Does Table B-310.11 apply for theatrical dimming conductor derating? (lots-o-wire)

[bbaumer]

Hi.

I am working on replacing a dimmer board in a theater and adding a ton of new dimmer circuits in the process. Hundreds of dimmer circuits.

I haven't done this type of work before to speak of, at least not to this scale.

The existing system has several conduits with 20 to 30 dimmer circuits in each conduit, so 40 to 60 current carrying conductors. All #12 THHN/THWN. I asked the dimming equipment supplier about ampacity derating. He said he thought NEC Art. 520 had exceptions in it so we wouldn't have to go clear down to 35% per T 310-15(B)(2)(a).

I looked in 520 and see that there is an exception that allows any number of conductors as long as you don't exceed the fill requirement but don't see where the ampacity adjustment is addressed.

I do see that diversified loads, like what we have in this instance, have a different derating chart, Table B-310.11 located in Annex B.

The chances of having all the dimmer circuits at full load to any given batten or box is practically none.

Each light bar over the stage will have 22 to 36 circuits. Each bar fed with a big-a SO cord. Doesn't really make sense that I would have to run several conduits to the 36 circuit bar when they would all be terminated in the same box and feeding a 36 circuit SO cord.

Anyone here got experience with this?

 

[GoldDigger]

I looked in 520 and see that there is an exception that allows any number of conductors as long as you don't exceed the fill requirement but don't see where the ampacity adjustment is addressed.

The ampacity adjustment is considered separately, and with its statement that you should not use the adjustments for numbers of CCC smaller that 30 it is essentially taking away the increased allowance that you would get for smaller numbers. Not allowing higher ampacity values to be used, but not limiting the reductions.

Diversity is a definite issue, but I have not found a place where it is addressed.

 

[infinity]

520.6 will allow more than 30 CCC's in a wireway without derating. I see no such provision for separate conduits. Engineered load diversity may be another option.

520.6 Number of Conductors in Raceway. The number of
conductors permitted in any metal conduit, rigid nonmetallic
conduit as permitted in this article, or electrical metallic tubing
for circuits or for remote-control conductors shall not exceed
the percentage fill shown in Table 1 of Chapter 9. Where
contained within an auxiliary gutter or a wireway, the sum of
the cross-sectional areas of all contained conductors at any
cross section shall not exceed 20 percent of the interior cross-
sectional area of the auxiliary gutter or wireway. The 30-
conductor limitation of 366.22 and 376.22 shall not apply.

 

[texie]

520.6 will allow more than 30 CCC's in a wireway without derating. I see no such provision for separate conduits. Engineered load diversity may be another option.

I get the sense that the OP is not concerned about fill, but rather derating given that not all circuits are active at one time.

 

[GoldDigger]

520.6 will allow more than 30 CCC's in a wireway without derating. I see no such provision for separate conduits. Engineered load diversity may be another option.

Read the specific sections again please.

The adjustment factors in 310.15(B)(3)(a) shall be applied only where the number of current-carrying conductors, including neutral conductors classified as current-carrying under the provisions of 310.15(B)(5), exceeds 30.

So without 520.6, the adjustment factors will be applied only when the number of conductors is greater than 30. "Removing the 30 conductor limitation" could either be interpreted
1. as not applying the adjustments when the number is greater than 30 either OR
2. always applying the adjustments even when the number is less than 30.
Now look again at the table and notice that for numbers less than 30 the table shows an increase, not a decrease, in the ampacity.
You can hope that the handbook makes it clear which was intended, because the code itself certainly does not.


In any case, you are right that the sections only apply to two very specific kinds of raceway in the first place.

 

[hurk27]

520.6 will allow more than 30 CCC's in a wireway without derating. I see no such provision for separate conduits. Engineered load diversity may be another option.

Read the specific sections again please.

The adjustment factors in 310.15(B)(3)(a) shall be applied only where the number of current-carrying conductors, including neutral conductors classified as current-carrying under the provisions of 310.15(B)(5), exceeds 30.

So without 520.6, the adjustment factors will be applied only when the number of conductors is greater than 30. "Removing the 30 conductor limitation" could either be interpreted
1. as not applying the adjustments when the number is greater than 30 either OR
2. always applying the adjustments even when the number is less than 30.
Now look again at the table and notice that for numbers less than 30 the table shows an increase, not a decrease, in the ampacity.
You can hope that the handbook makes it clear which was intended, because the code itself certainly does not.


In any case, you are right that the sections only apply to two very specific kinds of raceway in the first place.

The second sentence in 520.6 is only if you are using an auxiliary gutter or a wire way, there is a period after the "Chapter 9." I do not see anything in the first sentence that removes the derating factors required in 310.15 for regular conduit which the OP is using, 520.6 contains two different requirements for two very different types of raceways.

520.6 Number of Conductors in Raceway. The number
of conductors permitted in any metal conduit, rigid nonmetallic
conduit as permitted in this article, or electrical metallic
tubing for border or stage pocket circuits or for
remote-control conductors shall not exceed the percentage
fill shown in Table 1 of Chapter 9.

Where contained within
an auxiliary gutter or a wireway, the sum of the crosssectional
areas of all contained conductors at any cross
section shall not exceed 20 percent of the interior crosssectional
area of the auxiliary gutter or wireway. The 30-
conductor limitation of 366.22 and 376.22 shall not apply.

The partial quote from 366.22 does not apply the the OP use of round conduit.

 

[GoldDigger]

The second sentence in 520.6 is only if you are using an auxiliary gutter or a wire way, there is a period after the "Chapter 9." I do not see anything in the first sentence that removes the derating factors required in 310.15 for regular conduit which the OP is using, 520.6 contains two different requirements for two very different types of raceways.



The partial quote from 366.22 does not apply the the OP use of round conduit.

I agree that nothing in 520.6 affects round conduit, nor do either of the 30 wire provisions have any effect on round conduit either. I am simply trying to understand what 520.6 actually does when auxiliary gutters or wireways are in use.

The reference to 30 wires does not affect in any way the fill limits either, as you noted, but it does potentially affect derating.

Although my question about the effect of 520.6 does not apply directly to the OP's problem, I think it is related enough to be explored in this thread.

 

[infinity]

376.22(B) is for adjustment factors in metal wireways. Since (B) does not apply when 520.6 is applicable then derating is not required for the conductors in a wireway (520.6 states that the 30 CCC adjustment factors do not apply). The 20% fill would still apply because it's explicitly mentioned in 520.6.

376.22 Number of Conductors and Ampacity. The num-
ber of conductors and their ampacity shall comply with
376.22(A) and (B).
(A) Cross-Sectional Areas of Wireway. The sum of the
cross-sectional areas of all contained conductors at any cross
section of a wireway shall not exceed 20 percent of the inte-
rior cross-sectional area of the wireway.
(B) Adjustment Factors. The adjustment factors in
310.15(B)(3)(a) shall be applied only where the number
of current-carrying conductors, including neutral conduc-
tors classified as current-carrying under the provisions of
310.15(B)(5), exceeds 30. Conductors for signaling circuits
or controller conductors between a motor and its starter and
used only for starting duty shall not be considered as
current-carrying conductors.

520.6 The 30-
conductor limitation of 366.22 and 376.22 shall not apply.