Table 430.250 – accuracy at 480 V versus 460 V

[Dark Sparky]

We are looking to standardize the voltage used at our engineering firm. For example, the Electrical Department typically uses “480 V” while the Mechanical Department uses “460 V.” I understand that this is normal and expected, since Electrical is talking about Service/Supply voltage, and Mechanical is referring to Utilization voltage.

Nevertheless – to standardize and alleviate client confusion, we have decided to use one voltage across-the-board. We’ll all show “480 V.” (We’ve decided to standardize on 480, 277, 240, 208 and 120 volts. Discussing only 480 V here for simplicity.)

I am trying to confirm that this standardizing will not cause any problems. For example:

If mechanical has a 60 HP, 480 V, 3 phase motor. NEC table 430.250 tells me that the FLA is 77 amps.
I thus calculate that the VA is 460 V x 77 A x 1.732 = 61,349 VA.

The VA would be shown on a mechanical equipment schedule, along with “480 V” (the new standard voltage).
Since it’s standardized on 480 V, I’ll “lose” the knowledge that this equipment’s VA was originally calculated at 460 V.

I might then try to back-calculate the equipment’s current:
61,349 VA / 480 V / 1.732 = 74 amps.

So the equipment’s “actual” amperage is 77 (per NEC at 460 volts), but I’ve calculated it at 74 amps.

If I size my circuit using the FLA:
77 amps x 125% = 96.3 amps = #1 AWG wire
(I realize as an engineer I might say 95 amps ampacity of #1 wire is sufficient for this circuit, but I’ve chosen this size motor as an example to illustrate my point).

If I instead sized the circuit using the current draw calculated from 480 V:
74 amps x 125% = 93 amps = #2 wire

So is my circuit undersized if I calculate it off 480 volts? I have sized for a #2 circuit, when the FLA is 77 amps and I should use #1 wire. (I should really use the horsepower to locate the FLA on the table, and size my wire based off that FLA. But if I only know the VA, this scenario might occur.)

Add to this the verbiage from Table 430.250: “The currents listed shall be permitted for system voltage ranges of 110 to 120, 220 to 240, 440 to 480…”.

Are my fears unfounded? Can we standardize on 480 V, and assume that wire sized as indicated above is safe in either instance (i.e. that either #1 or #2 wire is acceptable for this motor)? Am I missing something or making this overly complex?

Thanks,
Jonathan

 

[jumper]

Am I missing something or making this overly complex?

Thanks,
Jonathan

For me, yes.

I use 460V since the chart says 460V.

Motors are always done this way AFAICT. NEMA specs I think.

T430.250 permits the various voltages and fluctuations, 440V-480V, but I was taught to use the table voltage listed.

 

[nhee2]

If I size my circuit using the FLA:
77 amps x 125% = 96.3 amps = #1 AWG wire
(I realize as an engineer I might say 95 amps ampacity of #1 wire is sufficient for this circuit, but I’ve chosen this size motor as an example to illustrate my point).

If I instead sized the circuit using the current draw calculated from 480 V:
74 amps x 125% = 93 amps = #2 wire

So is my circuit undersized if I calculate it off 480 volts? I have sized for a #2 circuit, when the FLA is 77 amps and I should use #1 wire. (I should really use the horsepower to locate the FLA on the table, and size my wire based off that FLA. But if I only know the VA, this scenario might occur.)

430.6 says to size conductors based on the table values. In your example the 60 HP motor conductors would be sized for 125% of 77A for any voltage between 440 and 480.

 

[Dark Sparky]

Thanks.

Thanks.

jumper & nhee2 - thank you both for the quick help. We will standardize our voltages (using Utilization voltages, 480V, etc.). A review does indeed indicate that 460V is what our "motor" manufacturers are using. Other mechanical equipment such as unit heaters, etc. appear to typically be 480V. So we'll use 460V (or other voltage as listed by the manufacturer, where appropriate) for calcs. Both of your insight was helpful in clarifying this.

Thank you!!

 

[iwire]

I find this chart helpful.

 

[tsparks1]

Here we have both 460v floating delta, 460v center grounded , 480v grounded phase and 480/277 wye. So must be sure how it is connected for correct voltage.


Sent from my iPhone using Tapatalk

 

[Smart $]

...
The VA would be shown on a mechanical equipment schedule, along with “480 V” (the new standard voltage).
Since it’s standardized on 480 V, I’ll “lose” the knowledge that this equipment’s VA was originally calculated at 460 V.

I might then try to back-calculate ...

If you're going to standardize, always use 480V in your calculations and you won't have these problems. I'd think that is the point of standardizing... and the problem you express is one of not standardizing.