Derating & OCPD

[dana1028]

Scenario: after derating, some #12 THHN conductors have an ampacity rating of 17.5A

What is the maximum OCPD permitted?

No, this is not a test question. My understanding is the conductors cannot be protected above their [derated] ampacity...which would be a 15A breaker.

But I thought I read a comment here from someone [it's been awhile] indicating you could use the 'next higher breaker' rule [240.4(B)] and thus put a 20A breaker on these conductors.

Which is it? Thanx

 

[480sparky]

Scenario: after derating, some #12 THHN conductors have an ampacity rating of 17.5A

What is the maximum OCPD permitted?

No, this is not a test question. My understanding is the conductors cannot be protected above their [derated] ampacity...which would be a 15A breaker.

But I thought I read a comment here from someone [it's been awhile] indicating you could use the 'next higher breaker' rule [240.4(B)] and thus put a 20A breaker on these conductors.

Which is it? Thanx

Only if all three conditions of 240.4(B) (1) - (3) are met. In most cases, (1) will not be met.

 

[augie47]

In addition to the 240.4(B) rules, remember, the conductor must be rated to carry the load.
If the load on the #12 s (derated) is greater than 17.5 amps, then you can't use the 20.

 

[dana1028]

Thanx, that's my take also.

It's been a long time so I don't remember the exact response to the 'next higher breaker' comment...but it's something that has stuck with me and I finally decided I wanted to get that clear.

 

[infinity]

So what is the load type and ampacity?

 

[Cold Fusion]

In addition to the 240.4(B) rules, remember, the conductor must be rated to carry the load.
If the load on the #12 s (derated) is greater than 17.5 amps, then you can't use the 20.

Augie -
I must have misunderstood your response. If the load is greater than 17.5A, then you can't use the derated #12 - doesn't have any thing to do with the 20A cb.

cf

 

[Cold Fusion]

Only if all three conditions of 240.4(B) (1) - (3) are met. In most cases, (1) will not be met.

My first response was, "huh?". But then I realized I don't do residential* and do little commercial and the only #12 would be lighting or small motors.

So I would say, "Of course you can use the next size up CB (a 20A) as long as the circuit is not a multi-outlet circuit for cord and plug connected portable loads.

cf

 

[infinity]

So what is the load type and ampacity?

My first response was, "huh?". But then I realized I don't do residential* and do little commercial and the only #12 would be lighting or small motors.

So I would say, "Of course you can use the next size up CB (a 20A) as long as the circuit is not a multi-outlet circuit for cord and plug connected portable loads.

cf

Yes that's correct. If your connected load is less than or equal to the final adjusted conductor ampacity of 17.5 amps and you're not feeding multi-outlet circuit for cord and plug connected portable loads then you can go up to the next standard sized OCPD.

 

[augie47]

ColdFusion, Rob expressed what I was trying so say far better than I (not unusual ).
In that the OP was asking about breaker sizes, I was trying to inpart,
without reagard to the mutlioutlet circuit, if he had a condcutor amapcity of 17.5, if his calculated load was above that, he could not use the "next larger size" overcurrent device (20).

 

[dana1028]

So what is the load type and ampacity?

Commercial kitchen - counter loads....I don't think any loads exceeded the 17.5 A rating.

 

[yired29]

Commercial kitchen - counter loads....I don't think any loads exceeded the 17.5 A rating.

Is that not a multioutlet circuit supplying receptacles? If so the wire would need a ampacity of 20 amps after derating.

 

[dana1028]

Is that not a multioutlet circuit supplying receptacles? If so the wire would need a ampacity of 20 amps after derating.

I agree...just double checking my little gray cells.

 

[Jim W in Tampa]

Commercial kitchen - counter loads....I don't think any loads exceeded the 17.5 A rating.

They are required to be 20 amps and if more than 1 on circuit could easily be 20 or even more for short period before breaker trips. If that is where your doing this then not only can't you use a 20 amp breaker but you can't use the 15 either. Up the wire to #10 or add another raceway. Even if load is under 17.5 and its a single receptacle it still is violation because a larger appliance might get plugged in.

 

[infinity]

They are required to be 20 amps and if more than 1 on circuit could easily be 20 or even more for short period before breaker trips. If that is where your doing this then not only can't you use a 20 amp breaker but you can't use the 15 either. Up the wire to #10 or add another raceway. Even if load is under 17.5 and its a single receptacle it still is violation because a larger appliance might get plugged in.

Jim are you saying that a less than 17.5 amp load on a single receptacle and on a 20 amp circuit is a violation?

 

[Jim W in Tampa]

Jim are you saying that a less than 17.5 amp load on a single receptacle and on a 20 amp circuit is a violation?

No not at all. I am saying he needs a 20 amp circuit and he does not have it with a 15 amp breaker. He also can not say its ok to breaker this after derated 17.5 amp circuit with a 20 amp breaker if it is a receptacle because we do not have a fixed load. And if its a duplex receptacle it could easily see 2 appliances that total over 20 amps.

 

[infinity]

No not at all. I am saying he needs a 20 amp circuit and he does not have it with a 15 amp breaker. He also can not say its ok to breaker this after derated 17.5 amp circuit with a 20 amp breaker if it is a receptacle because we do not have a fixed load. And if its a duplex receptacle it could easily see 2 appliances that total over 20 amps.

OK, so we agree that one single receptacle would be OK?

 

[dana1028]

OK, so we agree that one single receptacle would be OK?

One single 20A receptacle on a 20A circuit?

 

[Jim W in Tampa]

OK, so we agree that one single receptacle would be OK?

No , if it is a single it would need be a 20 amp single receptacle. And if we have that we must have a 20 amp breaker and he can not do that on wires that been derated to 17.5 amps. Only loop hole i see is to install disconnect and hard wire this appliance. Matters not if it is 1 amp or 17.5 . This load is not fixed because it is cord and plug.

 

[iwire]

No , if it is a single it would need be a 20 amp single receptacle. And if we have that we must have a 20 amp breaker and he can not do that on wires that been derated to 17.5 amps. Only loop hole i see is to install disconnect and hard wire this appliance. Matters not if it is 1 amp or 17.5 . This load is not fixed because it is cord and plug.

Jim, please cite a code section to back that up.

 

[infinity]

OK, so we agree that one single receptacle would be OK?

No , if it is a single it would need be a 20 amp single receptacle. And if we have that we must have a 20 amp breaker and he can not do that on wires that been derated to 17.5 amps. Only loop hole i see is to install disconnect and hard wire this appliance. Matters not if it is 1 amp or 17.5 . This load is not fixed because it is cord and plug.

Here's the relevant code section. Where does the load not being fixed fit in with a single receptacle?

240.4(B) Devices Rated 800 Amperes or Less. The next higher standard overcurrent device rating (above the ampacity of the conductors being protected) shall be permitted to be used, provided all of the following conditions are met:
(1) The conductors being protected are not part of a multioutlet branch circuit supplying receptacles for cord-and-plug-connected portable loads.
(2) The ampacity of the conductors does not correspond with the standard ampere rating of a fuse or a circuit breaker without overload trip adjustments above its rating (but that shall be permitted to have other trip or rating adjustments).
(3) The next higher standard rating selected does not exceed 800 amperes.