Derating conductors

[eager2learn]

This is the situation ! I have an auxillary gutter that has aprroximately 24 multi-wire branch circuits that extend from underground up one 2" emt conduit that is approximately 10' in length that hits another auxillary gutter then nipples through an 8" wall with a 2" nipple that goes into the panel where all circuits originate from. My question is, how do I apply the adjustment factors for this type of installation to figure out how many conductors I can get in the 10' 2" emt before I have to add more conduits. I'm really confused so lame man terms would be greatly appreciated.

 

[eager2learn]

All multi-wire branch circuits are #12 thhn on 20 amps. The panel is 120/208

 

[Little Bill]

All multi-wire branch circuits are #12 thhn on 20 amps. The panel is 120/208

Table C.1 in Annex C says you can have 101 #12 THHN in 2" EMT. According to your statement about having 24 mwbc you probably would have to count all the neutrals as CCC so that would be 72 CCC. Add the EGC's and that would be 96 total if your math is right on the amt. of circuits.

You can use Cpt. 9 Tables 4 & 5 also. It figures the same, 101 #12 THHN for 2" EMT.

 

[ActionDave]

The size of the raceway is irrelevant only the number of current, carrying, conductors (CCC) matters.

If you read 310.15(B)2a and skip over all the words between "raceway"and "raceways," in the second sentence it may make better sense. Then look closely at Exception No 2.

 

[Smart $]

Table C.1 in Annex C says you can have 101 #12 THHN in 2" EMT. According to your statement about having 24 mwbc you probably would have to count all the neutrals as CCC so that would be 72 CCC. Add the EGC's and that would be 96 total if your math is right on the amt. of circuits.

You can use Cpt. 9 Tables 4 & 5 also. It figures the same, 101 #12 THHN for 2" EMT.

Ummm... that would be NOT counting neutrals as CCC's.

72 CCC's get derated to 35%. If those are #12 THWN-2, their ampacity is 30A ? 35% = 10.5... and are not protected with a 20A OCPD.

If OP'er wishes to not upsize the conductors, 9 CCC's (or three 4-wire mwbc's) on 20A ocpd's is the max in one conduit.

Upsize to #10 THWN-2, and 40 CCC's (or thirteen 4-wire mwbc's) on 20A ocpd's is the max in one conduit (but have only a 16A circuit ampacity).

 

[Little Bill]

Ummm... that would be NOT counting neutrals as CCC's.

72 CCC's get derated to 35%. If those are #12 THWN-2, their ampacity is 30A ? 35% = 10.5... and are not protected with a 20A OCPD.

If OP'er wishes to not upsize the conductors, 9 CCC's (or three 4-wire mwbc's) on 20A ocpd's is the max in one conduit.

Upsize to #10 THWN-2, and 40 CCC's (or thirteen 4-wire mwbc's) on 20A ocpd's is the max in one conduit (but have only a 16A circuit ampacity).

Well did you ever wish you could erase instead of edit an entire post? I didn't notice it was 20A circuits, the rest of my reply I have no excuse for other than maybe staying up too late. You would count the neutrals as CC wouldn't you? I had just finished looking up something in Annex C and saw this and thought "ah, all the same size conductors, easy!"
Sorry for my ignorance.:roll:

 

[eager2learn]

I am aware that you can install 101 conductors in a 2" emt conduit but I don't understand how to derate the coductors according to the nec. This is the way I see it : Say I use every breaker in that 42 space panel. That would give me a total of 56 ccc. Since all the circuits are protected by 20 amp breakers, I would then take that 20 amps and multiply it by 35% which gives me 7 amps. What do I do from this point to figure out what size conduit I should use to accomodate my situation? I feel like I'm missing something. Like I stated above, lame man terms would be greatly appreciated.

 

[patriot]

Take your wire size and derate it. So if youhave #12 wire and you take it at 35%, that wire is only good for 7 amps. There are two ways to look at it, you either size your wire bigger or you get the # of current carrying cunductors in your raceway down. I would run 3/4" emt and put no more than 6 ccc in each pipe. Thats if all your wires are #12. Hope this helps.

 

[eager2learn]

Wait I think I got this - I'm derating the conductors not the circuit - So a #12 thhn conductor is rated for 30 amps under the 90 degree column - So I take the 30 amps and multiply it by 35% which gives me 10.5 amps - That won't work so I would have to bump up to the next wire guage verses adding conduit - which would put me at # 10 thhn which is rated for 40 amps which still wouldn't work - by process of elimination I would keep doing this until I get to 80% of a 20 amp circuit - Once I acheive this I size the conduit to the conductors - Obviously price is also another consideration

 

[ActionDave]

Wait I think I got this - I'm derating the conductors not the circuit - So a #12 thhn conductor is rated for 30 amps under the 90 degree column - So I take the 30 amps and multiply it by 35% which gives me 10.5 amps - That won't work so I would have to bump up to the next wire guage verses adding conduit - which would put me at # 10 thhn which is rated for 40 amps which still wouldn't work - by process of elimination I would keep doing this until I get to 80% of a 20 amp circuit - Once I acheive this I size the conduit to the conductors - Obviously price is also another consideration

Looks like you are getting it, but look at 310.15 B(4). You don't count the neutrals as current, carrying, conductors for the purposes of derating in a MWBC.

