Derating Feeders

[lbudden]

Question: When derating feeder conductors, does the number you finally derate to for the conductors have to be more than the breaker size of the circuit you are installing, or more than the calculated load?
For example, I have a #3 THHN 100 amp feed, 3 ph. with double neutral installed in a 2" conduit for bus duct (3 hots, 2 neutrals, 1 grd). I am wanting to pull another set of #3's in this conduit, making a total of 10 #3's and the #8 ground. The conduit is of sufficient size, and the neutrals are not current carrying conductors since they only carry the unbalanced load (and not a lot of non-linear load).
So that leaves me with 4-6 Current Carrying Conductors, which I derate 80%. 310.16 shows 110 amps for #3 in the 90C column, multiplied by 80% is 88 amps.
There is an example in the 2005 NEC Handbook showing branch circuit calc. for a 20 amp circuit, and it shows only needing the conductor to be above 16 amps, or 80% of the 20 amp breaker allowed loading after doing all the derating/temp. adjustments. It uses 210.19 A 1 to allow this.
Is this the same for the feeder in my example? Do I just need to have a conductor above 80 amps (or 80 % of the 100 amp breaker)?
Thanks for any advise!!!

 

[Dennis Alwon]

Question: When derating feeder conductors, does the number you finally derate to for the conductors have to be more than the breaker size of the circuit you are installing, or more than the calculated load?
For example, I have a #3 THHN 100 amp feed, 3 ph. with double neutral installed in a 2" conduit for bus duct (3 hots, 2 neutrals, 1 grd). I am wanting to pull another set of #3's in this conduit, making a total of 10 #3's and the #8 ground. The conduit is of sufficient size, and the neutrals are not current carrying conductors since they only carry the unbalanced load (and not a lot of non-linear load).
So that leaves me with 4-6 Current Carrying Conductors, which I derate 80%. 310.16 shows 110 amps for #3 in the 90C column, multiplied by 80% is 88 amps.
/QUOTE]
I didn't check you numbers however, If I understand you correctly, your #3 wire is now only rated for 88 amps so you cannot protect the circuit with a 100 amp breaker you could use a 90 amp breaker (article 240.4 (B) as long as your calculated load is more than 88.

 

[don_resqcapt19]

Why is there a double neutral if there is little non-linear load? Also you can't use a parallel neutral if the conductors are #3.
Don

 

[LarryFine]

Why is there a double neutral if there is little non-linear load? Also you can't use a parallel neutral if the conductors are #3.

I would use the second neutral for the new conductor set.

 

[dduffee260]

So that leaves me with 4-6 Current Carrying Conductors, which I derate 80%. 310.16 shows 110 amps for #3 in the 90C column, multiplied by 80% is 88 amps.

I thought you were only allowed to use the column which had the lowest rating in which the termination point was? I thought alot of the breakers were only rated in the 60C column so that was which column that the wire rating had to be from. Is this not correct? What is the correct way to rate your wire per the lug termination spots?

 

[Dennis Alwon]

I thought you were only allowed to use the column which had the lowest rating in which the termination point was? I thought alot of the breakers were only rated in the 60C column so that was which column that the wire rating had to be from. Is this not correct? What is the correct way to rate your wire per the lug termination spots?

Most termination are at 75 degree but derating can be done from the 90 degree rating as long as the OCP is not more than the 75 degree rating except where allowed in article 240.4 (B) and other areas of the code.

Romex must be rated at 60 degree

 

[infinity]

Why is there a double neutral if there is little non-linear load? Also you can't use a parallel neutral if the conductors are #3.
Don

I see this quite often, a drawing that calls for a 200% neutral. So a panel is fed with #3 or #2 and parallel neutrals. A better option would be a #3/0 neutral.

 

[don_resqcapt19]

Trevor,

So a panel is fed with #3 or #2 and parallel neutrals.

The code does not permit the installation of conductors smaller than 1/0 in parallel. There is an exception but it only applies to existing, not new installations, and even using the exception you can't parallel the #3s.

310.4 ... Exception No. 4: Under engineering supervision, grounded neutral conductors in sizes 2 AWG and larger shall be permitted to be run in parallel for existing installations.
FPN: Exception No. 4 can be used to alleviate overheating of neutral conductors in existing installations due to high content of triplen harmonic currents.

Don

 

[infinity]

Yes, the panel is fed with #3 or #2 conductors and a parallel set of neutrals. I'm aware of the #1/0 parallel restriction but from what I've seen in the field some other electricians are not aware of that restriction.