Voltage Drop

[Jimmy7]

We were talking on the job today about voltage drop. I was always told that rule of thumb for 120 volt circuits is 120’. Some of the houses we work on are very large, so it seems like it doesn’t take long to reach 100’. I even have one building contractor who doesn’t want us to drill across bays, he would rather go over the top which creates longer runs. Do you guys drill across bays for receptacles? Voltage drop really depends on load, correct? If so, LED lighting circuit could travel quite a long distance, correct? How long would you run a 20 amp circuit for receptacle?

 

[zbang]

Drop depends on current and resistance (size & length of the wire) -- when applied you get drop(voltage) = current(amps) times resistance (ohms). 14 g copper is about 2.5 ohms per 1000ft, 12g is about 1.6 ohms/1000ft.

Assuming a 10 amp load, 120 volts at the panel, and 100' from the panel (200' of wire); 12g gives a drop of about 3.3% (4 volts), 14g would give about 5.2% or 6.2 volts You can play with number on the calculators at https://www.calculator.net/voltage-drop-calculator.html and several other sites.

How long would I run a 20 amp circuit? As long as it needs to be; might run something larger than 12g, though (I've seen 20 amp circuits on 6g, they were 500-600' long).

 

[Fred B]

I've seen 20 amp circuits on 6g, they were 500-600' long).

Not bad, had one over a thousand ft, 120V 15A had a 2/0 CU for for a wifi repeater on a campus. Now try landing that on a 15A breaker, and receptacle on the other end.

 

[zbang]

That's an awfully large circuit for what's probably less than a 100 watt load. (I suspect pigtails were involved in the terminations .)

 

[ptonsparky]

Not bad, had one over a thousand ft, 120V 15A had a 2/0 CU for for a wifi repeater on a campus. Now try landing that on a 15A breaker, and receptacle on the other end.

Sometimes Solar & wind with battery would seem to be cost effective.
No doubt that would have been designed to excess as well.