Derating and conduit

[terabitdan]

I'm uncertain about when derating applies.

I need to move a subpanel with 7 circuits, (3) 15 amp, (3) 20 amp and (1) 30 amp. I'm thinking of running 3/4" conduit from between the two locations, maybe 1" for future expansion, with THHN. Either size conduit will be below fill limits. Does de-rating apply in this situation?

Dan


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[rburke65]

Well don't feel bad. Lots of electricians are confused by derating including myself.

 

[Bert_]

Is the run of conduit greater than 24"? Do you have more than 3 current carrying conductors in the conduit? If both of these are true then you need to derate.

I'm going to assume you have 14 current carrying wires, thats 50%. If you have multi-wire circuits then the current carrying conductor count could be less.

For THHN you can start with the 90* ampacity for derating purposes. But the end result cannot be higher than the 60* ampacity for small circuits or 75* ampacity if the terminals are rated 75*

14 thhn 25A x .5 = 12.5A max load (can still go on a 15A breaker)
12 thhn 30A x .5 = 15A max load (15A breaker)
10 thhn 40A x .5 = 20A max load (20A breaker)
8 thhn 55A x .5 = 27.5A max load (can go on 30A breaker)

To put all the circuits in the same pipe you will have to go up one size of wire for each circuit.
Or you could run 2 pipes

 

[Cmreschke]

I'd run 2 conduits. 3/4 and put the 15 amp and 30 amp ckts in one, 20 amps in the other.

 

[terabitdan]

Thanks Bert, that's very helpful And clear explanation. Appreciate your idea Cmreschke. The conduit in this case will be roughly 65' with no multi-wire branch circuits, so 2 conduits makes sense and if I reading the tables right can use the standard wire gauge for each circuit.

So, another example with romex; if cables are stapled alongside of each other, not stacked, does de-rating apply? Assume more than 24" with 4 12/2 cable runs? What about the same example through holes in joists, not tied together or bundled in any way?


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[zmaxmotorsports]

Is your pipe going to be surface mounted or is it going to be inside of a wall?
If it's going to be surface mounted I'd just bite the bullet and run it in 1 1/4" and be done with it instead of doing everything twice.

 

[walrus]

Is your pipe going to be surface mounted or is it going to be inside of a wall?
If it's going to be surface mounted I'd just bite the bullet and run it in 1 1/4" and be done with it instead of doing everything twice.

How does the larger conduit stop derating?

 

[zmaxmotorsports]

How does the larger conduit stop derating?

You noticed the part where I said to avoid doing everything twice?
I'd rather run a bigger run of conduit than to have to run 2 pieces of 3/4,avoid doing things twice and much easier to pull wire through.

 

[mm08822]

Also consider which circuits can be re-fed from the direction you are moving the panel to where the homeruns would be shortened.
Also consider refeeding the first outlet of the circuit from the new panel - depends on accessibility and distancel.
Maybe you could use an additional recept somewhere and that existing homerun cable is long enough to get to the new recept location.
This ideas aren't all cure-all, but maybe you can lower your conduit conductor count(s) so you can get up into the 70 or 80% derate factor. Maybe it is now one conduit only.

 

[terabitdan]

Thanks mm, the panel had 13 circuits when we started the migration, but I've found myself having to drill holes in joists along the same path for all of them. That's when using conduit and a junction box started to dawn on me.

Having always thought that full capacity was the only factor for conduit, the few mentions of derating on this excellent forum had me puzzled.

There's a return air duct about 12" from the walls on one side, conduits there would be completely hidden and easy to surface mount, less holes, less cleanup sounds like a better way for circuits I do need to extend.

There are a couple more circuits which will be consolidated, like lighting the the laundry room combined with the rest of basement lighting circuit. Similarly the 30 amp circuit will probably go away, it goes to my old garage which is being converted to living space. No much call for 30 amps in a bedroom!


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[Dagny]

If this is your residence don't worry about it. Here we have an exception to NEC allowing us to stuff as many romex cables as will fit into shorter lenghts of conduit instead of using romex connectors. I have never seen these even warm.

 

[sberry]

I run 2 pipes, don't own any where the branches are larger. Keeping it to a single size is simple. I wouldn't want another collection of fittings, ko's are a problem for diy, boxes too.

 

[Cmreschke]

You noticed the part where I said to avoid doing everything twice?
I'd rather run a bigger run of conduit than to have to run 2 pieces of 3/4,avoid doing things twice and much easier to pull wire through.

All due respect but imo, your mistaken here.

Easier to run 2 3/4" conduits than it is to run 1 11/4". Easier to pull smaller sets of wire than it is to pull 1 big set.
Cheaper to run smaller conduits over 1 bigger. Cheaper to pull 2 smaller sets as opposed to a larger set of bigger conductors due to derating.

Running parallel conduits isn't doing ot twice. Again imo.

 

[zmaxmotorsports]

All due respect but imo, your mistaken here.

Easier to run 2 3/4" conduits than it is to run 1 11/4". Easier to pull smaller sets of wire than it is to pull 1 big set.
Cheaper to run smaller conduits over 1 bigger. Cheaper to pull 2 smaller sets as opposed to a larger set of bigger conductors due to derating.

Running parallel conduits isn't doing ot twice. Again imo.

It's not a big deal,everybody has their own way of running things.
Go with which ever way works best/easiest for your individual situation.

 

[sberry]

I tend to agree with that. Z probably has all the tools and extensive know how, maybe even a collection of some fittings. We saw a job on here a while back it would have been 2x as easy to use all 1/2 on the job, same for air lines. A guy can collect a few fittings and may have a chance to use some extra, get 3 sizes and they sit forever and didn't gain much.
There is a likelihood that not every wire in a run needs to go the length of a run or the pipe for that matter, as I mention about another thread, instead of choking a bunch of wire thru 3/4 a guy could as easily run another pipe less than a piece from the panel for a welder circuit, no confusion, easy to move a few ft if needed, simple as dirt, made it single circuit in pipe.
Most could run 10 for all the 240 needs unless it is going to an electric range, ,makes it simple again to use a single size wire and no derate needed.