Short Circuit Rating on Panelboard

[paul75041]

We are looking at a older GE Panelboard 480volt 3phase 4wire system. Type CCB. The Nameplate states the following. "Short Circuit Current Rating 35,000 RMS Symmetrical Amperes 480volt Max." We are going to install a 35,000 AIC rated breaker in the panelboard. The building Engineer says it takes a 100,000 rated circuit breakers. Engineer says 35,000 is The RMS value not the AIC rating. The question is, "What is the Difference between RMS and AIC."

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[ November 22, 2004, 07:45 PM: Message edited by: charlie b ]

 

[ron]

Re: Short Circuit Rating on Panelboard

The short circuit current rating is 35KA and so a 35kAIC breaker is acceptable.
Although 35kAIC seems like an oddball rating, I haven't heard of 35k but it is possible.

 

[charlie b]

Re: Short Circuit Rating on Panelboard

?RMS? stands for ?Root Mean Square.? It is a means of describing the magnitude of an AC voltage or an AC current, in terms of an equivalent DC component. Almost every number you see will have already been converted to the RMS value. 120 volts, 208 volts, 480 volts, 15 amp branch circuits, 20 amp branch circuits: all of these are RMS values.

?AIC? stands for ?Ampere Interrupting Capacity.? It is a means of describing the amount of fault current that a breaker can withstand, and still be able to perform its function of opening (thus terminating the fault). If you have a breaker rated for a 35,000 AIC, then the 35,000 amps is an RMS value. If you have a breaker rated for a 100,000 AIC, then the 100,000 amps is also an RMS value.

The only requirement is that the amount of fault current that is available at the location of the breaker must not be higher than the AIC rating of the breaker. For a new construction, someone has to determine the amount of fault current available at the location of every panel. But if you have an existing panel, and it is rated for 35,000 amps, then you don?t need to install a breaker with a fault current rating any higher than 35,000 amps.

 

[charlie]

Re: Short Circuit Rating on Panelboard

The short circuit current rating is indeed 35 kAIC. If the available fault current is 100,000 amperes, the panelboard isn't going to cut it.

RMS is Root Mean Squared and is the way we measure AC. AIC is Amperes Interrupting Capacity or how much current an overcurrent device can interrupt under fault conditions.

 

[poweringtech]

Re: Short Circuit Rating on Panelboard

Are short circuit ratings rated in RMS or in peak to peak?

 

[bphgravity]

Re: Short Circuit Rating on Panelboard

Check out this booklet, everything is in RMS: Bussman Short Circuit Booklet

 

[poweringtech]

Re: Short Circuit Rating on Panelboard

Thanks bryan for the link info.

well appreciated

poweringtech

 

[wirenut1980]

Re: Short Circuit Rating on Panelboard

I have seen equipment giving nameplate information on the AIC ratings in symmetrical or asymmetrical. The way I understand it, the symmetrical rating is the AC component of the fault current, and the asymmetrical rating the AC and the DC components of the fault current added together. What if one has calculated the available symmetrical AIC rating, but the equipment rating is given in asymmetrical, what is the way to derive the symmetrical rating from the asymetrical rating given by the manufacturer?

 

[jim dungar]

Re: Short Circuit Rating on Panelboard

All UL ratings for 600V equipment will be for symmetrical fault current (actually RMS currents).

The only time I have seen assymetrical ratings was for medium voltage equipment and for protective relay applications (basically utility stuff).

 

[bob]

Re: Short Circuit Rating on Panelboard

Jim
I thought the MFG rating was now the Ampere Interrupting Capacity. The symmetrical fault current and the Assymetrical fault current were rated differently.

 

[flip]

Re: Short Circuit Rating on Panelboard

OK, so the old panel states 35K.

I guess you are installing a breaker in the panel or replacing the Main breaker.

If it is the main breaker, has the service itself changed in such a way that it may have more fault current available?? Thus being the reason the engineer has asked for larger??????