20 amp circuits

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Hwire

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How many 20 amp circuits may be in a conduit on 20 amp breakers before it is necessary to derate the breaker?
 
roger

roger

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Two if they are both Multiwire Branch Circuits.

Roger
 
roger

roger

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You should probably re-word your question to something along the lines of "how many current carrying conductors may be in a conduit ..."

Roger
 
roger

roger

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480sparky said:
How does one derate a breaker?

Welcome to the forum!
Click to expand...

I read right over the "derate a breaker" Doh!

Roger
 
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realolman

Senior Member
Hwire said:
How many 20 amp circuits may be in a conduit on 20 amp breakers before it is necessary to derate the breaker?
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You don't have to derate the breaker ...you have to derate the conductors if there are more than three current carrying conductors in the raceway or cable 310.15 B 2.

As Pierre said... with #12 it doesn't make much difference until you get up above 9

20 / 30 =.66

from Table 310.15 B 2 a: 9 # 12 conductors derate @ 30 x .7
 
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dnem

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realolman said:
You don't have to derate the breaker ...you have to derate the conductors if there are more than three current carrying conductors in the raceway or cable 310.15 B 2.
Click to expand...

Yep, derate the conductor not the breaker. . And derate based on number of ccc, current carrying conductors not total number of conductors.

480sparky said:
In order to derate any conductor, we must first know what type of insulation is covering it.
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True, but it's safe to assume 90?C conductor insulation.
Also assume 75?C lugs. . For SquareD, Seimens, Eaton, GE they rate even their small amp breaker lugs 75?C, "listed and marked otherwise", 110.14(C)(1).
Let's also assume that there's no issue with 240.4(B)(1) [cord + plug branch circuit] and you can use the "one up" allowance.

With those assumptions, if you put some time into running scenarios of the interaction between 240.4(B), 240.4(D), 240.6(A), T310.15(B)(2)(a), + T310.16 you find out the following:

14gauge can stay on 15amp breakers up to 30ccc.
12gauge can stay on 20amp breakers up to 9ccc.
10gauge can stay on 30amp breakers up to 9ccc.
8gauge can stay on 50amp breakers [except 334.80 + 338.10(B)(4)(a)] up to 3ccc.
8, 6, 4, 3, 2, 1gauge, 4/0, 250, 350, 500, 600 can all stay on their 75?C ampacity up to 3ccc.
1/0, 2/0, 3/0, 300, 400 can all stay on their 75?C ampacity up to 6ccc.

And of course all this changes when you're dealing with aluminum conductors.
 
iwire

iwire

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dnem said:
14gauge can stay on 15amp breakers up to 30 ccc.
Click to expand...

You may want to check that.

14 AWG CU @ 90 C = 25 amps

30 CCCs gets a .45 hit. resulting in 11.25 amp conductors.

IMO 14 gauge can stay on 15 amp breakers up to 9 CCCs.
 
don_resqcapt19

don_resqcapt19

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The issue of the overcurrent protection of the derated wire hinges on its use. As long as the adjusted ampacity is equal to or greater than the load and the load does not consist of multiple outlets for cord and plug connected loads you can round up to the next standard size OCPD.
 
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dnem

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post #11
iwire said:
You may want to check that.

14 AWG CU @ 90 C = 25 amps

30 CCCs gets a .45 hit. resulting in 11.25 amp conductors.

IMO 14 gauge can stay on 15 amp breakers up to 9 CCCs.
Click to expand...

post #12
don_resqcapt19 said:
The issue of the overcurrent protection of the derated wire hinges on its use. As long as the adjusted ampacity is equal to or greater than the load and the load does not consist of multiple outlets for cord and plug connected loads you can round up to the next standard size OCPD.
Click to expand...

post #9
dnem said:
..... Let's also assume that there's no issue with 240.4(B)(1) [cord + plug branch circuit] and you can use the "one up" allowance. .....
Click to expand...

240.6(A) Standard sizes 1, 3, 6, 10, 15, 20, 25 .....
You figure it 3 ways and take the lowest of the 3.
1] wire, start at 90?C and adjust for cccs and ambient
2] lug, almost always 75?C
3] raceway, usually only a 60?C issue on NM or SE [334.80 + 338.10(B)(4)(a)]

As long as the lowest of those 3 ampacities makes it as high as 10.1amps, then the "one up" allowance lets you stay on 15amp breakers.

PS
. .I know that assuming that there is no issue with 240.4(B)(1) [cord + plug branch circuit] is a risky assumption for 14 + 12gauge. . For cord + plug branch circuits, I agree with you that you have to limit 14gauge to 9 so that your ampacity doesn't drop below 15.

Maybe I shouldn't be making so many assumptions on a public message board where people will surf thru and say, "Hey, I can put 30ccc in a pipe that are 14gauge on 15amp breakers whenever I want to !"
 
iwire

iwire

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dnem said:
Maybe I shouldn't be making so many assumptions on a public message board where people will surf thru and say, "Hey, I can put 30ccc in a pipe that are 14gauge on 15amp breakers whenever I want to !"
Click to expand...

I think that is a wise thought.
 
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Steve G

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It is entirely possible to derate the breaker. It would be replacing the 20 amp breaker with a 15. With 9 current carrying conductors your 20 amp breaker with #12 THHN is good for 21 amps. As soon as you get to 10 current carrying conductors your #12 THHN is only good for 18 amps. Your options are to increase the wire size or DERATE THE BREAKER SIZE.
 
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Steve G

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I think my figure may be slightly incorrect as I am not looking at my code book. I think the percentage from 9 to 10 CCC goes from 70 to 50 percent which would be 21 amps to 15 amps for #12 THHN. I am pretty sure that is correct and it would change my previous post from 18 amps to 15 but the end result would be the same. Increase wire size to #10 or decrease breaker size to 15 amps. Sorry for the confusion and so many posts.
 
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dnem

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Steve G said:
It is entirely possible to derate the breaker. It would be replacing the 20 amp breaker with a 15. With 9 current carrying conductors your 20 amp breaker with #12 THHN is good for 21 amps. As soon as you get to 10 current carrying conductors your #12 THHN is only good for 18 amps. Your options are to increase the wire size or DERATE THE BREAKER SIZE.
Click to expand...

You can certainly change a breaker to another size but you're not derating the breaker. . The 20amp breaker is still good for 20amps, it has not been derated. . If you change it to a 15amp, then the 20amp has been taken out, not derated. . The 20amp will continue to be 20amp and the 15amp will continue to be 15amp.

The conductors which were originally rated for a certain amperage, let's say 12gauge THHN, 90?C, 30amps. . Because of the number of cccs in a raceway or because of ambient temp, you derate the wire. . The very same wire is still there but now it's good for a lesser amount of amperage.

The amperage for the same item changed. . That's derating.

If you really want to talk about derating a breaker rather than changing out a breaker, you have to shift subjects and talk about continuous load. . UL breaker marking guide item #36 under 100% continuous rated says, "Unless otherwise marked for continuous use at 100% of its current rating, a circuit breaker is intended for use at no more than 80% of its rated current where in normal operation the load will continue for 3 hours or more.". . Now that's derating a breaker because it's the same breaker but now its only good for 80% because your load is continuous.

But on this thread we haven't brought up continuous load. . We're only talking about derating wire and/or changing the breaker.
 
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