Increase wire size for voltage drop

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Hv&Lv

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Several electricians have told me that they increase there conductor one size for every 100'. I talked to one today that told me they were going to increase one wire size for every 75' to account for voltage drop. There aren't any calculations done, just an increase based on length.
My question is; where does this come from? Anyone else use this "rule of thumb"? (as I have heard it called)

(I guess this should be in calculations forum)
 
I'm an engineer and I calculate voltage drop before specifying a larger cable than the NEC would otherwise require. The rule of thumb I've heard is that a calculation should be performed if the length of the circuit in feet exceeds the voltage, i.e. at 480V the calculation is not required and the conductors do not need to be oversized if the length is under 480 feet.
 
I would guess that if you did some sample VD calcs for the maximum circuit load the 100' mark would be the point where you would exceed the recommended VD percentage.
 
The scenario: I ask about the wire size so I know what size lugs to bring to connect the wires to the XF spades. The electrician tells me he has 175 amps, he was going to use 3/0 wire, but he had to use 350 because the length was 275'. I asked him why he was upsizing the conductor because of length alone without a calculation. He informed me that is was in the code book. I asked him what year and which article. The only mention of voltage drop I can find is FPN's and Chapter 6.

This isn't the only time I have heard this, or the only electrician I have heard this from. Sure sounds like a waste of conductor material...
 
JD you say that if you have a 480 volt motor that is just say 350' from circuit breaker the voltage drop would be okay?
1.73x12.9x350x(say 14amp)/6530(#12)=16 volts which is 3.5% is this okay? I don't know I'm asking
 
according to MH voltage drop calculator, on a 240 volt system with a 275 ft run at 175 amps you would maintain a 3% voltage drop with a 3/0 cU.


Other than some specific areas such as Art 647, voltage drop is only addressed as a non-enforceable FPN in the NEC.
 
Hv&Lv said:
The scenario: I ask about the wire size so I know what size lugs to bring to connect the wires to the XF spades. The electrician tells me he has 175 amps, he was going to use 3/0 wire, but he had to use 350 because the length was 275'. I asked him why he was upsizing the conductor because of length alone without a calculation. He informed me that is was in the code book. I asked him what year and which article. The only mention of voltage drop I can find is FPN's and Chapter 6.

This isn't the only time I have heard this, or the only electrician I have heard this from. Sure sounds like a waste of conductor material...
Click to expand...

I think it is kind of dumb to "guess" what wire size you should use. Anytime someone says "rule of thumb" that means they are guessing.

Some installations are common. A 20 Amp breaker feeding a #12 cu wire which supplies a bedroom circuit 75 feet away does not normally require a voltage drop calculation. On the other hand, if this same installation supplies a motor load 75 feet away, then there would be a question if the wire size was big enough. I would need to check the motor nameplate and do calculations.

If there is any question about what the load will be, it is time to do voltage drop calculations.
 
Hv&Lv said:
Several electricians have told me that they increase there conductor one size for every 100'. I talked to one today that told me they were going to increase one wire size for every 75' to account for voltage drop. There aren't any calculations done, just an increase based on length.
My question is; where does this come from? Anyone else use this "rule of thumb"? (as I have heard it called)
Click to expand...

This rule of thumb works OK for 120 volt 15, 20 and 30 amp circuits.

It will not work for feeders, higher voltages etc.

Of course it is not perfect but VD is not required to be accounted for anyway. (At least not by the NEC except for a couple of specific items)

Also many of our prints have specifications from the EE that tells us to increase one size per 100'.
 
I'm sure the 1 min. it takes to do a proper calc would MORE than make up for the extra money unnecessarily spent on oversized wire(and possibly conduit too) based on guessing.
 
When I do the electrical design for buildings, I use a similar rule of thumb. But I use it in the opposite sense. If the length of a 120V circuit (branch or feeder) looks like it will be less than 100 feet, I do not bother considering voltage drop. If it looks like it could be close, or is obviously over that length, then I do a VD calculation. For complex distribution systems, I will use a computer model that includes all cable sizes and lengths. That will tell me whether there is a VD problem anywhere in the building, and I use that program, not a thumb rule, as the basis for increasing any wire sizes.
 
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