Should I upsize the wire on a 250ft run?

[Shak180]

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.

 

[tom baker]

Its easy to determine the power lost in volt drop and convert that into $$$. Run some numbers to find the sweet spot.

 

[LarryFine]

Unless you're running everything simultaneously, load diversity means even less loss.

That's one main advantage of using a feeder and panel instead of individual circuits.

 

[ptonsparky]

How often will these all be cycling at the same time? Next to never or a tad more than occasionally?

 

[ptonsparky]

Larry beat me...

 

[James L]

If you're running a 2- or 3-awg 250 feet, it's not a 100-amp circuit. It's only about a 75-amp circuit.

Granted, the OP shows 71 amps if everything ran at once. But I wouldn't run a circuit of that size that far, and call it 100 amps.

Practically speaking, it'll work

 

[Shak180]

If you're running a 2- or 3-awg 250 feet, it's not a 100-amp circuit. It's only about a 75-amp circuit.

Granted, the OP shows 71 amps if everything ran at once. But I wouldn't run a circuit of that size that far, and call it 100 amps.

Practically speaking, it'll work

Why would it be a 75 amp circuit?

 

[Shak180]

How often will these all be cycling at the same time? Next to never or a tad more than occasionally?

I'm only installing (2) 50 amp receptacles (each a dedicated circuit) so unless they plug in a splitter only 2 devices should be running at a time

 

[Shak180]

Unless you're running everything simultaneously, load diversity means even less loss.

That's one main advantage of using a feeder and panel instead of individual circuits.

I just briefly looked up load diversity. Man I have a lot to learn about building electrical infrastructure

 

[electrofelon]

Does it say copper only in the job specs? I would never run copper that far unless someone told me to.

 

[electrofelon]

Doesn't a plasma cutter need an air compressor?

 

[James L]

Why would it be a 75 amp circuit?

Because of voltage drop. If you want a 100 amp feeder with less than 3% drop, you would need a 1/0

 

[LarryFine]

Why would it be a 75 amp circuit?

As a design parameter more than a code limitation, I think.

 

[petersonra]

Why would it be a 75 amp circuit?

Why would it be a 75 amp circuit?

Because the total sum of all the loads is only 71 amps according to the poster, so even though he's running conductors with 100 ampacity, his load is only 71 amps maximum if all of them are on simultaneously at full load. I would be calculating the voltage drop at 71 amps instead of 100. My personal opinion is this is a perfectly acceptable situation.

 

[winnie]

The unstated question is: what voltage drop can be tolerated?

NEC suggests a max of 2% for feeders and 3% for branch circuits, max 5% sum. This is a suggestion not a requirement. In this case you have a long feeder and very short branch circuits, so I expect the total voltage drop for each circuit to be rather less than 5%.

The only question is if the equipment has any strict voltage drop requirements. The heater will certainly tolerate large voltage swings. I'd check the equipment manuals to see what voltage range they tolerate, but I suspect your design is fine as is.

Jon

 

[James L]

Because the total sum of all the loads is only 71 amps according to the poster, so even though he's running conductors with 100 ampacity, his load is only 71 amps maximum if all of them are on simultaneously at full load. I would be calculating the voltage drop at 71 amps instead of 100. My personal opinion is this is a perfectly acceptable situation.

Yet putting it on a 100 amp breaker and calling it a 100 amp feeder? That's not cool

Calculating at 71 amps is great, if it's put on an 80-amp breaker (if available) and called an 80-amp feeder

 

[electrofelon]

Yet putting it on a 100 amp breaker and calling it a 100 amp feeder? That's not cool

Calculating at 71 amps is great, if it's put on an 80-amp breaker (if available) and called an 80-amp feeder

Disagree. Everyone does this all the time: run a standard sized feeder/service but use a lesser number to calc V drop. I'll run a resi service 200 feet in 4/0 AL and still call it a 200 amp service. We also use larger equipment sometimes just for the larger lugs.

 

[ptonsparky]

We put a less than 200 amp wire on a residential service and call it 200.
I see no reason not call this a 100 amp feeder.

 

[ActionDave]

Yet putting it on a 100 amp breaker and calling it a 100 amp feeder? That's not cool....

It's not uncool, it's normal.

 

[infinity]

Because the total sum of all the loads is only 71 amps according to the poster, so even though he's running conductors with 100 ampacity, his load is only 71 amps maximum if all of them are on simultaneously at full load. I would be calculating the voltage drop at 71 amps instead of 100. My personal opinion is this is a perfectly acceptable situation.

I agree it's a 100 amp feeder. Calling anything else is nonsense.