Should I upsize the wire on a 250ft run?

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I certainly get a lot more calls from customers about GFCIs tripping than I do about customers getting shocked and the GFCI tripping. Fact of the matter is I only remember two instances of it happening. My brother with his metal skill saw and mud. Before and after the 1st repair.
 
I believe the NEC only allows a total of 5% voltage drop between the incoming service and end of branch circuit. I always used ohms law even though its not really for AC circuits. Years ago a fellow sparky was installing think it was around 9 or 10 watt post lamps every 100' on his long driveway. He retrofitted low wattage think it was 35. or 50 watt HID lamps.First light was think 150' from his garage. His sparky dad insisited that #10 copper was plenty big enough. I sat down and figured out to maintain less then a 5% voltage drop to last lamp he needed #8 copper to first post then #10 copper to next 5 post or something then #12 guage rest of way. Anyway when he was done using my cheater conculation the voltage drop was within 1 volt AC where I calculated it for furthest lamp. If it was my property I would go with a larger guage. Would use Aluminum conductors along with anti oxide paste and proper torque wrench.
 
ptonsparky said:
I certainly get a lot more calls from customers about GFCIs tripping than I do about customers getting shocked and the GFCI tripping. Fact of the matter is I only remember two instances of it happening. My brother with his metal skill saw and mud. Before and after the 1st repair.
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Only got socked once while using a GFCI. I was drilling old rivets off my van when a piece of rivet entered drill housing and nicked insulation. Was so quick that I hardly felt it.
 
garbo said:
I believe the NEC only allows a total of 5% voltage drop between the incoming service and end of branch circuit. I always used ohms law even though its not really for AC circuits. Years ago a fellow sparky was installing think it was around 9 or 10 watt post lamps every 100' on his long driveway. He retrofitted low wattage think it was 35. or 50 watt HID lamps.First light was think 150' from his garage. His sparky dad insisited that #10 copper was plenty big enough. I sat down and figured out to maintain less then a 5% voltage drop to last lamp he needed #8 copper to first post then #10 copper to next 5 post or something then #12 guage rest of way. Anyway when he was done using my cheater conculation the voltage drop was within 1 volt AC where I calculated it for furthest lamp. If it was my property I would go with a larger guage. Would use Aluminum conductors along with anti oxide paste and proper torque wrench.
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Thats a could thing. Otherwise the lights will not trip on a ground fault remaining energized.

As mentioned VD is not mandated in the NEC, just recommended.
 
garbo said:
I believe the NEC only allows a total of 5% voltage drop between the incoming service and end of branch circuit. I always used ohms law even though its not really for AC circuits. Years ago a fellow sparky was installing think it was around 9 or 10 watt post lamps every 100' on his long driveway. He retrofitted low wattage think it was 35. or 50 watt HID lamps.First light was think 150' from his garage. His sparky dad insisited that #10 copper was plenty big enough. I sat down and figured out to maintain less then a 5% voltage drop to last lamp he needed #8 copper to first post then #10 copper to next 5 post or something then #12 guage rest of way. Anyway when he was done using my cheater conculation the voltage drop was within 1 volt AC where I calculated it for furthest lamp. If it was my property I would go with a larger guage. Would use Aluminum conductors along with anti oxide paste and proper torque wrench.
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Those voltage drop values are in informational notes following 210.19(A) for branch cicuits and 215.2(A) for feeders.

90.5(C) tells us those notes are informational/explanatory in nature and not enforceable code content. Therefore those 3 and 5% values are not a requirement but rather just a suggestion.
 
Shak180 said:
I'm going to be running a 100 amp circuit over to a subpanel that's going to have (2) 50 amp 3 phase circuits coming out of it. I bid the job using 3 awg copper but now I'm wondering if I should use 2 awg copper. What do y'all think? Will the 3 awg copper be sufficient? There's a 2.2% voltage drop using the 3 awg which doesn't seem bad.

The loads being plugged in are an electric heater that pulls 36 amps, a welder that pulls 23 amps, and a plasma cutter that pulls 12 amps.
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In my experience, it's generally a good rule of thumb to upsize conductors for every 250ft of length, without performing voltage drop calculations. But, I'd recommend just doing the calculation as it's good to practice and you'll know exactly what wire size you can get away with!
 
