Available Fault Current and Short Circuit Interrupt Rating

R

Rotato

Member
Location
Indiana
Occupation
Controls Engineering Technician
So, my company purchased a wax melting machine today and they want to integrate it into a machine that they want built "in-house". Problem is, the people that do the planning and buying have knee-jerk reactions to just buying whatever and then handing it off and telling someone to "make it work".

My first observation is the nameplate on the wax melter states that it has only 1,500A of Interrupt capacity. It is 230VAC single phase and needs 24Amps. Our transformers out on the floor give us 120/208 are 75KVA. So, without looking at the transformer impedance value I am just going to go ahead and assume that we are significantly higher than that (probably 5kA - 6kA).

What are my options at this point.
Do I have to install another transformer 7.5KVA based on the load requirements? 24A * 230V * 1.25(Safety Factor) = 6900W
Can I use Current Limiting Fuses? Or would this fall in line with needing "Engineering Supervision" (licensed PE)?
Does NEC 110.22 and 240.86 apply here?

Any advice would be appreciated. Thank you.
 
IMHO.... Since the nameplate of the equipment has 1,500 amps of interrupting capacity, The circuit feeding the equipment should have a ISC of 1500 amps or less. I would consider a lower KVA transformer for the situation. You could also use Eatons FC2 Fault current calculator to establish a SCC value.
 
It will not take very much #10 conductor to drop several kA of fault current. You should use one of the on-line calculators to determine your worst case fault current at your control panel terminals.

Will the wax melting machine function correctly with the reduced wattage you will have supplying it with only 208V or will you need to boost the voltage up to 230V? Using a standard 7.5kVA isolation transformer will reduce your available fault current significantly., howevera buck-boost isolation transformer will have little effect.
 
I hopped onto the EATON's F2C and punched some numbers in. I had no idea that conductor runs dropped it that much . I am blown away😮. I just threw a 5% impedance into the calculator for the transformer.

So infinite bus theory 75kVA, 208VAC, 3ph, 5%, with -10% error gave me 4,627A
Added 10AWG, 60', 1 conductor per phase, 3 conductor cable, Copper, brought it down to 1,379A

I have to say that changed my perspective on things. I will definitely do some more research on this. Thank you for pointing me in this direction.
 
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