Unusual Electrical Load
Unusual Electrical Load
(OP)
I have a furnace type tool that is being designed. It's electrical load profile is 480 volts, 3 phase, 200 amps constant load, but when the tool process is in use, the current spikes to 840 amps for 6 seconds out of every minute. This process lasts maybe an hour or two, then the tool would be back to 200 amps for another hour, then process begins again. Is there any way the breaker or the cable could be sized smaller than the 840 amps you would normally size for?






RE: Unusual Electrical Load
Breakers and cables usually can take six seconds overload on a reglar basis, provided the duty factor isn't too high. At 10 % duty cycle, it would probably not be necessary to use anything 'thicker' than what you would use for the steady state load. One size up is probably sufficient.
Many motor loads with high inertia have the same characteristics with higher current peaks than you have. But, it depends on what code you have to obey.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Unusual Electrical Load
Will a cable rated for 300 amps be protected within code by a 300A breaker? Yes.
Check the breaker trip curves. Also look at the IEEE Brown Book on cable protection where it describes short term overloads.
Since it is a furnace load, you may have to upsize the cable for ambient temperature derating anyway.
RE: Unusual Electrical Load
You should also look at the reset time to be sure you don't ratchet into a trip.
RE: Unusual Electrical Load
RE: Unusual Electrical Load
Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.
RE: Unusual Electrical Load
You've already got some comments about the breaker. I'd have to look at an actual breaker curve to see what the trip levels are.
RE: Unusual Electrical Load
With 840 A 6 seconds and 200 A for 54 seconds you will have 326 A thermal load for cable and also for the breaker.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Unusual Electrical Load
The ETAP program used to have a cable heating module that predicted operating temperatures from varying loads. When we ran intermittent repetitive loads through it, the cable temperature ratcheted up and stabilized at the same temperature as when we ran the equivalent I2t continuous amps through it.
Still, use a minimum of 125%-140% margin and check for ambient temperature derating. If you use parallel cables, add an extra 5-10% derate for unequal load sharing.