6AWG circuit derating / correct size OCD

[quakerj]

I have a 3/4 EMT conduit run that is going to contain two 6ga wires for a welder outlet, and three 12ga wires for a MWBC quad-receptacle.

I know more than 3 conductors require derating, what is the appropriate breaker size for the 6ga circuit? I probably don't *need* more than 30A here, just trying to future proof in a sense. All wires are THHN, 12ga is solid. Any issues with conduit fill?

 

[mike93lx]

Welders have special wiring requirements based on duty cycle, so you are very likely significantly oversizing the wiring. A typical 50a home welder can be hooked up with #12.

 

[sparky 1971]

If you're using copper THHN/THWN the max breakers are going to be 60 for the #6 and 20 for the #12. This is after derating, which is 80%.

 

[billconner]

If you're using copper THHN/THWN the max breakers are going to be 60 for the #6 and 20 for the #12. This is after derating, which is 80%.

Isn't that 80% for continuous loads? I can't imagine a welder being a continuous load.

 

[jeepxj]

isnt this under NEC 630?

 

[quakerj]

Welders have special wiring requirements based on duty cycle, so you are very likely significantly oversizing the wiring. A typical 50a home welder can be hooked up with #12.

Oversizing the wire was sort of intended in this location as at some point in the future it may change from a welder circuit to something else entirely, like a mini-split or who knows what. I just wanted a big beefy circuit in at least one spot for the future unknowns, in the meantime it can be an extra place for a welder receptacle. The wire cost nothing, it was left over from a project I did years ago.

Sounds like up to 60A is okay if I ever need it?

 

[mike93lx]

How long is the run?

If any decent distance, would a subpanel be more useful? Then you can easily have multiple receptacles off it. Keep the welder and add hvac, or an ev

 

[quakerj]

How long is the run?

If any decent distance, would a subpanel be more useful? Then you can easily have multiple receptacles off it. Keep the welder and add hvac, or an ev

25 feet or so is all.

 

[mike93lx]

25 feet or so is all.

OK, nothing terrible, but I'd consider a sub. Even a small one, like a 12 space would give you a ton of flexibility. Just add a neutral. Then you can run your Mwbc off that too

 

[sparky 1971]

Isn't that 80% for continuous loads? I can't imagine a welder being a continuous load.

The question was what is the largest size breaker that can be used for the #6 with that many wires in the conduit. #6 THHN is rated for 75 amps, the adjustment factor is 80% meaning the largest breaker that can be used for #6 is a 60. I left out that a welder receptacle is a 6-50 so it's limited to a 50 amp breaker so #8 would work and I would probably use #10, but those weren't part of the question. Continuous/non continuous has nothing to do withhow many wires are in a conduit.

 

[jeepxj]

The question was what is the largest size breaker that can be used for the #6 with that many wires in the conduit. #6 THHN is rated for 75 amps, the adjustment factor is 80% meaning the largest breaker that can be used for #6 is a 60. I left out that a welder receptacle is a 6-50 so it's limited to a 50 amp breaker so #8 would work and I would probably use #10, but those weren't part of the question. Continuous/non continuous has nothing to do withhow many wires are in a conduit.

if it was a larger conduit would the same derate apply? or is it a # of conductors in there triggering it?

 

[sparky 1971]

Conduit size doesn't matter

 

[wyliesdiesels]

if it was a larger conduit would the same derate apply? or is it a # of conductors in there triggering it?

Its number of wires in the pipe that triggers the deration

 

[mike93lx]

Conduit size doesn't matter

But at what size is it no longer "conduit"? Dun dun dun

 

[wyliesdiesels]

Isn't that 80% for continuous loads? I can't imagine a welder being a continuous load.

Theres no discussion here about continuous loads. The 80% deration factor is due to # of wires in pipe...

 

[exranger06]

If the wires/conduit see ambient temps over 86 degrees F, you'll have to derate them some more (310.15(B)).

 

[jeepxj]

Its number of wires in the pipe that triggers the deration

so even if i put 10 #14s into a 4" derate still occurs? interesting.

 

[mike93lx]

so even if i put 10 #14s into a 4" derate still occurs? interesting.

They'll likely all be piled on top of each other, regardless

 

[jeepxj]

They'll likely all be piled on top of each other, regardless

oh duh. the problem is with the inner conductors in the bundle not having good airflow around them. i think.

 

[sparky 1971]

But at what size is it no longer "conduit"? Dun dun dun

The one and only time it's been an issue for me was about 15 years ago. I got dinged for having six #14 and two #12 in a 2" that was about five feet long and didn't derate.

 

[mike93lx]

oh duh. the problem is with the inner conductors in the bundle not having good airflow around them. i think.

It's really just a Big Wire conspiracy to make us buy larger conductors. Big Conduit is in on it too

 

[billconner]

Theres no discussion here about continuous loads. The 80% deration factor is due to # of wires in pipe...

Get it. I mistook the 80% for the connected load on a circuit.

Thank you!

 

[alfredeneuman]

But at what size is it no longer "conduit"? Dun dun dun

Unlimited conduit size. But if it's classified as a metal wireway as many as 30 conductors can be run without derating. 31 will revert to full derating for the number

 

[mike93lx]

Unlimited conduit size. But if it's classified as a metal wireway as many as 30 conductors can be run without derating. 31 will revert to full derating for the number

So thhn in a culvert pipe is cool?

 

[dcg9381]

If the wires/conduit see ambient temps over 86 degrees F, you'll have to derate them some more (310.15(B)).

I wonder if they take that into consideration in every single exterior conduit run in Texas?

 

[exranger06]

I wonder if they take that into consideration in every single exterior conduit run in Texas?

They should be! I'm in CT where the temps hardly ever go above 85*, and even I take it into consideration. I looked up the highest recorded temperatures in my area and base the derating off of that. Temps between 96-104* get derated to 91%, which is what I use. It's those rare, unusually hot times when you need the extra ampacity the most.

 

[quakerj]

Thank you fellas for all the answers and I certainly enjoy reading the extra debate.

I'll probably go with a 40A breaker for now because I happen to have one on hand. In this corner of the shop, it's not perfectly suited for HVAC but could be used as such. For an EV it's in the perfect location. I used #6 because I didn't have to buy it, not because it was really needed.

The rest of the shop is run in #12, which will suit all my 240V locations (lift, air compressor, welder outlets) just fine. I might need to add a conduit run in the future for HVAC if I want to put it in the back, but that shouldn't be a huge ordeal.

Just for clarification, many of my #12 branch feed wires leave the panel out of 3/4 EMT that is about 1' long before they enter a junction box. I don't have to derate there, correct? By the time I'm done, the pipe will have about 15 conductors, some of which aren't considered current carrying. I should mention the #6 leaves out a different pipe, so these are all #12's, think I'm allowed up to 16 in that 1' section if I'm not mistaken.