permitted.

Jon

Reply to
Jon Elson

If you run your wire thru conduit, that is your ground leg. Your breaker box and all is grounded to the ground rod. You should only need to four wires in the conduit.

Please note I am not an electrician. But I done a lot of my own wiring. (no fires yet!)

Randy

power/stinger

Reply to
Randy H.

"Chief McGee" wrote in news:eQTNc.204404$Oq2.30164@attbi_s52:

Sounds like 120/208 to me...(though I'm 15 years rusty, I work on Traffic Signals these days)

Nope. Run a 3 phase circuit for your machine, 3 hots and a ground. Fed off the proper size 3 phase breaker.

Run a seperate circuit in a seperate conduit off say a 15 amp or 20 amp single pole circuit breaker. Yes, some machines have a built in transformer for a couple reasons, one to reduce the higher voltage to single phase voltage for control contactors, single phase coolant pumps and even lighting but you will find that there is a fuse usually on either the primary or secondary side, sometimes both to protect the lower current (amp) circuits. Put a light and related wiring on a 30amp 3 phase breaker, and something goes wrong, the wiring will likely fry before a breaker trips. That's why I suggest you run a dedicated circuit just for your machine lighting.

See the above, Also, I believe Jon Elson to be correct. the NEC requires a seperate ground conductor. You can't rely on the conduit itself for a ground. Why? Because they have found that electricians sometimes forget to tighten the screws or lock nuts to keep the ground continuous, conduit pulls apart, guess what, no safety ground.

Yup for reasons states above, need a seperate ground now, you should be running flexible metallic conduit to the compressor, you need the seperate ground in that for the compressor, you could run heavy SO cord too. As long as it was 4 conductor.

You have doubts, you should run the conduit, and call an electrician to finish the job. Running the conduit is a good bit of work, let him pull and terminate the wire. My disclaimer, my advice may or may not be accurate, consult a licensed electrician for help....gawd I hate that....

Marty

Reply to
Marty Escarcega

| What you have is an open delta. You can have a look at a wiring diagram | of the bank of transformers at | |

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| | Take a look at the diagram that is open wye-open delta. Don't concern | yourself with the primary or open wye. But the secondary open delta is | just what you have.

Not necessarily. He could have the Scott 4 wire.

Reply to
phil-news-nospam

Reply to
Don Young

Don't worry, with that attitude you'll have your big fire any day now. :-/

Using the conduit as the sole source of ground has not been legal under NEC for many years - but it /used/ to be legal (since they didn't really worry about branch circuit grounding at all), so many people assume it still is. We've since learned that conduit joints get loose or sprung (and breaks the ground circuit) and nobody sees it or fixes it.

All power circuits that you run or re-run now should have a green safety ground wire run along with the power wires - if you are changing a section of wire in the middle of an old conduit run, bond your new ground wire to the boxes at each end, using a self-drilling sheet metal screw if the box doesn't have provisions for a grounding screw. And leave a pigtail of spare ground wire so you can connect to it when you rewire the circuit further.

If you take the green wire back to the main panel where the neutral and safety ground are bonded together, you can connect ground wires to a vacant hole on the Main neutral bar. If it is a sub-panel, you can't - you need to bond to the can, or add a separate grounding bar.

You can use a reduced gauge ground wire as compared to the power wires (like a #12 ground wire with #10 or #8 circuit wires) but it still has to be large enough to trip the breaker or blow the fuse on a ground fault without melting in the process.

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Reply to
Bruce L. Bergman

True - but putting in one or two extra wires is far cheaper than repulling it all later. Stick wires for all three phases and a neutral in that conduit now, and you future-proof yourself.

"Work Smarter, Not Harder."

If you need to get a bigger 3-phase air compressor later - or get a mega big one and reuse that old power line for something else, and that something else needs 3-phase and/or a neutral, you're ready.

And if not, in 30 years when they tear down the building you take the copper in for recycling. It'll be worth more as scrap than you paid new by then.

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Reply to
Bruce L. Bergman

Pretty scary huh?

See 250.118(2) thru 250.118(9) and 250.122

Reply to
Greg
250.118 Types of Equipment Grounding Conductors. The equipment grounding conductor run with or enclosing the circuit conductors shall be one or more or a combination of the following:

