Motor Contribution to AIC

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dcsva@aol.com

Senior Member
Location
Virginia
I have been studying AIC calculations and came across a statement in the "Electrical Plan Review"
(that I got off the web)that said...
"A practical estimate of motor short circuit contribution is to multiply the total motor FLA (in amps) by 4. Values of four to six are commonly accepted."

I have found that I have many more things to learn about AIC, but I was wondering about the validity of this statement...Commonly accepted by whom?

[ October 27, 2003, 03:06 PM: Message edited by: dcsva@aol.com ]
 

nosparks

Member
Location
Massachusetts
Re: Motor Contribution to AIC

I do not have an answer to your question. However maybe you can help me with the address of the website that you reference. I have recently put out for bid a proposal for a short circuit coordination study, and I too am constantly learning. Thanks in advance for your help.
 

dcsva@aol.com

Senior Member
Location
Virginia
Re: Motor Contribution to AIC

thanks awwt,

Nosparks,
I can't find the web site...I printed it out and forgot to save the site.Sorry
The Bussmann site from AWWT is very similiar.
 

gwz2

Senior Member
Location
Indiana
Re: Motor Contribution to AIC

Not sure of the exact dates - - -
In 1984 Bussmann used the total of all motor FLA with a multiplier of 6.

About 1990 they used a multiplier of 5.

In 1996 and since, they've used a multiplier of 4, unless it was recently changed.

On a installation with a lot of large motors, the Available Short Circuit Current ( ASCC ) does get a big boost from the motor contribution.

Be sure to look at NEC 240.86(B).

gwz2
 
Re: Motor Contribution to AIC

dcsva,

Disclaimer - I am not an expert in this field, but only have a layman's understanding.

In simple terms, when you lose voltage to a rotating induction motor, it becomes an induction generator. Since you do not have an exciting voltage, the motor contribution rapidly decays, reaching zero over a few cyles. The rate of decay depends on a number of motor design parameters, including the rotor subtransient reactance.

Also, the location of the fault enters into the analysis since the circuit impedance (including cables and transformers) will reduce the fault current. Unless you are talking about large motors above 200 horsepower, the effective contributions wind up being pretty small.

The whole issue of short circuit analysis is complex, and usually involves the engineers.
 
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