voltage drop

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rick1booth

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as general rules of thumb (which there are plenty), i was always told to use the larger size wire (#10) for the first segment of the circuit from the breaker and then downsize on the second part of the circuit. today i was told that was wrong, you start off with the smaller size wire and increase wire size the further from the breaker you go ie: larger wire closer to the load. my specific issue is some site signage that is 500' + from the source. size is 120v, amperage unknown. i was going to run #8's about halfway and finish with 10's. thoughts???
 
If there are multiple loads that occur at different points, then your original thinking is correct. Big wire first. If there’s only one load for the whole run, it doesn’t matter.

Voltage drop is about current going through the impedance of the conductor.
 
Whoever told you that is "an idiot". Voltage drop depends on wire size and current, so you want the larger sizes were there is more current to minimize the drop. And there is no earthly reason to use two sizes in a single run (that adds more work and a point of failure), if you need to run 500' from panel to sign with nothing in between, make them continuous runs (you may need/want a pull box in the middle though). Oh, and depending on load you may actually want 4g (check this yourself).

If there is already going to be a stop in the middle (2- 250' runs, end-to-end), I can run a smaller gauge in the second one since the load on that would be less. Making up numbers and assuming two 5amp loads-
first 250' segment, 10 amps total load, #8 copper
second 250 segment, 5 amps total load, #10 copper
 
zbang said:
Whoever told you that is "an idiot". Voltage drop depends on wire size and current, so you want the larger sizes were there is more current to minimize the drop. And there is no earthly reason to use two sizes in a single run (that adds more work and a point of failure), if you need to run 500' from panel to sign with nothing in between, make them continuous runs (you may need/want a pull box in the middle though). Oh, and depending on load you may actually want 4g (check this yourself).
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As opposed to 3g or the newer faster 5g??
 
zbang said:
newer, faster, doesn't get the nearly the distance (and I don't think 5 gauge copper is readily available, so 4 it is).

kingpb- I was trying to stay within the forum language guidelines :D
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Sorry, I saw the humor in that 4g statement. I assumed everyone was as warped as I am...
 
rick1booth said:
as general rules of thumb (which there are plenty), i was always told to use the larger size wire (#10) for the first segment of the circuit from the breaker and then downsize on the second part of the circuit. today i was told that was wrong, you start off with the smaller size wire and increase wire size the further from the breaker you go ie: larger wire closer to the load. my specific issue is some site signage that is 500' + from the source. size is 120v, amperage unknown. i was going to run #8's about halfway and finish with 10's. thoughts???
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You are better off using a uniform size as much as practical for the circuit in question. It simplifies your calculation, and it simplifies your installation mechanics. You may need to step-down the conductor to fit in the terminals of the equipment in question, but if that is necessary, you do that either in the equipment, or immediately adjacent to it, so that the shorter size is negligible.

If there is only one load on the circuit, and for some reason you did intend to wire half the circuit length in size A, and the other half of the circuit in size B, the sequence of size A and size B doesn't make a difference. Series resistance addition is commutative, like addition in general.

Where you encounter the possible desire to have the larger size closer to the source, is when you work with a circuit that feeds several different loads. The amperes "peel off" every time a load is connected. Suppose you fed three 10A loads on the same circuit, one located every 50 ft. The first 50 feet carries 30A of current. The next 50 feet carries 20A of current. The final 50 feet carries 10A of current. If you unify the size of wire, the first 50 ft will experience 3x the voltage drop of the final 50 ft, and 1.5 times the voltage drop of the middle 50 ft. So if you did need to increase the size, you would increase the first section before you look at increasing it in the other two sections.
 
GoldDigger said:
Not covered by that analogy is the requirement that the minimum size portion still be properly protected by the OCPD.

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This analogy applies to piping and ductwork as well. There are local factors and distance factors, that affect the sizing of piping and ductwork. The local factor is keeping the speed of the fluid within reasonable limits, and the distance factor is mitigating frictional pressure drop.
 
Carultch said:
This analogy applies to piping and ductwork as well. There are local factors and distance factors, that affect the sizing of piping and ductwork. The local factor is keeping the speed of the fluid within reasonable limits, and the distance factor is mitigating frictional pressure drop.
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My understanding is round duct will have less airflow resistance than similar CSA square or rectangular duct, which is not so much a factor for low volts low frequency in such analogies, bends in the run don't have the same sort of effects for electric as they do for air either.
 
kwired said:
My understanding is round duct will have less airflow resistance than similar CSA square or rectangular duct, which is not so much a factor for low volts low frequency in such analogies, bends in the run don't have the same sort of effects for electric as they do for air either.
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You have never seen the skidmarks left by the electrons going around sharp corners? :)
 
Anyway you do it is fine as long as the combined impedance of the ground and hot wire allow enough current for the breaker to open during a short circuit.
 
GoldDigger said:
You have never seen the skidmarks left by the electrons going around sharp corners? :)
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Haven't seen them in the turns in air ducts either.

Have seen air conveying systems have holes worn by abrasion of moving material at elbows many times though.
 
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