switchgear

[sleepy]

friends i have question?
we have an existing site, we have a transformer next to the building that is rated at 29k aic
the switchgear inside the building is rated at 65k, all the outgoing panels are rated at
65k aic, we adding a 400amp breaker in switchgear and panels, the electrician wants
to put in 42kaic rated equipment. i say no
any help out there
thanks

 

[zog]

friends i have question?
we have an existing site, we have a transformer next to the building that is rated at 29k aic
the switchgear inside the building is rated at 65k, all the outgoing panels are rated at
65k aic, we adding a 400amp breaker in switchgear and panels, the electrician wants
to put in 42kaic rated equipment. i say no
any help out there
thanks

What is your available fault current?

 

[charlie b]

Here's the key:

. . . we have a transformer next to the building that is rated at 29k aic

That is not a precisely worded statement. Are you saying that the available fault current, the most that that transformer is capable of putting out, is 29K? If so, then any equipment located downstream (at least until you get to another transformer, at which point the game starts over) need be rated no higher than 29K. So if I have correctly interpreted your situation, I would not object to a 42K rated breaker.

 

[mayanees]

include motor contribution...

include motor contribution...

Here's the key:That is not a precisely worded statement. Are you saying that the available fault current, the most that that transformer is capable of putting out, is 29K? If so, then any equipment located downstream (at least until you get to another transformer, at which point the game starts over) need be rated no higher than 29K. So if I have correctly interpreted your situation, I would not object to a 42K rated breaker.

The rating of the switchgear would need to include the 29kA from the transformer, plus spinning load contribution from any motors.
That can safely be approximated at 25% of the transformer contribution, which yields a total switchgear fault current of 36,250 amps - still satisfied by the 42kA breaker.

The only other caution I would make is to ensure that the 65kA gear wasn't put in for a reason - like the transformer is old and will eventually be replaced with a larger unit that will need the 65kA rating.

John M

 

[weressl]

The rating of the switchgear would need to include the 29kA from the transformer, plus spinning load contribution from any motors.
That can safely be approximated at 25% of the transformer contribution, which yields a total switchgear fault current of 36,250 amps - still satisfied by the 42kA breaker.

The only other caution I would make is to ensure that the 65kA gear wasn't put in for a reason - like the transformer is old and will eventually be replaced with a larger unit that will need the 65kA rating.

John M

Yes and no. The load growth is not necessarily given. I see it not to materialize more often that I do not. Transformer lasts much longer than switchegear.