We have this ongoing debate at work and get different answers. It concerns proper breaker rating for derated wires. I had an inspector tell me that if my #10wires were derated to 16amps I could not protect those wires with a 20amp CB. I said we were using the next size up rule since there are no 16amp breakers available. He said that if those circuits were to be put to where 18amps of load were being used then these wires would eventually heat up potentially creating a hazard. He made us change the 20amp CB's to 15amps and passed it. Is there anywhere in the code that states that you can not over protect a wire and you must use the next lower sized breaker?
Breaker sizing derated wires
- Thread starter Hoodood
- Start date
- Status
- Location
- Connecticut
- Occupation
- Engineer
It depends on whether the conductors are part of a branch circuit supplying more than one receptacle...see 240.4(B)(1).
If yes, then no you can't use a 20A CB.
If no, then yes you can use the next size up as long as the load is 16A or less.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
You could use the next size up rule for more than single receptacle circuits. As David stated look at 240.4(B)(1). In your example was the load 18 amps?
Thanks for the 240 reference. It should create quite the discussion tomorrow. A number of our foreman have been putting up to 40 CC's in 2 1/2" conduit. I have been installing only 20 in 1 1/2" conduit. They all say their's passes everytime using the next breaker size up rule. I always try to cover my bases especially after having to change out breakers before. I tell them it only takes 1 time and you never know when that will come! Thanks again!
- Location
- New Jersey
- Occupation
- Journeyman Electrician
What is the load on each circuit and what is it feeding?
Thanks for the 240 reference. There is no known capacity rating since the circuits are of multiple duplex receptacles. So, 240.4 (B) (1) says no but 240.4(B) (3) says yes but 240.4 states ALL must be met. My assumption is that this circumstances does not meet code. It should create quite the discussion tomorrow. A number of our foreman have been putting up to 40 CC's in 2 1/2" conduit. I have been installing only 20 in 1 1/2" conduit. They all say their's passes everytime using the next breaker size up rule. I always try to cover my bases especially after having to change out breakers before. I tell them it only takes 1 time and you never know when that will come! Thanks again!
Unknown capacity
Unknown capacity
As Ive stated it's a multiple Receptacle circuit so the capacity is unknown. That's why the inspector failed it to begin with. Although (B) (3) allows under 800amps to use the next highest size (B) (1) states multiple receptacles do not apply. So, seeing that not all the conditions under 240.4 have not been met I must use the lower sized breaker (15amps). One, on top of his game, inspector unlike those whom have let this pass!
Unknown capacity
As Ive stated it's a multiple Receptacle circuit so the capacity is unknown. That's why the inspector failed it to begin with. Although (B) (3) allows under 800amps to use the next highest size (B) (1) states multiple receptacles do not apply. So, seeing that not all the conditions under 240.4 have not been met I must use the lower sized breaker (15amps). One, on top of his game, inspector unlike those whom have let this pass!
Cow
Senior Member
- Location
- Eastern Oregon
- Occupation
- Electrician
Are you able to use 310.15(A)2 & Exc?
- Location
- New Jersey
- Occupation
- Journeyman Electrician
You can use the exception for derating also. Probably doesn't apply to your installation unless you're only talking about a small section in a long run of the raceway.
Since you have multi-receptacle circuits you cannot use the next size up rule. inspectors overlooking it does not make it code complaint.
- Location
- Tennessee NEC:2017
- Occupation
- Semi-Retired Electrician
If you're only putting 20 CCC in the conduit then it should derate to 20A and the 20A breaker would be acceptable.
from the table, 10-20 conductors adjustment is 50%
50% of #10 @ 40A = 20A
This would be if you only had to adjust for the number of CCC and not temperature adj.
- Status