Derating & OCPD

[augie47]

I agree with Jim W, if it's a receptacle then the conductor would need to be 20 amp. Other than a fixed load, how do you assure there won't be a 20 amp load on a conductor rated at 17.5 amps.
As far as Code, IMHO, 210.19(A)(1) and (A)(4) required the conductor to have an ampacity sufficient for the load served. With a 20 amp outlet, you can have a 20 amp load.

 

[infinity]

I agree with Jim W, if it's a receptacle then the conductor would need to be 20 amp. Other than a fixed load, how do you assure there won't be a 20 amp load on a conductor rated at 17.5 amps.
As far as Code, IMHO, 210.19(A)(1) and (A)(4) required the conductor to have an ampacity sufficient for the load served. With a 20 amp outlet, you can have a 20 amp load.

Augie, for a single receptacle I can put a 50 amp receptacle on a 20 amp circuit so I disagree with your logic. 210.21(B)(1).

240.4(B) is very specific in that it says "multioutlet branch circuit supplying receptacles" A single receptacle is not a multioutlet so the next size up is permitted. One other thing to keep in mind is that the conductor, a #12 THHN is rated for 30 amps so there is no safety issue with the conductor burning up.

 

[augie47]

I don't disagree with that, but are you saying I can put a 20, or 50 amp, receptacle on a conductor de-rated to 17.5 amps and put that on a 20 amp breaker ?
It would seem to me that 210.19: "(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served."
would not allow that.
Fixed load, I can see, but with a receptacle, the load could be 20 amps.

 

[iwire]

I don't disagree with that, but are you saying I can put a 20, or 50 amp, receptacle on a conductor de-rated to 17.5 amps and put that on a 20 amp breaker ?
It would seem to me that 210.19: "(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served."
would not allow that.
Fixed load, I can see, but with a receptacle, the load could be 20 amps.

A receptacle is not a load.

I agree with you it may not be what was intended or even a good idea but I agree with Rob about the requirements as written.

 

[infinity]

I don't disagree with that, but are you saying I can put a 20, or 50 amp, receptacle on a conductor de-rated to 17.5 amps and put that on a 20 amp breaker ?
It would seem to me that 210.19: "(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served."
would not allow that.
Fixed load, I can see, but with a receptacle, the load could be 20 amps.

But how do you know what the load is on a 50 amp single receptacle on a 20 amp circuit? The way 240.4(B) is written I don't see how it would be prohibited to use the next size up rule.

 

[Jim W in Tampa]

Augie, for a single receptacle I can put a 50 amp receptacle on a 20 amp circuit so I disagree with your logic. 210.21(B)(1).

240.4(B) is very specific in that it says "multioutlet branch circuit supplying receptacles" A single receptacle is not a multioutlet so the next size up is permitted. One other thing to keep in mind is that the conductor, a #12 THHN is rated for 30 amps so there is no safety issue with the conductor burning up.

The 30 amp rating is for derating only and it has been derated to 17.5 amps. You do not now have a wire that can be used for more than 17.5 amps. If this is a SA receptacle circuit it needs be a 20 amp circuit and we no longer have that. A 20 amp single receptacle is assumed it might carry 20 amps and it would accept a 20 amp plug. There might be one loophole in that you could breaker this single at 15 amps and then use a 15 amp receptacle.
To allow a 20 amp breaker defeets the purpose of derating anything.

 

[infinity]

The 30 amp rating is for derating only and it has been derated to 17.5 amps. You do not now have a wire that can be used for more than 17.5 amps. If this is a SA receptacle circuit it needs be a 20 amp circuit and we no longer have that. A 20 amp single receptacle is assumed it might carry 20 amps and it would accept a 20 amp plug. There might be one loophole in that you could breaker this single at 15 amps and then use a 15 amp receptacle.
To allow a 20 amp breaker defeats the purpose of derating anything.

Jim, you can use a 50 amp single receptacle on a 20 amp circuit, do we assume that because it's a 50 amp device that will have to carry 50 amps? If we can't go up to the next standard size why do we need 240.4(B)?

 

[curt swartz]

The 30 amp rating is for derating only and it has been derated to 17.5 amps. You do not now have a wire that can be used for more than 17.5 amps. If this is a SA receptacle circuit it needs be a 20 amp circuit and we no longer have that. A 20 amp single receptacle is assumed it might carry 20 amps and it would accept a 20 amp plug. There might be one loophole in that you could breaker this single at 15 amps and then use a 15 amp receptacle.
To allow a 20 amp breaker defeets the purpose of derating anything.

Jim, why does this have to be a 20 amp circuit? Please provide a code reference.

 

[augie47]

I don't disagree with that, but are you saying I can put a 20, or 50 amp, receptacle on a conductor de-rated to 17.5 amps and put that on a 20 amp breaker ?
It would seem to me that 210.19: "(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served."
would not allow that.
Fixed load, I can see, but with a receptacle, the load could be 20 amps.