(2) Rigid metal conduit. (3) Intermediate metal conduit. (4) Electrical metallic tubing. (5) Flexible metal conduit where both the conduit and fittings are listed for grounding. (6) Listed flexible metal conduit that is not listed for grounding, meeting all the following conditions: a. The conduit is terminated in fittings listed for grounding. b. The circuit conductors contained in the conduit are protected by overcurrent devices rated at 20 amperes or less. c. The combined length of flexible metal conduit and flexible metallic tubing and liquidtight flexible metal conduit in the same ground return path does not exceed 1.8 m (6 ft). d. The conduit is not installed for flexibility. (7) Listed liquidtight flexible metal conduit meeting all the following conditions: a. The conduit is terminated in fittings listed for grounding. b. For metric designators 12 through 16 (trade sizes 3/8 through 1/2), the circuit conductors contained in the conduit are protected by overcurrent devices rated at 20 amperes or less. c. For metric designators 21 through 35 (trade sizes 3/4 through 11/4), the circuit conductors contained in the conduit are protected by overcurrent devices rated not more than 60 amperes and there is no flexible metal conduit, flexible metallic tubing, or liquidtight flexible metal conduit in trade sizes metric designators 12 through 16 (trade sizes 3/8 through 1/2) in the grounding path. d. The combined length of flexible metal conduit and flexible metallic tubing and liquidtight flexible metal conduit in the same ground return path does not exceed 1.8 m (6 ft). e. The conduit is not installed for flexibility. (8) Flexible metallic tubing where the tubing is terminated in fittings listed for grounding and meeting the following conditions: a. The circuit conductors contained in the tubing are protected by overcurrent devices rated at 20 amperes or less. b. The combined length of flexible metal conduit and flexible metallic tubing and liquidtight flexible metal conduit in the same ground return path does not exceed 1.8 m (6 ft). (9) Armor of Type AC cable as provided in ...

250.122Table 250.122 Minimum Size Equipment Grounding Conductors for Grounding Raceway and Equipment Rating or Setting of Automatic Overcurrent Device in Circuit Ahead of Equipment, Conduit, etc., Not Exceeding (Amperes) Size (AWG or kcmil) Copper

15 14

20 12 30 10

After 30a you can start reducing the EGC

Reply to
Greg

I agree with the practice of running an equipment grounding conductor in the conduit for the same reasons you mention, but could you please cite the code article which prohibits the use of a conduit system as the sole equipment grounding conductor?

Reply to
Rick

Phil,

The only Scott connections I know about are to get 2 phase power from 3 phase. If you look at the bottom of this page

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you can see diagrams of three, four and five wire Scott connections.

If you know of any Scott connections which provide 3 phase power, I would like to learn about them.

Don

Reply to
Don Murray

Does any utility *ever* provide Scott two-phase power, except for a few rare cases of feeding very old stuff?

You could configure two transformers with both primaries and secondaries in a Scott configuration and wind up with what amounts to 3 to 2 to 3 phase transformation. I have never heard of this being done with pole pigs even though you only need two of them, but someone here mentioned pad transformers are sometimes built like this internally.

Reply to
Michael Moroney

Don sez: "Not necessarily. He could have the Scott 4 wire."

Yep! Seems like this could be true -- Scott 2 single-phase circuits derived from 3-phase. How about this Peter H. What say you?

Bob Swinney

Reply to
Robert Swinney

Er, for giggles, PECO and Con Ed list "120/240V 2ø 5-wire" in their servce conditions {but not for new work in any had the slightest concern.} PECO also lists existing 2400V 2ø 3-wire, and Con Ed lists 1950V 2ø.

There is an intersting article on CF Scott and his invention in IEEE Industry Applications Magazine, Nov-Dec 2002, pp.6-8

"In considering the marked advantage of the two-phase system for distribution and of the three-phase system for transmission, it occurred to me that that a combination of the two systems night secure the advantages of both, and I have worked out a simple and effective method of accomplishing this result.?"

Reply to
s falke

What is this supposed "marked advantage" of two-phase for distribution?

Reply to
Michael Moroney

Thanks guys for the info. I can see why they made the changes in the code as a good idea. I will take my comment as not having any fires as being lucky on my part.

My building was built in 1954. I'm sure I will have a lot to change around.

Thanks again,

Randy Hansen SC Glass Tech Scam Diego, Comi-fornia

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Reply to
Randy H.

| Does any utility *ever* provide Scott two-phase power, except for a few | rare cases of feeding very old stuff?

I've seen them. It was a big (probably 200 kVA) can doing the 120/240 volt part and a small (probably 10 kVA) doing the 208 volt part. They probably needed a "little bit of three phase" and a lot of 120/240.

| You could configure two transformers with both primaries and secondaries | in a Scott configuration and wind up with what amounts to 3 to 2 to 3 | phase transformation. I have never heard of this being done with pole | pigs even though you only need two of them, but someone here mentioned | pad transformers are sometimes built like this internally.

I've seen a number of 2 pole pig setups. I did not check out most of them. But in one case where I did, they were using a three phase motor for a pump.

I've seen 2 core Scott-T in dry-type as well.

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See at bottom. They say it cancels triplen harmonics magnetically. I haven't confirmed that.

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Just a picture

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See pages 27, 30, and 32 (diagrams 1 and 4)

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phil-news-nospam

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