AIC Calculations

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MA_PE

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I just started using my PE and I have two questions, one regarding feeders and the second regarding AIC Calculations. I have been given a set of drawings and an existing feeder, 3 phase conductors size 2 awg are to be reused. The feeders are going to feed a Computer Room Air Conditioning unit. There will be a fused disconnect prior to the CRAC. I have 2 questions, they are as follows:

1. The CRAC will only draw approximately 70 amps. Since the existing conductors are #2 awg I assume that these feeders are protected by a circuit breaker which is much greater than 70 amps. At first I thought that maybe we would need to downsize the circuit breaker but since we are going to supply a fused disconnect at the CRAC (sized for the equipment based on manufacturers specs) then the existing breaker should be OK. Is this correct?

2. In order to determine the AIC rating of the fuses in the fused disconnect I will need to know the following:

a. AIC rating of the circuit breakers on the line side of the CRAC, meaning the feeder breaker and the main breaker in the distribution panel.

b. Impedance of the transformer feeding the distribution panel.

c. KVA Rating of the transformer feeding the distribution panel.

d. Secondary voltage of the transformer feeding the distribution panel.

e. Approximate conductor length feeding the CRAC.

Any help would be appreciated.

Thanks
 
Are you planning to use a series rated combination of your new fused switch and the breakers in the CRAC?
 
That is a good question. I have been in electrical controls for a long time and recently took a part-time position where I would be using my PE. I also have my electrical license so I am relying on a lot of the knowledge I gained as an electrician. Right now I am working on relatively small projects so I am doing soem research to ensure that I understand exactly what I am doing and that what I am doing is correct. What would be the advantage of useing a series rated combination?
 
MA_PE said:
That is a good question. I have been in electrical controls for a long time and recently took a part-time position where I would be using my PE. I also have my electrical license so I am relying on a lot of the knowledge I gained as an electrician. Right now I am working on relatively small projects so I am doing soem research to ensure that I understand exactly what I am doing and that what I am doing is correct. What would be the advantage of useing a series rated combination?
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Advanatage is cheaper cost of the breakers due to the fact that a smaller AIC rating is required on some of the breakers. The disadvantage is more difficult to design, and I believe the series rated protection system needs to be listed together in order to be accepted.
 
MA PE

#1 correct

#2 If you are short on time, fully rate the new equipment, then catch up with

your homework. Not a bad idea to fully rate on a large AC unit like this, even

more so if on a short run. jmo.
 
Assuming limited motor contribution (e.g. short circuit contribution) from the low voltage side of the transformer, simply take the KVA of the transformer and divide by the impedance of the transformer. This will give you your short circuit KVA.

Then divide by your voltage and 1.73 (if it's 3 phase) and you will have your maximum (practically speaking) available short circuit current.

Make sure your equipment is rated for no less than this value. If it is close, further detail may be needed to ascertain what's going on.
 
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