Nominal Voltage versus Utilisation Voltage

[IanKeith1960]

I am a UK based electrical engineer working on a project for Exxon in Chad, Africa. The plant appears to have been built to USA/NEC standards.
The nominal voltage quoted at the plant MCC is 480V. Motors connected to this MCC are rated at 460V. Can anyone explain this apparent discrepancy? Is it normal in the US to connect a 460V motor to a 480V supply? Is the difference accounted for via voltage drop in supply cable and voltage tolerance of the motor?
In the UK a 400V rated motor would be connected to a 400V supply.
Any reference to an appropriate NEC clause would be appreciated.

 

[c-h]

Re: Nominal Voltage versus Utilisation Voltage

Huh? No takers on this one?

Better state upfront that I'm not the really the right guy to answer this.

You are correct, a 460V motor goes with a 480V nominal voltage.

The use of a "utilization voltage" is discouraged in the IEC world, and a 400V motor goes with a 400V system.

In the NEMA world, motors (among other things) are rated at a "utilization voltage" that is lower than the nominal voltage. Your presumtion that this is to take voltage drop into account is correct.

The table:

115 = 120V
230 = 240V
460 = 480V (US standard)
575 = 600V (Canadian standard)

P.S. If you are going to do any more than a very limited amount of electrical engineering on this plant, you may need to seek the assistance of an American engineer.

 

[steve66]

Re: Nominal Voltage versus Utilisation Voltage

I might add that since motors draw more current at a lower voltage, sizing the wires (cables) and overcurrent protection for the current drawn at the lower voltage gives a safer and more robust installation.

It seems like the IEC focus is more on saving materials and not sizing things any larger than they have to be.

Steve

 

[charlie b]

Re: Nominal Voltage versus Utilisation Voltage

Originally posted by IanKeith1960: Any reference to an appropriate NEC clause would be appreciated.

All I can suggest is Table 250.430. You'll see that 460 volts is in the Table, and 480 is not.

 

[oliver100]

Re: Nominal Voltage versus Utilisation Voltage

In the UK a 400V rated motor would be connected to a 400V supply.

Most of the European electrical motors are rated 380 volts and it is within 5% of the nominal voltage. The 400 volts is the distribution voltage of the utility companies, compensating for evntual excessive voltage drops. I think UK employs the same practice.

 

[bphgravity]

Re: Nominal Voltage versus Utilisation Voltage

The NEC is an installation standard that uses a nominal value for voltages. Other standards and publications such as ANSI and NEMA are product standards that may use different ratings.

Another example would be receptacle outlets. Receptacles are rated at 125V and 250V, yet are connected to 120V and 240V nominal systems. The notes to the motor FLC tables in 430 shows explains this as well.