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.
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Chief McGee wrote:

Chief,
What you have is an open delta. You can have a look at a wiring diagram of the bank of transformers at
http://murrayranch.com/Electricity.htm
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
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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

for
driven
meter
and
bar in

machine. AT

power/stinger
base
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wrote:

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.
--<< Bruce >>--
--
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Electrician for Westend Electric - CA726700
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box
in
(no
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?
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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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On Thu, 29 Jul 2004 16:09:01 GMT, Bruce L. Bergman put forth the notion that...

Oh really? I've been licensed since 1974, and that's news to me.
--
Checkmate

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wrote:

Okay, that was slightly overstated. ;-) Nothing outright, but there are a bunch of restrictions in Section 250 on using conduit for ground that you have to consider (and handymen never do) like the restrictions on using liquidtight and aluminum flexible conduit as the grounding conductor (conduit length and circuit ampacities).
But the more important part is that you really can NOT trust the conduit on old installations to be made up properly and a reliable grounding path, unless you want to crawl the attic and open the walls to inspect every inch of it... Fittings get loose, couplings get sprung inside walls where they can't be seen, "handyman" installers get creative when they don't have the right transition couplings (using plastic parts and big wads of tape)...
When in doubt, take a green grounding conductor back to the main with your other new conductors, and bond it to the boxes on your way - it's cheap insurance. And they'll rarely argue if you go a bit above and beyond the minimums that the codes call for.
Especially on very old houses that started out with an A-base meter, 30A fused disconnect and knob-and-tube through the attic, and have been upgraded piecemeal over the years with steel rigid, steel flex, alflex, Wiremold, XO breakers, Pushmatic breakers, Zinsco breakers... I've had lots of fun on those Electrical Archaeology projects.
Even on newer installations within the last 20 years I've had fun with ground faults and bad conduit grounds. Especially with cast zinc fittings that started a bit loose and burned themselves way loose - you wiggle the fitting and get a big spark across the gap...
(There's a /reason/ I listen to a portable AM radio at work, the static will let you pick up on stuff like that - If you wiggle the fitting and the radio goes staticky, start digging...)
--<< Bruce >>--
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Electrician for Westend Electric - CA726700
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| What you have is an open delta. You can have a look at a wiring diagram | of the bank of transformers at | | http://murrayranch.com/Electricity.htm | | 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.
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snipped-for-privacy@ipal.net wrote:

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
http://murrayranch.com/Electricity.htm
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
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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.
--
-Mike

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wrote:
| 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.
http://www.marcustransformer.com/techspecs.htm See at bottom. They say it cancels triplen harmonics magnetically. I haven't confirmed that.
http://www.marcustransformer.com/images/27rev.pdf Just a picture
http://www.dongan.com/products/pg26-33.pdf See pages 27, 30, and 32 (diagrams 1 and 4)
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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
wrote:

diagram
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Chief McGee wrote:

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

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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
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You do not need the neutral for the air compressor or three phase loads. The neutral is needed only for 120volt lighting and convenience outlets. Ground conductors are needed for everything and must be kept seperate from the neutral all the way back to the main service entrance panel. Don Young

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wrote:

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.
--<< Bruce >>--
--
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Electrician for Westend Electric - CA726700
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