Derating in Conduit Exceptions

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killer76

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I see that Table 31.15(B)(2)(a) indicate the adj factor
however I have been told that an exception will allow more than 3 ccc in a conduit

my example
I have a single phase loadcenter (6) 20 amp circuits
(4) circuits to (4) 20a duplex receptacles
(1) circuit to interior lights (4) 4' 2 lamp 40 watt fluorescent
(1) circuit to exterior lights (4) 70 watt HPS flood lights

a hot, neut, and gnd for all circuits #12 AWG THHN in a 1 inch conduit over 24 inches

from what I know I have in this conduit 12 ccc so I should now derate to 50%
according to the table

the building is manufactured to customer specification we are an OEM
I have been told that I could reduce the size of the breakers or run separate conduit

I wanted to know if there is such an exception to this table that will allow the above ccc to be in one (1) inch conduit
or if there is an exception due to the fact we are an OEM manufacture

I am getting so many mixed comments and would like to know if there is such an exception

One other question I asked this before and wanted to get further advice to gain more knowledge of this and other sections of the code any good teaching books that anyone can offer would be great, I plan to sign up at a local collage for code training soon

Thanks
 
If each circuit had it's own grounded conductor you would have 12 CCC's. That would require a 50% derating.

30 amps (.5)= 15 amps maximum OCPD.


If you could re-circuit using shared neutrals you may get the CCC's down to 6 conductors which would require a 80% derating.

30 amps (.8)= 24 amps maximum OCPD
 
I don't see an exception to allow you to run 12 CCC in 1"PVC without derating. Your options are as you said. More than one conduit or derate your wires.

Since you are using #12 thhn it will be rated 30 amps so at 50% you could put it on a 15 amp breaker.

The wiser move would be to use 2 smaller conduits with 6 CCC in each. Thus with a derating to 80% you could still use a 20 amp breaker on #12 thhn.
 
4-wire wye MWBC's you could do it in 1/2" IMO. Not available everywhere, but in most commercial applications.

Otherwise #10's in 1" to a point where you can reduce wire and conduit size. (I think... not calc'ing fill... I'm missing something???)
 
Dennis Alwon said:
I don't see an exception to allow you to run 12 CCC in 1"PVC without derating. Your options are as you said. More than one conduit or derate your wires.

Since you are using #12 thhn it will be rated 30 amps so at 50% you could put it on a 15 amp breaker.

The wiser move would be to use 2 smaller conduits with 6 CCC in each. Thus with a derating to 80% you could still use a 20 amp breaker on #12 thhn.
Click to expand...


Actually I said to use one conduit with shared neutrals and derate at 80%.
 
Wire amperage allowable vs. Circuit breaker

Wire amperage allowable vs. Circuit breaker

Ok I have the Derating down.....well better

However I need some help on this

the #12 thhn I see you have 30amp and then

I read the section under 240.4(e) (g) and have been challenged were this is allowed to figure the wire to be rated at 30amps,

It this specific to the breaker size only

I have been using the 30amp for #12 wire derating the conductors as given in the examples,

now asked to submit how I can use #12 for 30amps
after reading the section, 240 I could use some help

the circuits are typical lighting and receptacles, maybe a same hp fan

I am not sure what section under (e) or (g) to look for this application

Thanks again
 
Derating for more than 3 ccc's start in Article 310.15(B)(2)(a). This refers you to Table 310.15(B)(2)(a). That table states, "Percent of values in tables 310.16 through....". The value of #12 THHN is 30. You derate from 30.

From your original post, I see you have 6 circuits in a single phase panel. If each circuit has it's own grounded conductor, you have 12 CCCs. 30amps * 50% = 15 ampacity. Minimum PVC is 1" when grounding wire is included (Table C.9)

If you change to three MWBCs, you then would have six ungrounded with three grounded (neutral) conductors. This gives you a total of nine current-carrying conductors. 30amps x 70% = 21 amps capacity. This way, you could use a 20amp breaker per 240.4(D). Minimum PVC is ?".
 
