short circuit calc

[gnu_mike]

I need to do a short circuit analysis for a new hotel, and all the examples I've seen side step the issue of addressing fully the procedure WITH motors. This includes the Cooper Bussmann pdf book EPR-1, which in Ex. 1 of the problems lists "Motor Contribution*=", and asks you to assume that 50% of the load comes from motors. None of the problem solutions address motor contribution.
So what is the procedure?
At the transformer secondary we add all motor FLA's and multiply by five and add this to the transformer short circuit amps?
How do you account for the X/R ratio? Do you need to?
The comment I got back from the plan reviewer states "... show the calculated short circuit amps at all protective devices downstream from the main protective device and that all protective devices are rated to safely interrupt the available short circuit current."
I wonder what " main protective device" they mean? The switch ahead of the panelboard? The OCPD in the panelboard?
The reviewer listed code section 240-83(c) as requiring this. This seems a stretch when you read that section.

Thanks,
Mike

 

[rbalex]

Re: short circuit calc

240.83(C) is a stretch but 110.9 and 110.10 are on point.

In general, IEEE 141(Red Book)would have you ignore all motors 50hp and below for interrupting purposes and, in absence of specific motor data, use roughly 6 X FLA for motor contributions above 50hp. For most small systems that's adequate for momentary too.

I've found for "quick and dirty" calcs, 4 X transformer secondary rated Amps gives a good first approximation for motor contribution that allows for reasonable future expansion.

[ June 02, 2004, 05:03 PM: Message edited by: rbalex ]

 

[gnu_mike]

Re: short circuit calc

Thanks, Bob. Looks like I'll have to buck up and spring for the Red Book. $140!

-Mike

 

[bob]

Re: short circuit calc

I've found for "quick and dirty" calcs, 4 X transformer secondary rated Amps gives a good first approximation for motor contribution that allows for reasonable future expansion.

Bob
I do not understand this method. If you have a transformer that has an FLA of 1000 amps then the motor contribution is 4000 amps. I don't see a relationship here.

 

[rbalex]

Re: short circuit calc

Well, I did say it was "quick and dirty."

Basically, it's an experience thing. If motors 50hp and below can be ignored,as IEEE 141 recommends, using 4 X transformer secondary Amps for motor short-circuit contribution for initially selecting interrupting duties will usually cover most current and reasonable future additions. This is not the only contribution - transformer through-fault is also a consideration.