Electrician Talk banner
1 - 7 of 7 Posts

·
Registered
Joined
·
15 Posts
Discussion Starter · #1 ·
I was hoping to find someone here that wouldnt mind sharing some info on the relationship between the short circuit current rating and interrupter rating of overcurrent devices and equipment. I looked at the definition of Interrupting rating in the codebook and am a little unsure if I understand it correctly. For example, If I am installing a service for a residence, does this mean the main breaker in my panel has to have an interrupter rating equal to or higher than the short circuit current rating provided from the utility ?And does that mean the maximium current that can be provided to a service before the utility side trips out? I try not to be too lazy and learn this stuff on my own but since I've found this site I have learned alot from reading the posts and seem to understand things better from the people here at the forum. Any info or links will be greatly appreciated.
Thanks in advance
 

·
Wyome
Joined
·
757 Posts
i'm not sure what you meant with the 2nd question, but I can answer the first. Yes your service equipment and service disconnect have to be rated for the short circuit available from the utility. That all depends on transformer size, wire size, and distance from the transformer. Poco here will give us that calculation. Makes it way easier. A lot of service equipment is now rated for 22,000 aic. We seldom need more than that, but there have been occasions when we have had to buy special breakers or fuses.
 

·
Registered
Joined
·
15 Posts
Discussion Starter · #3 · (Edited)
i'm not sure what you meant with the 2nd question, but I can answer the first. Yes your service equipment and service disconnect have to be rated for the short circuit available from the utility. That all depends on transformer size, wire size, and distance from the transformer. Poco here will give us that calculation. Makes it way easier. A lot of service equipment is now rated for 22,000 aic. We seldom need more than that, but there have been occasions when we have had to buy special breakers or fuses.
Thank you for the reply,
What I meant in the second question was, if the utility side was to reach their max short circuit current available will their fuses or protection trip out? Also I noticed on alot of the breakers we use there is a 10,000 aic rating, so would this mean it would be a violation to use them for branch circuits in a main panel if the utility had 20,000 short circuit current rating?
Again, Thank you very much for the reply, I appreciate all the info.
 

·
Ax grinder
Joined
·
2,053 Posts
The breakers and other electrical equipment must be rated for the maximum avaliable short circuit current, throughout the system. So you need to do a short circuit caluclation for the entire system to verify that the equipment installed is capable of withstanding the avaliable short circuit current.

The utility side of the transformer will trip when the primary current reaches the trip threshold, but this will allow enough current to flow on the secondary side to destroy equipment that is not rated for the avaliable short circuit current.

Sections 110.9 and 110.10 of the NEC gives us the requirments for AIC rating of breakers and SCC ratings of equipment.

Chris
 

·
Registered
Joined
·
15 Posts
Discussion Starter · #5 ·
te12co2w and Raider1,
Thank you both for the info, I feel I have a fairly clear understanding now on this subject. Sometimes when I have questions like this the higher ups at our company give a very brief explanation or none at all and say thats just how it is, but I need and want to learn these things for my own piece of mind to know Im doing code compliant and safe installations.
thanks again
 

·
Wyome
Joined
·
757 Posts
You're welcome. We are all happy to be of any help we can. This is a pretty good forum and usually there are very knowledgeable folks there to help with most any situation you can find yourself in. Sometimes the reply might not be immediate but there is almost always someone around to help.
 
1 - 7 of 7 Posts
Top