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480sparky said:
If you change to three MWBCs, you then would have six ungrounded with three grounded (neutral) conductors leaves you with a total of nine current-carrying conductors.
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I would not bother counting the neutral as a CCC as Trevor pointed out.

30 amps (.8)= 24 amps maximum OCPD
 
killer76 said:
One other question I asked this before and wanted to get further advice to gain more knowledge of this and other sections of the code any good teaching books that anyone can offer would be great, I plan to sign up at a local collage for code training soon

Thanks
Click to expand...

Mike Holt offers Understanding the National Electrical Code volumes one and two.

IAEI (International Assocation of Electrical Inspectors) offers some very good books for code issues. Analysis of Changes is updated with every new code cycle. They also have, for residential applications, One & Two Family Dwelling Electrical Systems American Technical Publishers has a book by Michael I. Callanan and Bill Qusinich titled Electrical Systems based on the 2005 NEC. Another source is Stallcup's Illustrated Code Changes. Again, that one is updated every code cycle. There are others, of course, but these are some of the better ones I have and would recommend.

(edit- oops! forgot IAEI Soares Book on Grounding )
 
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IWire and Infinity.. So are you saying that if you run multiwire circuits you don't have to count the neutral as a current carrying conductor? I'm just trying to learn here as I have always been instructed to count it.
 
After looking at 310.15(B)(4) my question would be how do you apply (b) and (c) of that section. (a) allows you to not count it as a current carrying conductor but (b) and (c) say that you do depending on the transformer? So do you need to know the type of transformer to not count the neutral for derating purposes?
 
infinity said:
If you could re-circuit using shared neutrals you may get the CCC's down to 6 conductors which would require a 80% derating.

30 amps (.8)= 24 amps maximum OCPD
Click to expand...

True if it is 120/240 volt panel. If it is 120/208v wye-connected he would have to count the neutrals as current carrying in a multi-wire circuit.
 
ike5547 said:
True if it is 120/240 volt panel. If it is 120/208v wye-connected he would have to count the neutrals as current carrying in a multi-wire circuit.
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Not exactly true if the MWBC neutral did not contain harmonic currents. Take a look at 310.15(B)(4)(a). The OP mentions 6 circuits, two MWBC's (three circuits with a shared neutral) would only have 6 CCC's. The neutrals wouldn't count for derating purposes.


wireman71 said:
After looking at 310.15(B)(4) my question would be how do you apply (b) and (c) of that section. (a) allows you to not count it as a current carrying conductor but (b) and (c) say that you do depending on the transformer? So do you need to know the type of transformer to not count the neutral for derating purposes?
Click to expand...


(b) applies to a 3 wire MWBC circuit with a neutral from a wye connected system (i.e.-208/120Y or 277/480Y) In that case the neutral current will be about the same as the line to neutral current so it counts as a CCC.

(c)Applies to wye systems (as above) and the neutral would be counted as a CCC when the harmonic loading exceeds 50%.

So yes, you do need to know what type of supply system you have to in order to apply the provisions of 310.15(B)(4).
 
wireman71 said:
After looking at 310.15(B)(4) my question would be how do you apply (b) and (c) of that section. (a) allows you to not count it as a current carrying conductor but (b) and (c) say that you do depending on the transformer? So do you need to know the type of transformer to not count the neutral for derating purposes?
Click to expand...

Wireman, it's not as hard as you picture it to be.

310.15(B)(4)(c) is saying to count the neutral if you have harmonic loads
310.15(B)(4)(b) is saying to see if all phase conductors of a wye system are present in the conduit or jacket
310.15(B)(4)(a) is saying not to count the neutral if it "carries only the unbalanced current". . If all phase conductors are sharing the neutral and there's no harmonic loads, don't count it.

First determine if you have harmonic loads because if you do, then don't bother asking anymore questions, you have to count the neutral. . But if you don't have harmonics:

A single phase house panel has 2 phase bars behind the breakers. . If conductors from both phases are present in the conduit or jacket and sharing the neutral, you don't count the neutral. . Simple as that.

