voltage drop
- Thread starter d1reyad
- Start date
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- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
080430-0634 EST
d1reyad:
Dennis gave you a good answer.
Here is an example:
A 150 ft run of #12 wire with a 20 A QO breaker. The loop resistance is about (300/1000)*1.588 ohms = 0.476 ohms. On a 120 V supply and my DeWalt radial arm saw with a 12" blade the start up current is about 80 A and voltage drop at the saw is about 38 V. Time to reach full speed is about 5 seconds. I do not remember the unloaded current, but it might be 10 A. Assuming under load it is 15 A, then the voltage drop would be 7.1 V.
If I stop and start the motor several times with maybe 15 seconds off time I will trip the breaker. If I were doing a lot of work on and off, then this is unacceptable. The real solution is to change to 240 V and use the same wire.
But if I stay at 120 V and want to avoid tripping the breaker with the on-off cycles, then I would probably want to go to #8 or even #6 wire and this makes no sense vs changing to 240 V.
The reason the larger wire would reduce the unwanted trips is that motor start up time would be much shorter with less voltage drop.
If you simply had a lighting load of 15 A, then with the above 150 ft run of #12 your voltage drop would be about 7.1 V. Not too bad for lights. Note: this is a 6% drop.
Basically analyze your circuit, set some criteria, and do the calculations.
.
d1reyad:
Dennis gave you a good answer.
Here is an example:
A 150 ft run of #12 wire with a 20 A QO breaker. The loop resistance is about (300/1000)*1.588 ohms = 0.476 ohms. On a 120 V supply and my DeWalt radial arm saw with a 12" blade the start up current is about 80 A and voltage drop at the saw is about 38 V. Time to reach full speed is about 5 seconds. I do not remember the unloaded current, but it might be 10 A. Assuming under load it is 15 A, then the voltage drop would be 7.1 V.
If I stop and start the motor several times with maybe 15 seconds off time I will trip the breaker. If I were doing a lot of work on and off, then this is unacceptable. The real solution is to change to 240 V and use the same wire.
But if I stay at 120 V and want to avoid tripping the breaker with the on-off cycles, then I would probably want to go to #8 or even #6 wire and this makes no sense vs changing to 240 V.
The reason the larger wire would reduce the unwanted trips is that motor start up time would be much shorter with less voltage drop.
If you simply had a lighting load of 15 A, then with the above 150 ft run of #12 your voltage drop would be about 7.1 V. Not too bad for lights. Note: this is a 6% drop.
Basically analyze your circuit, set some criteria, and do the calculations.
.
brian john
Senior Member
- Location
- Kilmarnock, Va
- Occupation
- Retired after 52 years in the trade.
voltage drop on light poles
voltage drop on light poles
This may be alittle off topic, but can anybody tell me if this is ok? I have 3 light poles that are 120 volt, each drawing about 4 amps, the 1st is 250' from panel, 2nd is bout 400' from panel, and last is about 650' from panel. Is #8 copper good enough with the voltage drop, mainly for that last one, they are actually spliced in a underground jbox about 200' from the panel. Thanks for any advice.
voltage drop on light poles
This may be alittle off topic, but can anybody tell me if this is ok? I have 3 light poles that are 120 volt, each drawing about 4 amps, the 1st is 250' from panel, 2nd is bout 400' from panel, and last is about 650' from panel. Is #8 copper good enough with the voltage drop, mainly for that last one, they are actually spliced in a underground jbox about 200' from the panel. Thanks for any advice.
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
millspgh said:
If the last light is 650' from the panel then that light without the other loads would need a #6 in order to be within a 3% drop.
Try the online calculator
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