I'm stubborn
A 20 amp receptacle will allow a 20 amp load on a 20 amp breaker, a 50 amp receptacle will allow a 20 amp load on a circuit with a 20 amp breaker.
The breaker limits/allows the load. The maximum load could be 20 amps if there is a 20 amp breaker (on a fixed load such as a motor, the load is limited).
If the derated conductor is rated 17.5, 210.19 states maximum load woud be 17.5 so a 20 amp breaker would be prohibited

 

[iwire]

I'm stubborn

Me too! :grin:

If the derated conductor is rated 17.5, 210.19 states maximum load would be 17.5 so a 20 amp breaker would be prohibited

A receptacle is not a load so 210.19 is not violated unless someone plugs in a load that is more than 17.5 amps.

You cannot inspect for the future.

 

[augie47]

but with a receptacle, it's not future. we protect most circuits with receptacles based on the conductor ampacity. we put in 20 amp small appliance circuits with receptacles expecting a load up to 20 amps.

 

[augie47]

bob, to me "inspecting for the futurea" is a "what if"... what if someone changes out this piece of equipment, etc.
with recepacles, we install specifc wiring and receptacles "anticipating" the load.. designed for the maximum allowed anticipated load
that to me is not a "future" load

 

[iwire]

How is that any different from me installing a 30 amp fusible switch tapped of a 100 amp feeder with 14 AWG and 15 amp fuses?

The switch is desinged for 30 amp fuses and someone might install them.

 

[iwire]

but with a receptacle, it's not future. we protect most circuits with receptacles based on the conductor ampacity. we put in 20 amp small appliance circuits with receptacles expecting a load up to 20 amps.

210.19

"(1) General. Branch-circuit conductors shall have an ampacity not less than the maximum load to be served."

It says load, you cannot just decide that today a receptacle is a load. It is a device and by definition it is not a load as it does not use current.

 

[augie47]

How is that any different from me installing a 30 amp fusible switch tapped of a 100 amp feeder with 14 AWG and 15 amp fuses?

The switch is desinged for 30 amp fuses and someone might install them.

To me a difference. A layperson purchasing an appliance with a 20 amp plug would normally assume the 20 amp receptacle with a 20 amp breaker is designed to accept that load.
Upsizing a fuse, should, to most people ring a bell as "altering" a circuit.
Every homeowner is likely to plug in to an existing receptacle without thinking an electrician should be consulted, not so with upsizing fuses.
(I know it happens but we try to educate folks not to upsize fuses, we don't educate them to determine conductor ampacity)'

we may just have to agree to disagree since we are both stubborn

 

[Jim W in Tampa]

You would put in rejection clips on that 30 amp disco to stop that fuse from being increased.
Your giving the user a receptacle that has no load and able to receive a 20 amp load by simply using it. In this case your not likely to find anything that actually uses even 18 amps with a 20 amp cord as it likely would be a 30 amp plug. Our problem is it is not safe to plug in a load that is greater than 17.5 amps into this outlet. How will we stop this from happening ? Will the extra 2.5 amps cause a problem, not likely but it is possable. Does every counter receptacle (commercial) need to be a 20 ? If not then problem solved with 15 breaker, 15 receptacle and #12 wire

 

[infinity]

So what code section would you use to violate a #12 THHN conductor (with an adjusted ampacity of 17.5 amps) on a 20 amp OCPD with a 20 amp single receptacle? Before you answer take a look at 240.4(B) again and tell me how the above scenario does not meet the three conditions required to use the next standard size OCPD:

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.

 

[iwire]

we may just have to agree to disagree since we are both stubborn

We can and I don't entirely disagree with your thoughts of the 'layperson' but how about the fact a receptacle is a device and the NEC defines a device as something that does not use current. That being the case a receptacle cannot not called a load and 210.19 only applies to loads not devices. The requirements for devices are in different sections of 210.

 

[augie47]

So what code section would you use to violate a #12 THHN conductor (with an adjusted ampacity of 17.5 amps) on a 20 amp OCPD with a 20 amp single receptacle? Before you answer take a look at 240.4(B) again and tell me how the above scenario does not meet the three conditions required to use the next standard size OCPD:

Rob, To me Art 240 addresses overcurrent protection and Art 210 addresses the circuit. To me, 210.19 still requires the conductor to have an ampacity adequate for the load. ONCE you have selected that conductor, then you go to Art 240 and select the OCP device. Art 240 allows us to use that 20 amp breaker on any load (in this situation) up to 17.5 amps.
If we took Art 240 as the only limiting factor then we could put a 900 kcmil (ampacity = 520) on a 590 amp load and a 600 amp breaker (next size OCP) ignoring the 1000 and 1250 kcmil.
So, my thought is comply with both Art 210 for conductor selection and Art 240 for OCP.

 

[augie47]

We can and I don't entirely disagree with your thoughts of the 'layperson' but how about the fact a receptacle is a device and the NEC defines a device as something that does not use current. That being the case a receptacle cannot not called a load and 210.19 only applies to loads not devices. The requirements for devices are in different sections of 210.

I'm sorry but when I read that it supports my thinking. My concern is protecting the conductor. As long as the device is rated per 210.21 it's covered. My concern is that, by the receptacle being in the circuit, the breaker is the only limit to the end utilization equipment which can in this case exceed the ampacity of the conductor thus putting it in violation of 210.10.