Voltage Drop... Again

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Electron_Sam78

Senior Member
Location
Palm Bay, FL
Ok I thought I had a handle on voltage drop but I'm stumped. I'm helping someone with a calculation and something doesn't add up. We have 6 each 30 amp loads totaling 180 amps at 120 volts. Two 90 amp circuits (one 2-pole CB, sharing neutral) running 240 feet comes to 250 mcm for roughly 3% max VD with Al conductor at 120 volts.

But he suggested a "90 amp sub-panel" close to the loads and input it in the calculator using 240 volts as the system voltage and it output wire size of 2/0 Al for roughly 3% voltage drop for the same length run. This doesn't seem to add up to me but can't figure out why for the life of me. Maybe it's been too long of a day :blink:

If there were no VD to account for I'd run the same size wire to feed a 240 volt, 90 amp sub-panel as I would a 120 volt, 90 amp circuit. Should I input the total amperage at 120 volts (180a) then use 240? Grr I need some sleep. :sleep:
 

George Stolz

Moderator
Staff member
Location
Windsor, CO NEC: 2017
Occupation
Service Manager
Voltage Drop... Again

If you have 6 circuit breaker poles at 120V 30A, that would be the same as a 2-pole 90A at 240V. The 90A sub panel makes sense.

agugary6.jpg
 

Electron_Sam78

Senior Member
Location
Palm Bay, FL
Yeah I don't doubt the vd calculator I just couldn't figure out why the two scenarios were coming up with different results when the only difference was adding another circuit breaker in between. Now that I've had some sleep I realize I should have treated the original circuit as a 240 circuit instead of calculating at 120. Since each load is a small sub panel itself and I am using a shared neutral it is a feeder and a 240 volt calculation is needed.

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charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
You have it correct now. When you use a 2-pole breaker, you are dealing with 240 volts, not 120. It doesn't matter whether the 2-pole breaker serves a sub-panel or serves two sets of 90 amp loads. It is still a 240 volt supply.
 

Smart $

Esteemed Member
Location
Ohio
Thanks for the replies. It was a long day yesterday...

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Sorry but I have to change gears in this machine :blink:.

Whether you use 240V or 120V wire sizing is dependent on the likelihood of the loads being coincidental, or not...

Take for example the extreme where all loads on L1 are on, and all loads connected to L2 are off. The voltage drop percentage (ratio) will be twice that figured at 240V, i.e. I?R is the same at half the voltage.

I?R ? 240V = (I?R ? 120V) ? 2

If the loads can reasonably be expected to operate coincidentally, you can ignore the preceding :huh:.
 
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