 

[Little Bill]

I am aware that you can install 101 conductors in a 2" emt conduit but I don't understand how to derate the coductors according to the nec. This is the way I see it : Say I use every breaker in that 42 space panel. That would give me a total of 56 ccc. Since all the circuits are protected by 20 amp breakers, I would then take that 20 amps and multiply it by 35% which gives me 7 amps. What do I do from this point to figure out what size conduit I should use to accomodate my situation? I feel like I'm missing something. Like I stated above, lame man terms would be greatly appreciated.

Forget everything I said in my first reply to you. (post #3) If you already have all those 12awg for the 24 20A mwbc in the 2" emt you are already in violation. As Smart $ said, you can only have 9-12awg CCC in the raceway to be able to use the 20A OCPD (breaker). Table 310.15(B)(2)(a) shows the adjustment factors for more than 3 CCC in a raceway. 9 CCC would require a 70% adj. Then look at Table 310.16 for ampacities. You can use the 90 dg. column for derating. Using that, 12awg THHN is 30A, 70% of 30A is 21A. You would have to upsize your wire to get more than that in the 2" (or any size) emt to comply with the 20A OCPD. I don't understand your comment about 35% of 20A. That has nothing to do with determining amt. of conductors in a raceway. Just look at the 2 tables I mentioned and go from there. You might also have to look at Art. 366 for the Auxiliary Gutters.

Smart $ or anyone, correct me if I'm wrong and feel free to chime in with anything else that needs to be added.

 

[Smart $]

Wait I think I got this - I'm derating the conductors not the circuit - So a #12 thhn conductor is rated for 30 amps under the 90 degree column - So I take the 30 amps and multiply it by 35% which gives me 10.5 amps - That won't work so I would have to bump up to the next wire guage verses adding conduit - which would put me at # 10 thhn which is rated for 40 amps which still wouldn't work - by process of elimination I would keep doing this until I get to 80% of a 20 amp circuit - Once I acheive this I size the conduit to the conductors - Obviously price is also another consideration

Some finer points...

You mentioned early on these come from underground. You can't use thhn for underground because the conductors have to be wet location listed. That said, thhn nowadays is usually dual or triple listed thwn and or thwn-2. In order to use the 90?C column for derating underground circuits, the #12 must be listed as thwn-2. If it is just thwn listed, you have to start with the 75?C column value.

Also, while 80% of 20A is the magic number, so to speak... it is not determined that way. The reason you can use a conductor derated to 16A on a 20A breaker is because 16A is greater than the 15A standard ocpd (breaker/fuse) rating... for which 240.4(B) let's you go to the next higher standard rating. Additionally, your load cannot be greater than the derated 16A, even though the breaker rating is 20A.

 

[Smart $]

Looks like you are getting it, but look at 310.15 B(4). You don't count the neutrals as current, carrying, conductors for the purposes of derating in a MWBC.

A full mwbc... i.e. 4-wire mwbc. The neutral would be counted as a CCC in a 3-wire mwbc out of a 3? 4-wire panel.

 

[ActionDave]

Thank you for clarifying.

 

[Little Bill]

A full mwbc... i.e. 4-wire mwbc. The neutral would be counted as a CCC in a 3-wire mwbc out of a 3? 4-wire panel.

Smart $, that's why I ask in one of my posts about the neutral being counted because he said the panel was 208/120.

 

[infinity]

Also, while 80% of 20A is the magic number, so to speak... it is not determined that way. The reason you can use a conductor derated to 16A on a 20A breaker is because 16A is greater than the 15A standard ocpd (breaker/fuse) rating... for which 240.4(B) let's you go to the next higher standard rating. Additionally, your load cannot be greater than the derated 16A, even though the breaker rating is 20A.

The next standard size up rule cannot be applied if these are multi-receptacle circuits. So if your circuits are supplying more than one receptacle per circuit then the max OCPD would be 15 amps after derating.

 

[Smart $]

The next standard size up rule cannot be applied if these are multi-receptacle circuits. So if your circuits are supplying more than one receptacle per circuit then the max OCPD would be 15 amps after derating.

He did not say what purpose the circuits are for, but you are correct for any circuit which has multi-outlet receptacles. Thanks for pointing that out.

Question: Would this apply to a mwbc that has one receptacle on each leg?

 

[infinity]

Question: Would this apply to a mwbc that has one receptacle on each leg?

I would say no.

 

[Smart $]

I would say no.

That is my thought as well... but the Code is not specific, and I could see an inspector construing a conclusion of the opposite interpretation.

 

[eager2learn]

The panels that I have are 120/208 3 phase 4 wire. It appears that the only thing that I can do is upsize my conductors or add more conduit. So the only time I don't count the nuetrals as an CCC is when?