120 volt receptical from 208 3 phase panel

[mid tenn elect]

Hey all, I involve myself mostly with residential elect, but, was given a set of prints for a "clean room" at the AF base and asked if I would be interested in the elect work. I noticed the requirement for 3 each 120V, 20A receptical circuits. The panel, i was told, will be 208V, 3 phase, provided by the AF. Question is, how do I get 120V single ph from a 208V 3 ph panel ?

 

[celtic]

Won't 1 phase and nuetral = 120v?

 

[mid tenn elect]

That's my thought, but I don't deal with 208 or 3 ph enough to keep it in my head. By using a single 20 amp breaker, a neutral, and a ground, i should be able to provide 120 volt, single phase power to all circuits, correct ?

 

[mdshunk]

That's my thought, but I don't deal with 208 or 3 ph enough to keep it in my head. By using a single 20 amp breaker, a neutral, and a ground, i should be able to provide 120 volt, single phase power to all circuits, correct ?

Mid Tenn... divide 208 by 1.732 and see where that puts you.

 

[LarryFine]

By using a single 20 amp breaker, a neutral, and a ground, i should be able to provide 120 volt, single phase power to all circuits, correct ?

Correct, and not only that, you might even be able to use a shared neutral for all three circuits, much like two 120v circuits from 240/120 1ph.

 

[sparky59]

3-phase

3-phase

on 3-phase 208Y the voltage from either phase to neutral is 120 volts. I know you would think phase to phase should be 240 but 3-phase isn't like single phase.

 

[adrian33773]

I do a lot of commercial work. You would use three 20A single pole breakers. The breakers need to be right on top of each other so each one is on a separate phase. Use black, red, and blue ungrounded conductors. You only need one white grounded conductor and one green for the grounding conductor. Run all five wires (12 AWG) in 3/4" EMT to a junction box close to the clean room. Use 4" SQ X 2-1/8" Deep for junction box. Then branch in three directions from there with 1/2" EMT. Circuit #1 or 2 = black, white, green. Circuit #3 or 4 = red, white, green. Circuit #5 or 6 = blue, white, green.

 

[brian john]

The major area of concern is that the panel is 3 phase 4 wire, there are numerous 3 phase 3 wire panels out there......

 

[adrian33773]

Also, if they want each circuit to be dedicated or isolated, you would have to run a separate neutral and ground for each one. There's all kinds of ifs.

 

[e57]

The major area of concern is that the panel is 3 phase 4 wire, there are numerous 3 phase 3 wire panels out there......

Ah- Usually you can get the up-grade kit...

 

[infinity]

on 3-phase 208Y the voltage from either phase to neutral is 120 volts. I know you would think phase to phase should be 240 but 3-phase isn't like single phase.

That's because this is a 3 phase Wye system. A 3 phase, 4 wire Delta system would have a phase voltage of 240 volts.

 

[mid tenn elect]

I think it is a 3 phase, 3 wire panel. Thanks all of you, adrian hit the nail on the head, the guys building the clean room described the previous elect work as you did. Thank's a lot, this has been very helpful.

 

[haskindm]

If it is a 3-phase, 3-wire panel, then you do not have a neutral and only 3-phase loads may be served by the panel. You need to have a 3-phase, 4-wire panel in order to have a neutral to supply single phase 120-volt loads.

 

[Rockyd]

There's a great article on transformers in regard to practical operation and theory at this link.

 

[bsh]

If the panel is 3 phase 3 wire (a delta system)then you only have 208 volts available. You need to make sure it is 3 phase 4 wire (wye system). The fourth wire is the neutral from the transformer and will provide 120 volts between the neutral and any of the 3 phases.
I would consider a neutral for each circuit in case they have some harmonic generating load which could elevate the neutral current.

 

[don_resqcapt19]

bsh,
Unless you have a special transformer the 3 wire delta voltage will not be 208, it will be 240. 208 is a wye system voltage. Either system can be set up as a 3 phase 3 wire ungrounded system, and not have 120 available. Most of the time the wye is a grounded system, but not always and even if it is a grounded system, the grounded conductor is not always run to the panel.
Don

 

[LarryFine]

Most of the time the wye is a grounded system, but not always and even if it is a grounded system, the grounded conductor is not always run to the panel.

True, but it should be run to at least the main disconnect.

 

[don_resqcapt19]

Larry,

True, but it should be run to at least the main disconnect.

I agree, but the code only requires that for services, not for SDS.
Don

 

[mid tenn elect]

I looked at the prints a little closer today. There is nothing requiring 3 phase power in the clean room. Even the A/C units are single phase 240V 60A. I wonder why the need for 3 phase. And with all the 120V and 240V requirement, it must be a 4 wire system. I only saw the panel leaning against the wall, not hook up. You guy's have been very helpful, I'll be looking around and reading more in the future. Love this site.

 

[brian john]

Upgrade kit, what they supply a 4th conductor?