Voltage Drop

J

Jimmy7

Senior Member
Location
Boston, MA
Occupation
Electrician
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?
 
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).
 
zbang said:
I've seen 20 amp circuits on 6g, they were 500-600' long).
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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.
 
Fred B said:
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.
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I would like to know the situation where you needed WiFi a thousand feet from any electrical source?

Running some CAT 6E with some POE repeaters would have been 1/1000 of the cost!
 
I’m going to start to wire a new house that is approximately 40’ x 30’. The house has two floors with an unfinished basement. Up here in Massachusetts we strap the ceilings. The main panel will be located in the basement. I’m going to run #12 on 20 amp circuits for the receptacles, and #14 on 15 amp breakers for lighting which is mostly led wafer lights.
Would you be concerned with voltage drop at all on house this size?
Do you drill across bays, or do you go over the top to each receptacle on a wall?
 
Jimmy7 said:
Would you be concerned with voltage drop at all on house this size?
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Not at all. Try to locate the panel near the kitchen and/or HVAC.

Consider a sub-panel or two for remote concentrated loads.

Jimmy7 said:
Do you drill across bays, or do you go over the top to each receptacle on a wall?
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Interior walls, either. Exterior walls, up and over.
 
LarryFine said:
Not at all. Try to locate the panel near the kitchen and/or HVAC.

Consider a sub-panel or two for remote concentrated loads.


Interior walls, either. Exterior walls, up and over.
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Exterior walls would to make the insulation guys job a little easier?
 
40 x 30 with two floors isnt that big. I wouldn't consider voltage drop unless you have specific appliances with long runs.

I like what @LarryFine said panel near the kitchen if you can that is were a lot of circuits go.
 
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