208V 2-pole rated Compressor fed with 223V single-pole??

[blueboard]

I have a compressor that is rated for a 208V 2-pole(single phase) 30A circuit, being fed with a stepped down 277V circuit. So at the 2-pole breaker at the compressor I have 223V on one leg and 0 volts on the second leg. There should be a equal split of 115-120V on each leg. Has anyone seen this before? and is this operating on the thought process as though it was a 208V high-leg? and what would be the results on the compressor or motor under these conditions?

 

[GoldDigger]

I have a compressor that is rated for a 208V 2-pole(single phase) 30A circuit, being fed with a stepped down 277V circuit. So at the 2-pole breaker at the compressor I have 223V on one leg and 0 volts on the second leg. There should be a equal split of 115-120V on each leg. Has anyone seen this before? and is this operating on the thought process as though it was a 208V high-leg? and what would be the results on the compressor or motor under these conditions?

If the stepdown to 208V is coming from a transformer, it is possible that it is set up as a straight 208V two wire circuit with one side grounded. That is also what you would get if you applied a buck transformer to one leg of a 277 wye.
The NEC does not require an OCPD in the grounded conductor but allows it as long as any trip will open both grounded and all ungrounded conductors.

 

[blueboard]

If the stepdown to 208V is coming from a transformer, it is possible that it is set up as a straight 208V two wire circuit with one side grounded. That is also what you would get if you applied a buck transformer to one leg of a 277 wye.
The NEC does not require an OCPD in the grounded conductor but allows it as long as any trip will open both grounded and all ungrounded conductors.

So this might be standard procedure if the voltage available is 277V and what is needed is 208V, even though what the unit is designed for is an even split 208V circuit? How does the internal circuitry hold up under these conditions?

 

[just the cowboy]

Does it use a neutral coming in to it.

Does it use a neutral coming in to it.

Does the unit use 3 wires or 4 wires? If it uses 4 wires it probaly won't work, that would be 2 0v connections. If 3 wires it may work because it is not using any 120v. Does the name tag say 208-120v

 

[blueboard]

Does the unit use 3 wires or 4 wires? If it uses 4 wires it probaly won't work, that would be 2 0v connections. If 3 wires it may work because it is not using any 120v. Does the name tag say 208-120v

Feed is 277V (1 hot, 1 neutral, 1 ground) after xfmr it is 223V (1 hot, 1 neutral, 1 ground) but wires have been converted - instead of yellow for the hot and gray for the neutral, it turns to blue and red to look like 2 hots no neutral. This is what is run to compressor/motor. All roof top condensers are fed this way.

 

[GoldDigger]

Feed is 277V (1 hot, 1 neutral, 1 ground) after xfmr it is 223V (1 hot, 1 neutral, 1 ground) but wires have been converted - instead of yellow for the hot and gray for the neutral, it turns to blue and red to look like 2 hots no neutral. This is what is run to compressor/motor. All roof top condensers are fed this way.

As long as all of the breakers involved are full voltage rated and not slash (240/120) rated, the building wiring part of it should be OK. Only the equipment manufacturer can tell you with certainty whether their electronics can handle 208V to ground.

 

[J.P.]

As long as your transformer is splitting the phase you should be ok. If you made a 230v to ground or neutral I would say not ok.

I have been wrong before though. If it were me I would post a pic of the transformers diagram and the motors nema plate on here and let the clever guys look at it.

 

[kwired]

Is your transformer an autotransformer (buck-boost) or is it a separately derived system?

Either will work, I'm guessing you have a buck-boost as a full isolation transformer with 277 one side and 208 the other side would be kind of unusual (not impossible) and there would likely be some better cost or easier to find route to take.

If you have the buck - boost then the grounded conductor of the supply circuit should likely have remained white or gray all the way to your load- with a tap to the transformer, and the ungrounded supply conductor should have passed through the transformer coil to arrive with the new output voltage. Single pole overcurrent device on the supply side is all that is necessary for this setup.

Single pole overcurrent device is also all that is necessary with the separately derived system if the secondary is only a two wire system, but grounding one conductor of the secondary or ground fault detection is another issue that needs addressed.

 

[Jraef]

It boils down to two main issues.

1. Does the compressor control circuit use 120V that it is tapping from one line to neutral inside of the compressor controls? If so, this won't work at all. If the power goes ONLY to the motor, then it would not matter to the motor if one side is grounded, all that matters is the potential difference between the two leads. In fact this is exactly what 230V single phase power looks like in most other parts of the world outside of North America, and motors are exchanged back and forth all the time. So if its only the motor, read on.

2. Is your 208V compressor motor OK with 223V? It might be, but it might not, only the compressor mfr can tell you for sure. If its a hermetically sealed refrigeration compressor, likely not, if it's an air compressor with a standard off-the-shelf single phase motor 5HP or under, it likely is, or read the actual motor nameplate if you can see it. If the nameplate says 208/230V, you're good to go. If not, someone put in the wrong transformer.

 

[kwired]

It boils down to two main issues.

1. Does the compressor control circuit use 120V that it is tapping from one line to neutral inside of the compressor controls? If so, this won't work at all. If the power goes ONLY to the motor, then it would not matter to the motor if one side is grounded, all that matters is the potential difference between the two leads. In fact this is exactly what 230V single phase power looks like in most other parts of the world outside of North America, and motors are exchanged back and forth all the time. So if its only the motor, read on.

2. Is your 208V compressor motor OK with 223V? It might be, but it might not, only the compressor mfr can tell you for sure. If its a hermetically sealed refrigeration compressor, likely not, if it's an air compressor with a standard off-the-shelf single phase motor 5HP or under, it likely is, or read the actual motor nameplate if you can see it. If the nameplate says 208/230V, you're good to go. If not, someone put in the wrong transformer.

I guess I figured OP came across something that has already been operating as is, but yes the control circuit will matter. A 208, or a 208-240 contactor coil will still work with the way he described the setup - a 120 volt coil will not last very long.