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01-24-2009, 03:51 PM   #1
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## Derating

Hi guys,

I was just reading the post by jamie and I got to thinking....

For example when you are going to install general use circuits do you use (for a 15 amp breaker) 15 amps for the load calculation or would you use the 80% rule which would give you 12 amps for the circuit?

I'm guessing the 80% rule is for protection/avoidance of nuisance tripping of the breaker. I was always told it is best not to load up a circuit more than 80% of the breaker setting. If there is such a thing as the 80% rule where can you find it in the code book?

This is what I'm getting at: I want to install five 15 amp circuits using 14 AWG THHN in 1/2" EMT. Using Table 310.16 - 90 degree column 14 AWG is rated at 25 amps. If I derate 25 amps by 50% based on Table 310.15(B)(2)(a) that would give me 12.5 amps per conductor.

So now the conductors are rated at 12.5 amps and based on the 80% rule the circuit shouldn't be loaded more than 12 amps.

That being said can I safely utilize 15 amp breakers or would I be asking for trouble?

I hope that I didn't just confuse the heck out of you guys.

01-24-2009, 04:03 PM   #2
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The 80% rule comes from the requirement that a circuit be sized to 125% of the load. So, if a circuit is 125% bigger than the load, that must mean the load is only 80% of the circuit size.

A circuit must be sized to handle the load before any adjustment factors are applied. So, if you have a 13 A load, and after derating you determine the conductor ampacity is 12 A, then you would be in violation. But you can still use a 15 A breaker on conductors derated below 15 A, because we are allowed to use the next size up for anything under 800 A.

 01-24-2009, 04:17 PM #3 Member   Join Date: Dec 2008 Location: Southern Wisconsin Posts: 171 Rewards Points: 150 For general use circuits where there is no specific load what would be considered an acceptable load for calculations? 15 amp breaker = 12 amps?

 01-24-2009, 05:41 PM #4 Member   Join Date: Dec 2008 Location: Southern Wisconsin Posts: 171 Rewards Points: 150 You said it was okay to use the next size breaker but 240.4(B)(1) says it is not okay if the conductors being protected are part of a multioutlet branch circuit supplying receptacles for cord-and-plug-connected portable loads. If I had a box with a duplex receptacle installed in it would this be considered multioutlet? Or do you need more than one box with a receptacle(s)?
01-24-2009, 07:45 PM   #5
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Quote:
 Originally Posted by Steelhead You said it was okay to use the next size breaker but 240.4(B)(1) says it is not okay if the conductors being protected are part of a multioutlet branch circuit supplying receptacles for cord-and-plug-connected portable loads. If I had a box with a duplex receptacle installed in it would this be considered multioutlet? Or do you need more than one box with a receptacle(s)?
Dammit! It got me again. I forget that very same detail when I explained this to Jamie! You are right. Can't upsize the breaker if you derate below that. But you can and must upsize the wire.

And in residential settings, there is no limit to the number of receptacles allowed on a 15 or 20 A branch circuit. But in commercial calculations, we figure general receptacles at 180 VA.

Can't believe it stuck me again...

01-24-2009, 08:02 PM   #6
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Quote:
 Originally Posted by InPhase277 Dammit! It got me again. I forget that very same detail when I explained this to Jamie! You are right. Can't upsize the breaker if you derate below that. But you can and must upsize the wire. And in residential settings, there is no limit to the number of receptacles allowed on a 15 or 20 A branch circuit. But in commercial calculations, we figure general receptacles at 180 VA. Can't believe it stuck me again...
Hey, it just goes to show that we're all human.

Thanks for all the help!

01-24-2009, 09:00 PM   #7

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Quote:
 I'm guessing the 80% rule is for protection/avoidance of nuisance tripping of the breaker. I was always told it is best not to load up a circuit more than 80% of the breaker setting. If there is such a thing as the 80% rule where can you find it in the code book?
210.23 (A)(1) 80% rule for branch circuits supplying multi outlet branch circuits for cord and plug equipment.

There is also an 80% rule for a branch circuits supplying continuous loads.

The lowest protective device (circuit breaker) for residential branch circuits is 15 amps. Branch circuit conductors supplying more than one receptacle for cord and plug equipment shall have an ampacity of not less than the branch circuit rating.

Branch circuit rating = circuit breaker size = 15 amps minimum = conductors must have a minimum 15 amps of ampacity. 14 awg thhn = 25 amps I cannot derate those conductors to less than 15 amps if I'm serving multiple receptacles.

Be careful with drating conductors for example Thhn may use the 90C column but most thhn is dual rated as thwn. Thwn is a 75C wire so if I'm using dual rated wire thhn/thwn underground in conduit in a wet location I must derate using the THWN 75C column as my starting point. An often overlooked table is 310.13(A) this table gives you the insulation ratings of most electical wires that are not listed in 310.16. So in summary you derate according to the insulation rating of the wire in the application your using it.
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 01-25-2009, 01:31 PM #8 Member   Join Date: Dec 2008 Location: Southern Wisconsin Posts: 171 Rewards Points: 150 Thanks Stubbie for the info. You and InPhase were very helpful.
 01-25-2009, 01:44 PM #9 Member   Join Date: Jan 2008 Location: Oregon Posts: 116 Rewards Points: 75 A few options here: 1. Get rid of one circuit bringing your circuit total down to 4. 2. Break them up into 2 conduits. 3. Run 3/4 emt and use #12's instead. 4. Use MWBC's and reduce your CCC's down to 6.
01-25-2009, 01:52 PM   #10
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Quote:
 Originally Posted by Cow A few options here: 1. Get rid of one circuit bringing your circuit total down to 4. 2. Break them up into 2 conduits. 3. Run 3/4 emt and use #12's instead. 4. Use MWBC's and reduce your CCC's down to 6.

Ya, I also thought about this, but I was just making sure that I understood exactly what I could and could not do. Sometimes you have few options depending on the situation and I just needed some clarification. I do appreciate your help though.

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