new service 1

[axd38]

I am working on a building where there are two seperate services both to be removed. One is a three phase three wire 120/230V and the other is single phase 120/230V service. The change will get rid of both the previous services and will now be to a new service three phase/4W. The question is that there is a lot of motors that are single phase 230V or three phase 230V, can these motors remain, as in be derated? Is there any part of NEC that covers this?

 

[axd38]

Sorry I forgot to post that the new service is 120/208V three phase 4 wire.

 

[jraef]

The quick answer is that MOST 230V rated motors can run fine on 208V, but that is not true for all of them. You should survey each one and check with the manufacturers. For instance, NEMA design criteria call for a minimum of -10% voltage tolerance, so a motor designed specifically for 230V will run fine at 208V, but not necessarilly at 200V. But if you look at the nameplates of many newer 230V motors, it actually says "200-230V", meaning that it was designed to do exactly what you are planning, work in a retrofitted 208V 4 wire system.

"Venditori de oleum-vipera non vigere excordis populi"

 

[aolalde]

If the motors are manufactured under NEMA MG1, they should work into +/-10% of voltage fluctuation.

10% of 230V = 23 so 207 V is still into the voltage limits.

The problem is that your base design is marginal and you have no allowance for operating voltage drops.

The motor power should be de-rated proportional to the squared ratio of the applied voltage to the minimum acceptable Voltage. Another inconveniences are; the current will increase when the voltage is reduced and your winding and power lines will work with high resistive loses, pushing the motors to operate into the limits of allowed temperatures, these factors will shorten the life expectancies of insulations and bearings.

I recommend you to re-evaluate your project and select the best compromise of voltage supply.

 

[buzzp]

I inventory all the motors for nameplate data. There are lots of flavors out there and only the nameplate will tell you for sure what the motor voltages are. I bet you will have nameplate voltages all over the place depending on the age of the plant. Then use the 10% rule using the nameplate voltage.
Most likely, the current requirements will go up slightly due to the reduction of voltage.

 

[busbar]

 
It is somewhat unusual to “upgrade” to a lower-voltage electric service, although 208Y/120V is admittedly more modern and typically readily supported by utilities in commercial neighborhoods and smaller institutional facilities.

Demand kVA of the existing 230V equipment is not mentioned, and may affect the following suggestions. Where 240V service is to be maintained, there may be two approaches.

Acme Electric group B or federal Pacific T242T-S transformers at 240—208Y/120 volts may permit a 240V 3ø separately derived system. It is likely that the drytype manufacturer would recommend that the transformer 208Y/120 X0-neutral terminal be left unconnected to prevent circulating currents in the local wye winding. Primary and secondary overcurrent protection would be needed per NEC 240-3(e)&(f)+, 450-4+ and others.

Another possibility is three wye-connected 120x240—16/32 volt boost autotransformers, from the same manufacturers. Nota Bene for wye-autotransformer application—the 208Y/120 circuit MUST carry the neutral from the source to autotransformers’ common-connected shunt windings, but with NO neutral extended to downstream equipment or loads—as phases will operate at ~139V to neutral/ground. NEC 450-4+ and others reference autotransformer application. Effectively it will be a 240Y/139V system.

Keep in mind that in both cases 150°C-rise transformers typically run blistering hot when approaching nameplate load. In both cases, dedicated 240V 3ø distribution should have distinctly different and readily identifiable color coding for phase conductors— critical to avoid damage and load imbalance.

1ø 230V motors might be ‘staggered Aø-Bø, Bø-Cø and Cø-Aø on the 240V 3ø system (with no attempt to connect 115V devices.)