Load Calculation: 480V vs 460V

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BBadger

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When calculating three phase motor loads I've always used 480V times 1.732 to get VA. I've recently seen this done with 460V by an extremely respected engineer. Can anyone provide clarification on this.

Thanks
 

charlie b

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Location
Lockport, IL
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Retired Electrical Engineer
This is a debatable issue. Part of the problem comes from the fact that we are supposed to take the "running current" values from table 430.250 (instead of from the nameplate), and the fact that that table shows the voltage as 460. OK, we all know that the power system is rated for 480, and that (for voltage drop considerations, perhaps) the motors are rated at 460. My method is to take the tabulated current times 480 (then times 1.732), for two reasons. First, that is the voltage at which the system will operate. Secondly, it gives me a slightly higher (i.e., more conservative) value for the motor load in KVA. I figure that it is better to provide a little more capacity than to provide too little capacity.

That said, do I think that the other engineer is doing the math incorrectly? I do not. It is a valid method as well.
 

skeshesh

Senior Member
Location
Los Angeles, Ca
Many motor's are "rated" at 460 but of course allow of 10% voltage tolerance. I would say using 460V is not a bad idea since it takes into account possible voltage drop (depending on how far the motor loads are located) and also would result in a larger current (more conservative). Having said that, doing the calculation using 480V is not really incorrect.

Edit: Haha, Charlie beat me to the punch.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
My method is to take the tabulated current times 480 (then times 1.732), for two reasons. First, that is the voltage at which the system will operate. Secondly, it gives me a slightly higher (i.e., more conservative) value for the motor load in KVA.

I would say using 460V is not a bad idea since it takes into account possible voltage drop (depending on how far the motor loads are located) and also would result in a larger current (more conservative).
Well, both of you can't be correct about arriving at a more conservative (i.e., "better to big than too small") value for current by using 480v vs. 460v, unless you're each doing the math differently.

It seems that Charlie is using V and I to derive KVA, while Skeshesh is using V and KVA to derive A. I certainly agree that using the larger valu ofr ampacity is important when its close.
 
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BBadger

Member
Appreciate the responses. Do any of you have any thoughts on standardizing a voltage so there isn't confusion.

Also, wouldn't an engineer utilize the higher of the two voltages that would increase service size (thus creating a system that was inherently safer) due to the their responsibility to "...hold paramount the safety, health, and welfare of the public."

Thanks again.
 
220.5 Calculations.
(A) Voltages. Unless other voltages are specified, for purposes of calculating branch-circuit and feeder loads, nominal system voltages of 120, 120/240, 208Y/120, 240, 347, 480Y/277, 480, 600Y/347, and 600 volts shall be used.
 

skeshesh

Senior Member
Location
Los Angeles, Ca
You know Larry, now that I think about it maybe Charlie is right afterall. Most motors do have nameplate info (or the info can be requested from mfg) that shows the current information (FLA, MCA, etc.) so having an accurate account of the current or being given the size of the conductor by the mfg. negates my argument of using 460V to arrive at higher amps. Since the current is given using 480V will yeild higher KVA and thus be more conservatives when considering total load for the system.
 

charlie b

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Location
Lockport, IL
Occupation
Retired Electrical Engineer
It seems that Charlie is using V and I to derive KVA, while Skeshesh is using V and KVA to derive A.
From the wording of the question, I think that the mission is to calculate load for the purpose of sizing branch circuits, feeders, and services. If that is the case, then "amps" is an input value, obtained from a table, rather than a result.

 
This is a debatable issue. Part of the problem comes from the fact that we are supposed to take the "running current" values from table 430.250 (instead of from the nameplate), and the fact that that table shows the voltage as 460. OK, we all know that the power system is rated for 480, and that (for voltage drop considerations, perhaps) the motors are rated at 460. My method is to take the tabulated current times 480 (then times 1.732), for two reasons. First, that is the voltage at which the system will operate. Secondly, it gives me a slightly higher (i.e., more conservative) value for the motor load in KVA. I figure that it is better to provide a little more capacity than to provide too little capacity.

That said, do I think that the other engineer is doing the math incorrectly? I do not. It is a valid method as well.

The voltage drop itself is a load (I^2*R), so indeed, it is appropriate to use the supply voltage, not the utilization voltage.

Example: a 12AWG single phase circuit will produce ~200VA @ 16A, 250'
 

e57

Senior Member
Appreciate the responses. Do any of you have any thoughts on standardizing a voltage so there isn't confusion.

Also, wouldn't an engineer utilize the higher of the two voltages that would increase service size (thus creating a system that was inherently safer) due to the their responsibility to "...hold paramount the safety, health, and welfare of the public."

Thanks again.
Which calculation are you mentioning?

A higher voltage may result in a higher KVA to add to a service calc' - but still not result in a larger service size...

Conversely:

The higher - standard voltage would produce a calc' amperage result smaller than the utilization voltage result. The lower voltage would produce a calc' with a higher amperage result. (When seeking a conductor and OCP sizing)

Volts down = Amps up and vise versa... Volts up = Amps down...

But unless you have a stamp of your own - you should probably limit yourself to using the standard voltage and tables... The voltage present may or may not remain the same from time to time, and would you really want to have the finger pointed at you if say a "finger pointing party" started one day? (Then you just point at the tables if you stick to them...)

That said there are a few more factors and standard practices to conductor and service sizing other than just that - and will often result in branch/feeder conductor and service sizing much larger than actually required.
 

dicklaxt

Senior Member
Right ,wrong or indifferent,,,,,I'll go along with LZW because many ,many years ago(45 maybe) I had a NEC class instructor use the terms "source and utilization voltage",that made sense to me and being not educated enough to do anything but play follow the leader here I am.:)

dick
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
The voltage drop itself is a load (I^2*R), so indeed, it is appropriate to use the supply voltage, not the utilization voltage.

Example: a 12AWG single phase circuit will produce ~200VA @ 16A, 250'

I have to agree with Lazo on this one. Its very easy to forget the source must supply the losses as well as the loads. So if we do the calcuations with 460V, we are ignoring the losses the panel or utility must supply.

Edit: So I think the correct way to calculate the load would be to use motor currents from their 460V rating, but use the 480V supply voltage. I think thats what most engineers usually do anyway.

Steve
 
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