Conduit fill

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raydamesek

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I have six 20A 220V circuits to run. I want to put twelve #12 THHN conductors and one #12 THHN ground in Alflex conduit. I use a program that tells me that the fill for these 13 conductors is 28.70% for 3/4" conduit. (In practice I would use at least 1" conduit.) Since this fill is less than the 40% cited in Table 1 chapter 9, are there any other factors pertaining to derating of any sort that I should account for in the conduit. I'm not concerned with turns, boxes, etc. for this question.
 
Refer to table 310.15 (B) (2) (a) It looks like if you have 12 CCC's you need to derate 50%. That would mean a #12 is good for 15 amps.
 
In other words, to retain the 20a per circuit capacity, you must upsize to #10 wire, and figure your conduit size based on that.
 
LarryFine said:
In other words, to retain the 20a per circuit capacity, you must upsize to #10 wire, and figure your conduit size based on that.
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I understand that a dereating of 50% would apply. But exactly how do you do this? I.e., do I use a 10A breaker (50% of 20A)? Do I use #10ga THHN? What if the derating called for 70%? Exactly what needs to be done?
 
What if each device on the circuit can only draw 9A? Is this a 50% derating? Do I need a 10A breaker (50% of 20A)? What if I needed a 40% derating? What says going from 12ga THHN to 10ga THHN is 50%?
 
LarryFine said:
In other words, to retain the 20a per circuit capacity, you must upsize to #10 wire, and figure your conduit size based on that.
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Since #12 wire is good for 20A and #10 wire is good for 30A, it appears to me that that is only a 66.67% derating (20/30=.6667). Wouldn't I need #8 wire (40A) for a 50% derating (20/40=.5000) and then figure conduit size?

If not and #10 wire constitutes a 50% derating, where does that information come from? Where is it written that going from #12 to #10 is 50%?
 
You need to start with the maximum allowable ampacity for the conductors selected using table 310.16. For example #12 THHN is good for 30 amps. You then use table 310.15 (B) (2) (a) and apply the appropriate adjustment factor. In your case a 50% reduction in ampacity must be applied due to the number of conductors. 30 amp X 50% = 15 amps. This would then be the maximum allowable overcurrent device you are allow to protect the conductor with. There may also be additional factors such as ambient temperature that may further reduce the allowable ampacity of the conductors. Does that help at all?
 
shockin said:
You need to start with the maximum allowable ampacity for the conductors selected using table 310.16. For example #12 THHN is good for 30 amps. You then use table 310.15 (B) (2) (a) and apply the appropriate adjustment factor. In your case a 50% reduction in ampacity must be applied due to the number of conductors. 30 amp X 50% = 15 amps. This would then be the maximum allowable overcurrent device you are allow to protect the conductor with. There may also be additional factors such as ambient temperature that may further reduce the allowable ampacity of the conductors. Does that help at all?
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This helps quite a bit. Thank you.

What if I have a 15A 220V simplex (vice duplex) receptacle on each of the six circuits where I could only plug in one device that draws 9A @220V. Would that satisfy this section of the code if the breaker is a 20A two pole unit? The units that get plugged in are stationary tread mills in a physical rehabilitation center and will not be moved and nothing else will "ever" be plugged in. Can I assume that even with this relatively low current draw (for a 20A breaker) this scenero does not meet code?

In practice, I have not been able to locate a suitable receptacle other than a 20A 220V simplex. I assume in this case, a 15A two pole breaker would satisfy this section of the code.

Thanks again. I appreciate your answers.
 
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mattsilkwood said:
the problem isnt your device, you can put a 50a welder rec on it if you like. but your trying to protect your 15a circuit with a 20a breaker
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Strictly speaking, Matt, You can only plug in a 15A device into a 15A receptacle and a 20A or 15A device into a 20A receptacle. I fully understand that.

The question was if I have a 20A breaker and a 20A receptacle but the unit that plugs in can only draw 9A and nothing else will ever be plugged in, does this satisfy the code? I don't think it does but I'm looking for confirmation.

I feel a 15A breaker with all else being equal will satisfy the code.
 
raydamesek said:
I feel a 15A breaker with all else being equal will satisfy the code.
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I agree, 15A OCPD for your #12's, with all derating applied would satisfy code. As far as I can tell from my chair anyway.
 
I agree with G Thomas, Larry Fine is correct. Using #10 THHN with a 20A breaker would satisfy the code.

I have done more research including talking to two forum experts who both say that using a 15A breaker with #12 THHN would also satisfy the code. This also agrees with K2500's comments.

I appreciate all the answers and the quick responses. Awesome forum! Thank you all.
 
Receptacles

Receptacles

Take a look at table 210.21(B)(2) and (3). Your receptacle rating cannot exceed your circuit ampacity. IMHO your situation would call for #12 wire on a 15A two pole breaker and a 15A 250V receptacle.
 
Lcdrwalker said:
Take a look at table 210.21(B)(2) and (3). Your receptacle rating cannot exceed your circuit ampacity. IMHO your situation would call for #12 wire on a 15A two pole breaker and a 15A 250V receptacle.
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A single receptacle is permitted to have a rating larger than the OCPD protecting it. Since we're discussing 220 volt loads I would assume that he's using single receptacles therefore 210.21(B)(2) and (3) would not apply.
 
infinity said:
A single receptacle is permitted to have a rating larger than the OCPD protecting it. Since we're discussing 220 volt loads I would assume that he's using single receptacles therefore 210.21(B)(2) and (3) would not apply.
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These will be six separate branch circuits with a 220V simplex receptacle on each one. I intend to use six two pole 15A breakers and six 15A simplex receptacles.

I am surprised to learn that I could use six two pole 15A breakers and six 20A simplex receptacles which according to Trevor would still be in compliance with the code.
 
raydamesek said:
These will be six separate branch circuits with a 220V simplex receptacle on each one. I intend to use six two pole 15A breakers and six 15A simplex receptacles.

I am surprised to learn that I could use six two pole 15A breakers and six 20A simplex receptacles which according to Trevor would still be in compliance with the code.
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The receptacle can be greater than the OCP but it cannot be less. Thus you could not have a 20 amp circuit with a single 15 amp recep.


art. 210.21(B) Receptacles.
(1) Single Receptacle on an Individual Branch Circuit. A single receptacle installed on an individual branch circuit shall have an ampere rating not less than that of the branch circuit.
Exception No. 1: A receptacle installed in accordance with 430.81(B).
Exception No. 2: A receptacle installed exclusively for the use of a cord-and-plug-connected arc welder shall be permitted to have an ampere rating not less than the minimum branch-circuit conductor ampacity determined by 630.11(A) for arc welders.
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