Data plate continuous amperage

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Electron_Sam78

Electron_Sam78

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Palm Bay, FL
Hello all,
I have a 400 Hz generator that has an output of "72 amps continuous" as actually labeled on the data plate. My question is should I install wire/OCPD for 90 amps (72 x 1.25) or is this the max continuous amperage already calculated on the data plate (57.6 x 1.25 = 72)?
 
Electron_Sam78 said:
Hello all,
I have a 400 Hz generator that has an output of "72 amps continuous" as actually labeled on the data plate. My question is should I install wire/OCPD for 90 amps (72 x 1.25) or is this the max continuous amperage already calculated on the data plate (57.6 x 1.25 = 72)?
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Your conductor and breaker need load calculated at 100% of non continuous load plus 125% of continuous load as minimum. There is no additional 125% to factor into what the generator can deliver, it can continuously deliver 72 amps. That means if all the connected load is continuous and is 72 amps, the generator can handle it - but when selecting conductor/breaker you must add another 25% unless it is a 100% rated breaker.
 
I looked at the info on the unit. It's a 20 kw with 3 phase 208 volt output. So my math works out 55.6 amps and that times 1.25 is 69.5. That's close enough for me to say that the 72 is the engineered allowance for 20000 watts x 1.25ish. So if all load(s) is continuous I would wire and protect for 72 amps. So I'm thinking I need fast-acting 80 amp fuses and #4 THHN copper. The loads are all electronics testing receptacles for avionics repair lab no motors or anything with high starting current.
 
to convert from kW to kVA you need to consider power factor also, that may be why your "close enough" figure isn't so exact.
 
I'm sure you're right. I was just rough calculating. I just assume theirs was a more precise calculation based on engineered testing and material.

Do you agree with my OCPD of 80 amp fast acting fuses?
 
I recommend basing the conductor & OCPD selection on the current the loads will consume, not the current the generator is capable of delivering.

If you use a circuit breaker, make sure it's rated for 400 Hz. A solenoid will have a substantially higher impedance at the higher frequency and might not develop enough magnetic force to actuate the mechanism at the desired current.

Depending on the prime mover, overcurrent protection for the generator may be unnecessary. If it's driven by a 60-Hz electric motor, (that's not oversized) the protection in the motor control center should suffice, or the motor overload settings can be calibrated to the generator's overload/short-circuit capacity. If it's driven by an engine, (again, that's not oversized) the engine will exhaust its torque rise and the speed will drop, possibly reducing the current to a safe level even if the generator output is shorted.
 
Electron_Sam78 said:
Hello all,
I have a 400 Hz generator that has an output of "72 amps continuous" as actually labeled on the data plate. My question is should I install wire/OCPD for 90 amps (72 x 1.25) or is this the max continuous amperage already calculated on the data plate (57.6 x 1.25 = 72)?
Click to expand...
In my experience it's common for generators to be rated for 0.8 PF.
20kW or roughly 25kVA
 
Note that 72*1.25 is exactly 90 which is the next standard size. It shouldn't be 80. Also Google and find an application guide for naval ships (or whatever it happens to be) using 400hz because this will change the trip time of the breaker and may be require derating. Not sure of the effects on fuses but likely will change things too.

Sent from my SM-N920V using Tapatalk
 
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