Derating for ccc's

[Frank DuVal]

Square D QO line (even QOB) comes in 10 amp size also.

 

[Smart $]

Square D QO line (even QOB) comes in 10 amp size also.

Good to know, but still not a standard breaker rating per the NEC.

 

[charlie b]

Ummm... he said OCPD. 15A is the smallest standard breaker. Fuses are OCPD's and the smallest standard is 1A.

I should have read the second sentence of 240.6. Mea culpa (that's Latin for "my bad").

 

[Carultch]

We have to be careful how we word this type of circuiting. What you stated is not true for 3-wire with neutral circuit of a 208Y/120V 3Ø 4W, or simply a 120/208V 1Ø 3W circuit, or this part of a MWBC. The neutral is always counted as a CCC in this case, even though it is only carrying the imbalance of line-to-neutral loads.

That would be an example of where the neutral HAS TO carry a part of the return path of the current, even when the loads are completely balanced.

The idea is that if the ungrounded conductors' currents alone can add up to zero (as vectors), when the load is completely balanced, then the neutral doesn't count.
If the ungrounded conductors need a neutral to add the current up to zero (as vectors) to complete the return path, then neutral does count.


In your profile picture, the three phases add up as vectors to zero, indicating that the three phase wires can carry the full round trip path of the current. If you ditch the blue phase, as is the case with 1ph/3W branch circuit, the white wire must carry the current of the blue wire, for current to add up to zero.

 

[Smart $]

That would be an example of where the neutral HAS TO carry a part of the return path of the current, even when the loads are completely balanced.

The idea is that if the ungrounded conductors' currents alone can add up to zero (as vectors), when the load is completely balanced, then the neutral doesn't count.
If the ungrounded conductors need a neutral to add the current up to zero (as vectors) to complete the return path, then neutral does count.


In your profile picture, the three phases add up as vectors to zero, indicating that the three phase wires can carry the full round trip path of the current. If you ditch the blue phase, as is the case with 1ph/3W branch circuit, the white wire must carry the current of the blue wire, for current to add up to zero.

Correct. I aware that you understand it. I'm just saying we have to be careful how we state it to other people... and not everyone understands vectors and how they add up.

 

[kwired]

And here is the basis for that "magic number":

• 9 CCCs gives a derating factor of 70%.
• The 90C column tells us that a #12 THHN has an ampacity of 30 amps.
• 70% of 30 is 21 amps.
• 21 is larger than 20 (i.e., the ampacity of #12 THHN at 75C), so we assign an ampacity of 20 amps.
• This allows us to still use a 20 amp circuit breaker with #12 THHN wires and with 9 CCCs.
• If there were 10 or more CCCs, then the derating factor of 50%, multiplied by the 90C ampacity of 30 amps, would put you below 20. Thus, you can't still use a 20 amp breaker with #12 THHN wires and with 10 or more CCCs.

It is worth noting that this reasoning does not work if the wire does not have a 90C rating. So if you are using THWN wire (that does not also have a THHN designation), then the process shown above would tell us that the magic number is 6 CCCs.

And now I must disagree with Action Dave, with regard to the 15 amp branch circuits. For #14 THHN wire and a 15 amp circuit breaker, the magic number is 6, not 9. As a homework assignment, use the process shown above to confirm, or refute, my claim. As another homework assignment, find the magic number for 30 amp circuits and #10 wire.

THWN-2 is 90 deg conductor also.

I disagree with 6 being the magic number for 14 AWG (with 90 degC conductors).

14 AWG @90C is rated for 25 amps.
9CCC gives 70% derating factor.
25 x .7 = 17.5. They taught us in my school that 17.5 was greater then 15, but maybe that was wrong?

10 AWG would only have 28 amps after adjustment for 9 CCC's - may be fine for most applications but we generally don't have as many applications using 30 amp circuits that supply multi-outlet receptacle outlets like we have with 15 and 20 amp circuits.

 

[Frank DuVal]

If the blue wire current goes to zero, it is no longer a current carrying conductor! No way can you get all 4 wires in a 120/208 4 wire circuit to carry full rated current unless there are harmonics to deal with. Go ahead and draw the vectors. This is why rubber cord tables for 4 and 5 conductors have the same amp rating. For 4 wires, they are thinking 3 phase and ground. For 5 wires, they are thinking 3 phases, neutral and ground. Same total current whether 4 or 5 conductors.

 

[Smart $]

... This is why rubber cord tables for 4 and 5 conductors have the same amp rating. For 4 wires, they are thinking 3 phase and ground. For 5 wires, they are thinking 3 phases, neutral and ground. Same total current whether 4 or 5 conductors.

Perhaps by the manufacturer. You won't find that in the NEC. What if someone used the 5 conductor cord for two, two-wire circuits?

 

[MD84]

Question on derating for switch loop. I have six switches that I need to feed. Each switch is fed by a 20A Ocpd. I will be running #12thhn. There will be 12 wires. Six wires feeding the switches and six wires coming back out to the lighting. I was considering running all wires in one conduit.

Do I have to derate the wire 50%? If so I will need to run two conduits or run #10thhn.

I am guessing I have to treat it as 12 ccc's but was curious for other opinions and/or confirmation.

 

[luckylerado]

Question on derating for switch loop. I have six switches that I need to feed. Each switch is fed by a 20A Ocpd. I will be running #12thhn. There will be 12 wires. Six wires feeding the switches and six wires coming back out to the lighting. I was considering running all wires in one conduit.

Do I have to derate the wire 50%? If so I will need to run two conduits or run #10thhn.

I am guessing I have to treat it as 12 ccc's but was curious for other opinions and/or confirmation.

I would agree that it is 12 CCC.

Cannot do a single switch and a contactor?

 

[MD84]

Thx. The customer would like to switch each string of lights on independently.