DIY Home Improvement Forum banner

1 - 5 of 5 Posts

·
Registered
Joined
·
247 Posts
Discussion Starter #1
hi i am adding a subpanel to my shed with 60 amp service. the distanc from the original panel to the subpanel is 200 feet. what is the allowable voltage drop? what size wire is necessary to achieve this?
 

·
Registered
Joined
·
9,024 Posts
Three percent voltage drop is considered the acceptable limit.

You would need 4 gauge copper wire.

4 gauge wire: 0.000292 ohms per foot or 0.117 ohms for the 400 foot round trip. Times 60 amps to get the voltage drop of 7.02 volts.

Assuming the total load is somewhat balanced between the two hot wires of the 120/240 volt feed, we may use 240 as the reference voltage and 3% of that is 7.20 volts. This leaves a small margin for the resistance and voltage drop of the 12 and 14 gauge branch circuit wires coming out of the subpanel. (In any portion of a circuit including a piece of wire, voltage dropped equals current flowing times resistance)

For a somewhat shorter distance the rule of how much heat per foot is produced in the cable and for 60 amps you use a 6 gauge copper wire whose resistance is .0.000465 ohms per foot.
 

·
Registered
Joined
·
247 Posts
Discussion Starter #3
so of you take a wire and run it too far what basically happens. Ex. you take a number 12 wire that would normally pull 20 amps and have it on a 20 amp breaker, and run it out to an extreme distance and then try to pull 20 amps. will the breaker throw or will the wire get hot and possibly start a fire?
 

·
Scared Electrician
Joined
·
715 Posts
so of you take a wire and run it too far what basically happens. Ex. you take a number 12 wire that would normally pull 20 amps and have it on a 20 amp breaker, and run it out to an extreme distance and then try to pull 20 amps. will the breaker throw or will the wire get hot and possibly start a fire?
well volts begin to fall off say u start with 120v then 119 118 and soon you have too little volts to do anything but cause excess heat in equipment and burn it up.
 

·
Registered
Joined
·
9,024 Posts
The more amperes are drawn, the warmer the wires will get but the heat is distributed over the length of the wire and the temperature at any point on the long wire will be the same as along a short wire, all other things including amperes being equal.

Motorized equipment, upon receiving fewer volts than it likes, is likely to draw more amperes to perform a given task and the motor itself can heat up more.

If the total number of amps actually drawn is less than the breaker rating, the breaker will not throw.

So you can run an extremely long 12 gauge wire to power some incandescent lights and you have 20 amperes worth (actual draw, not stamped on the bulbs) and they will run indefinitely* getting say 80 volts and there is no fire hazard except you don't get as much light.

* "Continuous" usage, whatever that means, should not exceed 80% of the circuit wire rating. Lights and small appliances are not considered "continuous" loads.
 
1 - 5 of 5 Posts
Top