Jacobb951 said:
In my experience, it's generally a good rule of thumb to upsize conductors for every 250ft of length, without performing voltage drop calculations. But, I'd recommend just doing the calculation as it's good to practice and you'll know exactly what wire size you can get away with!
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voltage and load do factor into this.

If your load is near ampacity of conductor and is only 120 volts, VD may be pretty significant.

Same size conductor, length and amps but a circuit operating at 480 volts won't be nearly as significant of VD.
 
kwired said:
voltage and load do factor into this.

If your load is near ampacity of conductor and is only 120 volts, VD may be pretty significant.

Same size conductor, length and amps but a circuit operating at 480 volts won't be nearly as significant of VD.
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Since the VD specs are in percentages as long as you are working under 600 V this is not true. 5% is the rule whether it’s 120, 240, 480, or 600. At medium voltage the standards change a little bit but the principles are the same.

The biggest difference is that as cable sizes get above about #6 the reactance rather than the resistance starts driving things but the end result is the same. At 480 V a VD of 5% or 24 V is recommended while it is only 6 V at 120 V. UL and NEMA call for equipment to be designed to operate at 5% lower than the distribution voltage and to tolerate +10% / -15% and CBEMA recommends a much wider range for short time periods.
 
paulengr said:
Since the VD specs are in percentages as long as you are working under 600 V this is not true. 5% is the rule whether it’s 120, 240, 480, or 600. At medium voltage the standards change a little bit but the principles are the same.

The biggest difference is that as cable sizes get above about #6 the reactance rather than the resistance starts driving things but the end result is the same. At 480 V a VD of 5% or 24 V is recommended while it is only 6 V at 120 V. UL and NEMA call for equipment to be designed to operate at 5% lower than the distribution voltage and to tolerate +10% / -15% and CBEMA recommends a much wider range for short time periods.
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Maybe I didn't describe what I was trying to say very well?

take two conductors same size type and length and apply same current but different overall system volts to each of them. They both have same volts drop across them, Ohms law says it has to be the same.

If it is the suggested maximum of 24 volts @ 480 then you are fine.

If it is a 240 volt supply it still drops 24 volts but that is 10% drop.

If it is a 120 volt supply it would be 20% drop.

2400V and it is only 1% drop.

A little more real world application however means you need a particular load supplied, which voltage to select for the supply? Well if you choose 480 over 240 the current is going to be half what it is for 240 - can already use a smaller conductor for that reason alone. On top of that can possibly tolerate 24 volts drop in the circuit vs 12.
 
kwired said:
Maybe I didn't describe what I was trying to say very well?

take two conductors same size type and length and apply same current but different overall system volts to each of them. They both have same volts drop across them, Ohms law says it has to be the same.

If it is the suggested maximum of 24 volts @ 480 then you are fine.

If it is a 240 volt supply it still drops 24 volts but that is 10% drop.

If it is a 120 volt supply it would be 20% drop.

2400V and it is only 1% drop.

A little more real world application however means you need a particular load supplied, which voltage to select for the supply? Well if you choose 480 over 240 the current is going to be half what it is for 240 - can already use a smaller conductor for that reason alone. On top of that can possibly tolerate 24 volts drop in the circuit vs 12.
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Hence why utilities run higher voltages for transmission. Losses are proportional to the square of the current. For the same amount of power if I double the voltage I need half the current and I get 1/4 of the losses. But doing so has a cost. Going from 120 to 600 isn’t that big of a deal but going to 4160 where you need shielded cable and larger built to order equipment increases costs dramatically. It gets gradually worse and worse as voltage increases but the trade off is lower losses and current.

Recently dealt with a 900 HP 480 V motor. I forgot how many leads per phase in the motor but the starter is 4 500s per phase. The same motor at 4160 would be a single much more reasonable conductor per phase and maybe 120 A vs 1100. But the VFD would cost 300% more so they elected to stick with 480. We are stuck dealing with the crummy high common mode current from the drive.
 
paulengr said:
Going from 120 to 600 isn’t that big of a deal but going to 4160 where you need shielded cable and larger built to order equipment increases costs dramatically. It gets gradually worse and worse as voltage increases but the trade off is lower losses and current.
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Yes; one-time expense vs continuous expense of ongoing losses.

I often tell people that insulation is cheaper than conductor.
 
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