Receptacle Short Circuit Rating

[ron]

I have a 120/208V panelboard with a calculated 23,000A of 3 phase bolted fault current.
There is a receptacle installed next to and fed by the panelboard with 5' of #12 copper conductor.
The calculated fault current at the receptacle is approx 10kA.

Does anyone know what the withstand (short circuit) rating of a 15A receptacle is? Where can I find it?

 

[steve66]

I've always assumed (maybe incorrectly) that receptacles, light fixtures, and other utilization equipment is protected by the circuit breaker feeding them.

Steve

 

[G._S._Ohm]

Almost everything has an I squared T rating.
A 15 A receptacle can withstand 15 A for T = "infinity" and more current than that for less time. It can probably stand 400 A for 16 milliseconds without shortening its life too much, which implies a rating of 2600 (A^2)-seconds.
If the upstream breaker has an I squared T rating of substantially less than that of the receptacle, you're OK, in principle.

You can use this link
http://home.earthlink.net/~jimlux/hv/fuses.htm
to calculate the rating for copper wire and, with some difficulty, receptacles.
At least this will get you in the ballpark.

BTW, 120 V/23,000 A = a source impedance of 5 milliohms and the #12 adds 16 mOhms so I get a fault current of 120/0.021 = 5700 A, but this is just an approximation.

 

[ron]

This happens to be a 208V, 3 phase 15A receptacle.
Does the NEC responsibility for adequate short circuit ratings end before the utilization device cord?

 

[steve66]

The only requirements I know of are 110.9:

"Equiment intended to interrupt current at fault levels shall have an interrupting rating sufficient...."

And I believe I've heard there is a similar requirement for industrial control panels, and I know HVAC equipment often has maximum short current ratings.

But a receptacle is not intended to interrupt fault current levels, only normal operating current levels.

A receptacle may have an I^2*T rating, but I don't see any requirement for a receptacle to survive a short circuit. Hopefully, it doesn't explode and become shrap metal. But I don't think it has to be usable after a short circuit.

Maybe the manufacturers instructions limit the fault current that the receptacle can be connected to, or maybe the UL listing does. But I don't think the NEC does.

 

[G._S._Ohm]

Leviton says they have no idea [!!!] what will happen to a resi. receptacle when it passes thousands of amps.
I'm waiting on NEMA's reply.

 

[jim dungar]

The only requirements I know of are 110.9...

Don't forget 110.10, which deals with equipment being able to withstand the fault current.

 

[ron]

Where does 110.10 stop? Do we have to also find the short circuit rating of motors, modular furniture and light fixtures?

 

[dbuckley]

A bit of googling showed that Rockwell have some DIN rail mount sockets that look like a standard 5-15, and it has a 10KA rating, link here.

I seem to recall from my UK days that there was an assumption that a bolted fault would not occur in the first (I think) 1.5m of branch circuit, and that the 1.5m of branch circuit condutor would be an effective control on the maximum possible prospective short circuit current. Of course, having a socket hanging out the bottom of the panel made it a lot closer than 1.5m! I gather these days you would have to calculate it...

 

[steve66]

A bit of googling showed that Rockwell have some DIN rail mount sockets that look like a standard 5-15, and it has a 10KA rating, link here.

I seem to recall from my UK days that there was an assumption that a bolted fault would not occur in the first (I think) 1.5m of branch circuit, and that the 1.5m of branch circuit condutor would be an effective control on the maximum possible prospective short circuit current. Of course, having a socket hanging out the bottom of the panel made it a lot closer than 1.5m! I gather these days you would have to calculate it...

Yes, but those are designed for use in an industrial control panel. A typical receptacle is not.

110.10 says listed equipment installed in accordance with that listing meets the requirement. If the manufacturers instructions on a receptacle don't list a maximum fault current, and the UL listing doesn't specify a maxiumum fault current, I think you are good to go.

 

[hotblueelectric]

recptacls

recptacls

I

 

[G._S._Ohm]

NEMA says 'no interrupting current rating'.

Assuming an outlet is sort of made up of #14 copper wire, it will melt in 16 mS at 5000 A, = 400,000 (A^2)-seconds, so a protecting upstream breaker would have to trip at less than 5000 A within one cycle at 60 Hz.
At 600 A it'd take one second.

 

[richwaskowitz]

10 kA

10 kA

UL 508A Supplement SB (an allowable means for determining the SCCR of an industrial control panel) allows a default rating of 10 kA for UL listed non-GFCI receptacle outlets if not listed with a higher rating.

 

[G._S._Ohm]

This brings up an interesting point.
You have a 15 A receptacle with a bolted short connected with #14 copper.
Either the receptacle will melt and protect the wire or the wire will melt and protect the receptacle.
I guess I'd want option #1 and I'd live with the wire insulation being somewhat more brittle than it was before.

 

[ron]

If the 10kA fault occurred in a receptacle back box, or in a panelboard, we essentially get the same result.
If the code requires an adequate rating then we are stuck and have to run longer wire. #12 knocks down the current pretty fast.

I think Steve66 is the closest IMHO to showing that a rating for the receptacle is not required. That's really what I want, as then I will be asked for the SCCR for condensate pumps, and light fixtures, and .....

 

[jim dungar]

I think Steve66 is the closest IMHO to showing that a rating for the receptacle is not required. That's really what I want, as then I will be asked for the SCCR for condensate pumps, and light fixtures, and .....

A receptacle must withstand fault current flowing through it until a protective device operates, so the wiring on its load side may be considered.

Your condensate pump and light fixtures are loads/end use equipment, fault current does not flow through them. In fact they are typically involved in the fault.

A protective device needs to be rated for the current available on its line side (not the current flowing through it) because of the potential for an internal fault.

 

[steve066]

A receptacle must withstand fault current flowing through it until a protective device operates, so the wiring on its load side may be considered.

Your condensate pump and light fixtures are loads/end use equipment, fault current does not flow through them. In fact they are typically involved in the fault.

A protective device needs to be rated for the current available on its line side (not the current flowing through it) because of the potential for an internal fault.

After reading 110.10, i would agree that is the geneal intent. However, it also says equipment installed in accordianance with its listing is considered to meet the requirements. I searched through the UL white book for fault current, and mostly got hits for protective devices, and switches, and (suprising to me) power distribution blocks.

I also looked through the listing info. for receptacles, and thre was one IEEE standard referenced that I don't have access to. So maybe that standard lists a maximum fault current for a receptacle. But if not, than I think the receptacle automatically complies with 110.10. After all, we wouldn't have any other legal way to determine the withstand rating of the receptacle.

Also consider that a fault flowing through a receptacle is also flowing through a line cord. That's probably something like #18 or #16 wire. (I'm guessing here). I'm thinking that wire would burn in half before much damage happend to the receptacle. Maybe the receptacle manufacturers have even designed the receptacle to withstand more current than the typical #14 or #12 supply wire its connected too.