voltage drop: 2% or 3% or 5% ?

[Stevenfyeager]

I read various % voltage drops acceptable, 2,3, or 5 % I'm a little confused. In wire sizing for an underground run to a detached building, which is recommended? My customer asks for 60 amps to a barn, 160 feet away. Thanks !

 

[Dsg319]

Off the top of my head…. Voltage drop is not a requirement but rather a NEC recommendation. In my opinion a good one, sure voltage drop at your appliance might not be a big deal. But try clearing a fault on super long runs with dinky conductors.

2% on feeders, 3% on branch circuits. Total of 5% at the appliance. As for your situation I’m assuming you are doing a 240volt feeder to a distribution panel or car charger?

You would need your calculated load to plug in on your VD formula or if you want to be conservative you could plug in the rating of your OCPD which some people think is way overkill.

Below is a simple single phase formula to find the needed conductor size.

CM=2 x K x I x D / VD

*also EGC must be increased in proportion to the ungrounded conductors.

 

[ptonsparky]

Without doing the calculations we would consider using the max size wire that would fit the breaker. AL.

 

[tom baker]

The only article with a voltage drop requirement is 695 fire pumps. Otherwise it’s a recommendation. The connected load may be less than 60 amps. I would check the vd at different wire sizes and loads, get costs. As pointed out is it a car charger or some shop tools? And run large conduit for future

 

[packersparky]

The only article with a voltage drop requirement is 695 fire pumps. Otherwise it’s a recommendation. The connected load may be less than 60 amps. I would check the vd at different wire sizes and loads, get costs. As pointed out is it a car charger or some shop tools? And run large conduit for future

Not to be nitpicky, but 647.4(D) has a voltage drop requirement.

 

[gar]

211206-1941 EST

My comment is this:

If the customer is going to run a 2 HP radial arm saw with a 12" carbide blade, then they probably want not more than 10 V to possibly 15 V drop at the saw during startup. Starting current might be 80 to 100 A.

Otherwise starting takes too long.

.

 

[infinity]

Although not enforceable code this is what the NEC suggests:

210.19 Conductors — Minimum Ampacity and Size.
(A) Branch Circuits Not More Than 600 Volts.
Informational Note No. 1: See 310.15 for ampacity ratings of conductors.
Informational Note No. 2: See Part II of Article 430 for minimum rating of motor branch-circuit conductors.
Informational Note No. 3: See 310.15(A)(3) for temperature limitation of conductors.
Informational Note No. 4: Conductors for branch circuits as defined in Article 100, sized to prevent a voltage drop exceeding 3 percent at the farthest outlet of power, heating, and lighting loads, or combinations of such loads, and where the maximum total voltage drop on both feeders and branch circuits to the farthest outlet does not exceed 5 percent, provide reasonable efficiency of operation. See Informational Note No. 2 of
215.2(A)(1) for voltage drop on feeder conductors.

 

[retirede]

Depends on what the loads consist of.
If it’s something as simple as a stock tank deicer, it won’t matter, for example.

 

[tom baker]

Not to be nitpicky, but 647.4(D) has a voltage drop requirement.

No thats being picky, but 647 is not a commonly used article. Next time I will say there are only 2 articles in the NEC that require voltage drop compliance. 695 I can remember as in my career I had one fire pump installation and that was enough for me. Everywhere else in the water system we had gravity and millions of gallons of storage.

 

[oldsparky52]

I didn't know that article existed.

 

[ptonsparky]

I didn't know that article existed.

Well, I'm in the back row, just to the left of you.

 

[Rick 0920]

Using #4 AWG copper conductors would be sufficient.
CM = 2 x K x D x I / VD permitted
CM = 2 x 12.9 x 160' x 60A / 7.2 (3% of 240v)
CM = 34,400
Chap. 9 Table 8 - #4 cu. 41,740 cm

 

[winnie]

IMHO the NEC recommendations are a good starting point, but since the bulk of the length for any circuits fed will be in the 60A feeder, I'd likely use 3% for the feeder and 2% for the branches for a total of 5%.

If the loads can tolerate large voltage drop but will be used infrequently then I'd be comfortable with larger %, if they will be used frequently then I'd tend toward lower %.

If you have a mix of high starting current loads and lower more sensitive loads, (air compressor and computers in someone's hobby CNC shop) then I'd lean toward lower %.

-Jon

 

[tortuga]

The only article with a voltage drop requirement is 695 fire pumps. Otherwise it’s a recommendation. The connected load may be less than 60 amps. I would check the vd at different wire sizes and loads, get costs. As pointed out is it a car charger or some shop tools? And run large conduit for future

Not to be nitpicky, but 647.4(D) has a voltage drop requirement.

Not to be more nitpickey but others have pointed out on this forum that 110.10,110.4, and 110.(3)(B) could in theory come into play, if say the barn was 1000' away and you had so much voltage drop the system would not function correctly.

 

[petersonra]

I read various % voltage drops acceptable, 2,3, or 5 % I'm a little confused. In wire sizing for an underground run to a detached building, which is recommended? My customer asks for 60 amps to a barn, 160 feet away. Thanks !

How much of the sixty Amps is he actually going to use? 160 is not real far so the cost difference to upside to reduce vd may not be all that much.

I think people try to overthink this kind of thing and often end up spending more money on the time spent trying to justify what they already decided to do then it would cost to upside.

I would also point out this is probably a good place to consider aluminum conductors.