A 3phase wye panel has 3 phase bars behind the breakers. . If conductors from all 3 phases are present in the conduit or jacket and sharing the neutral, you don't count the neutral. . Simple again.

3phase deltas require a little thought.
1] Floating [ungrounded] deltas have no neutral.
2] Corner grounded have no neutral but have a white identified phase. . You always count that wire.
3] Center grounded. . If both non-high leg conductors are present in the conduit or jacket and sharing the neutral, you don't count the neutral.

Try this as a flow chart

Harmonics ?
. . if yes, count neutral
. . if no, continue:
. . . . Single or 3phase ?
. . . . . if single, then determine:
. . . . . . present and sharing, don't count neutral
. . . . . . not both present and sharing, count neutral
. . . . . if 3phase, then ask Wye or Delta ?
. . . . . . if Wye, then determine:
. . . . . . . present and sharing, don't count neutral
. . . . . . . all 3phase wires not both present and sharing, count neutral
. . . . . . if Delta, then ask floating, corner, or center ?
. . . . . . . if floating, then there is no neutral to consider
. . . . . . . if corner, then there is no neutral and the white grounded must be counted
. . . . . . . if center, then determine:
. . . . . . . . both non-high legs present and sharing, don't count neutral
. . . . . . . . both non-high legs not both present and sharing, count neutral
 
killer76 said:
now asked to submit how I can use #12 for 30amps
after reading the section, 240 I could use some help

the circuits are typical lighting and receptacles, maybe a same hp fan

I am not sure what section under (e) or (g) to look for this application

Thanks again
Click to expand...

For the lighting and recepts

Don't intermix the 2 requirements. . Determine ampacity both ways independent of each other and then you're required not to exceed the smaller of the 2 values you end up with.

requirement #1
240.4(D)
12 gauge max 20a

requirement #2
T310.16, T310.15(B)(2)(a), 240.4(B), 240.6(A)

Look at 3 scenerios:
scenerio A: 6 12gauge CCCs in a conduit or jacket
scenerio B: 9 12gauge CCCs in a conduit or jacket
scenerio C: 10 12gauge CCCs in a conduit or jacket

scenerio A: 6 12gauge CCCs in a conduit or jacket
requirement #1 = 20a
requirement #2
. . T310.16 = 30a
. . T310.15(B)(2)(a), 30 x .8 = 24a
. . 240.4(B) + 240.6(A), 24 is between 20 + 25 so you go to 25a
requirement #1 is lower so your answer is 20a

scenerio B: 9 12gauge CCCs in a conduit or jacket
requirement #1 = 20a
requirement #2
. . T310.16 = 30a
. . T310.15(B)(2)(a), 30 x .7 = 21a
. . 240.4(B) + 240.6(A), 21 is between 20 + 25 so you go to 25a
requirement #1 is lower so your answer is 20a

scenerio C: 10 12gauge CCCs in a conduit or jacket
requirement #1 = 20a
requirement #2
. . T310.16 = 30a
. . T310.15(B)(2)(a), 30 x .5 = 15a
. . 240.4(B) + 240.6(A), 15 is 15 so you have to stay at 15a
requirement #2 is lower so your answer is 15a

For the "same hp fan" go to 240.4(G)

David
 
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David,
310.15(B)(4)(c) is saying to count the neutral if you have harmonic loads
Click to expand...
No, that is not what the code says.
(c) On a 4-wire, 3-phase wye circuit where the major portion of the load consists of nonlinear loads, harmonic currents are present in the neutral conductor; the neutral shall therefore be considered a current-carrying conductor.
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Major portion is not defined in the NEC so we fall back to standard definitions and that would mean that more than half of the load would have to be non-linear.
Don
 
don_resqcapt19 said:
Major portion is not defined in the NEC so we fall back to standard definitions and that would mean that more than half of the load would have to be non-linear.
Don
Click to expand...

Good point


.
 
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