Voltage Drop

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JohnDS

Senior Member
Location
Suffolk, Long Island
Occupation
Electrician
Can someone just refresh my memory about voltage drop.

I totally forgot how to calculate voltage drop for a 240v circuit.

2KiL/CM=VD for 120v circuits

Is this formula different for 240v? If so, can someone be clear on the different formula instead of referring me back to the code please?
 
T

The Iceman

Senior Member
Location
Florida
Resistive methode for calculating voltage drop:
VD = 2 x L x R x I / 1000

Circular-Mil methode for calculating voltage drop:
VD = 2 x R x I / cm

Also see 210.19 FPN No. 4
 
480sparky

480sparky

Senior Member
Location
Iowegia
JohnDS said:
Thank you, so just to be sure, it doesn't matter whether it's 120 or 240, use this same formula?
Click to expand...

Yes. You'll just need to know what 3% (or whatever number you require) of your operating voltage works out to be.
 
H

haskindm

Senior Member
Location
Maryland
Voltage does not really matter in a voltage drop calculation. The calculation is the same for 12-volts, 24-volts, 120-volts, 208-volts, 240-volts, or 24,000-volts. The number of phases matter and the higher the voltage, the more volts you can afford to "lose" (3% of 240-volts is more than 3% of 12-volts) but other than that, voltage is not a consideration.
 
K

keepitsimplestupid

Member
Since this topic is fresh, I'm wondering what is the correct voltage to use in a 240/120 feeder to a sub-panel. 3% of 240 or 3% of 120.

It would seem that 240 V is not the correct voltage to use from a voltage drop standpoint. Depending on how the sub-panel was loaded, the 120 Volt loads could suffer.

I'm even seeing service entrance sizing based on the 240 volt values.

What's the correct way?

Does one use 240 V when no neutral is present in the load and 120 V, if it is?

Confused.
 
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