short circuit current

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nafis

Senior Member
Location
Palestine,tx
hi everybody
if you have two different loads ( motors and lights ) having the same voltage and wattage .

would a short circuit current be the same or
would be (higher/less) in one than the other and why ? Where in the code book addresses this issue?

thanks for your help in advance
 
zog

zog

Senior Member
Location
Charlotte, NC
The source determines the available short circuit current, however during a S/C motors contribute to S/C current so all other things equal in you question, the answer is motors, if that makes ant sense.
 
mivey

mivey

Senior Member
The motor contributes to the fault because it has energy that is stored in the rotational movement, like in a flywheel. As the fault occurs, the spinning motor will be dragged down. As it is dragged down, the rotational energy is converted back into electrical energy and is pumped into the fault along with the "normal" energy coming from the source.

Not a very scientific explanation but gets the gist of it.

I'm not sure about a code reference.
 
quogueelectric

quogueelectric

Senior Member
Location
new york
mivey said:
The motor contributes to the fault because it has energy that is stored in the rotational movement, like in a flywheel. As the fault occurs, the spinning motor will be dragged down. As it is dragged down, the rotational energy is converted back into electrical energy and is pumped into the fault along with the "normal" energy coming from the source.

Not a very scientific explanation but gets the gist of it.

I'm not sure about a code reference.
Click to expand...
Like the motor becomes a generator.
 
charlie

charlie

Senior Member
Location
Indianapolis
Motor Contribution

Motor Contribution

240.86(C) Motor Contribution. Series ratings shall not be used where
(1) Motors are connected on the load side of the higher-rated overcurrent device and on the line side of the lower-rated overcurrent device, and
(2) The sum of the motor full-load currents exceeds 1 percent of the interrupting rating of the lower-rated circuit breaker.
​
From the NEC Handbook:
One critical requirement limits the use of series rated systems in which motors are connected between the line-side (protecting) device and the load-side (protected) circuit breaker. Section 240.86(C) requires that series ratings developed under the parameters of either 240.86(A) or (B) are not to be used where the sum of motor full-load currents exceeds 1 percent of the interrupting rating of the load-side (protected) circuit breaker . . .
​
This is the only place in the Code that I could find a reference to the motor contribution. :smile:
 
mivey

mivey

Senior Member
coulter said:
Sounds like a like from Shaggy in my kid's "Scooby Doo" cartoons. Who's "Kemal"?

carl
Click to expand...
You got it. Kemal Amin "Casey" Kasem (yes, that one): the voice of Norville "Shaggy" Rogers
 
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