UPS SCCR

Status
Not open for further replies.
D

DAB41162

Member
Location
Mason, OH.
Occupation
Electrical design and Controls Engineer
Hello,

I recently ordered a UPS that doesn't have an internal OCPD and doesn't have an SCCR rating on its label, spec sheet, or operation manual.

After speaking with a manufacturer representative; they told me that the rating of the UPS is set by the external OCPD then went on to say
the rating is 1kA. How can this be? Wouldn't this mean it couldn't be plugged into a receptacle supplied by a standard breaker (10kA)?

Are there any codes that could be cited to justify the claim about using the rating of the external OCPD?

Thanks...
 
Is it required to have a short circuit current rating?

Lots of things that plug into receptacles don't have short circuit current ratings.

Do motors have short circuit current ratings?
 
DAB41162 said:
After speaking with a manufacturer representative; they told me that the rating of the UPS is set by the external OCPD then went on to say
the rating is 1kA. How can this be? Wouldn't this mean it couldn't be plugged into a receptacle supplied by a standard breaker (10kA)?
Click to expand...
The AIC rating of a protective device has no bearing on the amount of fault current at a downstream location.

I would not be surprised that the bolted fault current at a typical 15A or 20A branch circuit receptacle is always less than 1kA.
 
According to my research, per NEC, new requirements call for an SCCR rating for control panels. In my case, the UPS is fed from a branch circuit. This makes me think that to establish an SCCR for the control panel assembly, the UPS needs to be included in this determination. Attached is a whitepaper that makes me think this way.

I understand that something like a lamp or TV doesn't have an SCCR rating, however, if they have a UL marking, then they are tested using approved breakers in a typical residential breaker panel. (I would say.) That is to say they may not work after a trip, but are unlikely to blow up or start a fire.

Motors don't have an SCCR, but the devices that control them do and in an industrial environment, their control circuits (i.e. breakers, starters) do.

Please folks, I'm not trying to portray myself as having years of experience or a vast amount of knowledge so I may be wrong or misinformed. But that's why I'm here asking questions.

Thank you
 

Attachments

  • SCCR Whitepaper.pdf
    789.3 KB · Views: 3
NEC 110.10 has required short circuit ratings for installed equipment for a very long time. If it is plug-in style, as mentioned by Jim, the available short circuit current is probably very low and is not directly related to the upstream breaker's AIC rating.

110.10 Circuit Impedance, Short-Circuit Current Ratings, and Other Characteristics.
The overcurrent protective devices, the total impedance, the equipment short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit protective devices used to clear a fault to do so without extensive damage to the electrical equipment of the circuit. This fault shall be assumed to be either between two or more of the circuit conductors or be any circuit conductor and the equipment grounding conductor(s) permitted in 250.118. Listed equipment applied in accordance with their listing shall be considered to meet the requirements of this section.
 
jim dungar said:
The AIC rating of a protective device has no bearing on the amount of fault current at a downstream location.

I would not be surprised that the bolted fault current at a typical 15A or 20A branch circuit receptacle is always less than 1kA.
Click to expand...
I'm using a thermal magnetic breaker that magnetically trips (Ii rating) at 1450A. Doesn't that mean that the UPS could see more than 1kA, all be it for only a cycle or two?

Basically, how can I determine the SCCR of the branch without knowing the device's SCCR? I'm working with UL508A Supplement SB and examples to determine the SCCR of the entire control panel. It will be awesome if someone could help me out of this conundrum.

Thank you...
 
ron said:
NEC 110.10 has required short circuit ratings for installed equipment for a very long time. If it is plug-in style, as mentioned by Jim, the available short circuit current is probably very low and is not directly related to the upstream breaker's AIC rating.

110.10 Circuit Impedance, Short-Circuit Current Ratings, and Other Characteristics.
The overcurrent protective devices, the total impedance, the equipment short-circuit current ratings, and other characteristics of the circuit to be protected shall be selected and coordinated to permit the circuit protective devices used to clear a fault to do so without extensive damage to the electrical equipment of the circuit. This fault shall be assumed to be either between two or more of the circuit conductors or be any circuit conductor and the equipment grounding conductor(s) permitted in 250.118. Listed equipment applied in accordance with their listing shall be considered to meet the requirements of this section.
Click to expand...
Thank you Ron. I've read this, however, could you break it down a little? 110.10 appears a little convoluted for me to try to explain to someone else how I came up with a value.

Thank you...
 
zbang said:
How big is this UPS? I wouldn't expect a plug-in (like maybe less than a few KVA) to be SCCR rated. OTOH I would expect any UPS to have an OCPD somewhere.
Click to expand...
The UPS is 2700kW with a 208V input and does not have an internal OCPD according to the information on the datasheet and manufacturer's representative.
 
DAB41162 said:
I'm using a thermal magnetic breaker that magnetically trips (Ii rating) at 1450A. Doesn't that mean that the UPS could see more than 1kA, all be it for only a cycle or two?
Click to expand...
Maybe, maybe not.

The fault current available at the breaker line side terminals is not the same fault current that is available at the end of a branch circuit. The impedance of the conductors does a great job of limiting current. In fact it is not unusual to find 15 and 20A branch circuit protective devices never operating in their magnetic/instantaneous regions.
 
DAB41162 said:
Thank you Ron. I've read this, however, could you break it down a little? 110.10 appears a little convoluted for me to try to explain to someone else how I came up with a value.

Thank you...
Click to expand...
It means that all things electrical have to be rated to withstand the available fault current in the environment installed (adequate short circuit rating).
 
DAB41162 said:
Yes. 2700W
Click to expand...
2,700W at 208V single phase is a 20 ampere branch circuit. The small wire is a very high impedance and will reduce the available fault current to 1kA pretty fast (short run), which is why the small UPSs like this likely have no SCCR since it is irrelevant because you don't have enough fault current to make the evaluation worth it.
 
jim dungar said:
Maybe, maybe not.

The fault current available at the breaker line side terminals is not the same fault current that is available at the end of a branch circuit. The impedance of the conductors does a great job of limiting current. In fact it is not unusual to find 15 and 20A branch circuit protective devices never operating in their magnetic/instantaneous regions.
Click to expand...
Thank you
 
ron said:
2,700W at 208V single phase is a 20 ampere branch circuit. The small wire is a very high impedance and will reduce the available fault current to 1kA pretty fast (short run), which is why the small UPSs like this likely have no SCCR since it is irrelevant because you don't have enough fault current to make the evaluation worth it.
Click to expand...
Thank you
 
ron said:
It means that all things electrical have to be rated to withstand the available fault current in the environment installed (adequate short circuit rating).
Click to expand...
I don't see anything in 110.10 that says that Ron. I read it as saying if something has a SCCR, you need to design the circuit to not exceed it. I don't see anything saying equipment must have a SCCR.
 
electrofelon said:
I don't see anything in 110.10 that says that Ron. I read it as saying if something has a SCCR, you need to design the circuit to not exceed it. I don't see anything saying equipment must have a SCCR.
Click to expand...
My editorial summary as requested was what I said. This is the important piece for your question: the equipment short-circuit current ratings ...... shall be selected
 
110.10 also says this:

Listed equipment applied in accordance with their listing shall be
considered to meet the requirements of this section.
Click to expand...

That seems to suggest quite strongly that is the equipment does not have a SCCR you can just apply the instructions and you are code compliant.

In any case, something that is plugged in is not typically considered part of the premises wiring system and thus not really covered by the code.
 
As for UL 508a, uninterruptible power supplies are mostly used in control circuits and thus are not required to have a SCCR. This is not always the case, so you have to be careful.
 
Status
Not open for further replies.
Top