Voltage Drop Calculation for Panel

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treytries

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I'm trying to learn about how voltage drop is calculated for a panel. There is a lot of information on how it is calculated from a panel to a end device and that is relatively trivial. However I haven't found much information on how it is figured say from a switchgear to a remote panel.

So for example let's say I have a 200A 480/277V Panel roughly 750' away from the switchgear that feeds it. I would like to ensure that the voltage drop across this feeder is no more than 2% to ensure that I have the remaining allowance of 3% to figure for my end devices.

Hypothetical Loads:
A Phase Load: 19088VA
B Phase Load: 19088VA
C Phase Load: 19088VA

How would I go about calculating the size of my feed in order to keep my drop under 2%?

Thank you in advance. Any/All help is appreciated.
 
Ok. Thank you for your response. So I just have 2 questions then. First is in the case of the panel described would you use 480 or 277 as your voltage in that equation? And would you still use a 3 phase calculation even if some of the loads on that panel may not be 3 phase loads?
 
treytries said:
Ok. Thank you for your response. So I just have 2 questions then. First is in the case of the panel described would you use 480 or 277 as your voltage in that equation? And would you still use a 3 phase calculation even if some of the loads on that panel may not be 3 phase loads?
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So, you actually need to know something about the loads to properly answer the above questions, I believe.

If all of the loading is 3 phase loading, i.e. the neutral current will be 0 in all cases (and so may not be present), then you definitely use the 3 phase calculation based on 480V L-L and the VA from all the legs. The worst case %VD is that all three legs are seeing the full VA.

Conversely, if all of the loading is single phase loading, then the worst case %VD will be with all the L-N loads on one leg active, and all the other loads inactive. So you'd do a single phase calculation based on 277V L-N, using the VA just for the most heavily loaded leg. That will give you twice the voltage drop (assuming full size neutral) that you would have for the case of all legs at that same VA (which would match the earlier case). Basically because now the neutral is contributing voltage drop, too, rather than just the ungrounded conductor.

Thus if you have a mix of 3 phase and single phase loads, you could do 2 separate computations and add the resulting VDs. The worst case would be all the 3-phase loads active, along with just the single phase loads on the most heavily loaded L-N leg.

At least, that is what I conclude from my understanding; confirmation from someone who does these calculations professionally would be helpful.

Cheers, Wayne
 
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