15amp circuit - electrical load?


  #1  
Old 07-15-05, 08:42 PM
lordsoth0
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15amp circuit - electrical load?

Hi all,

I am finishing my garage that my father-in-law started a couple of years ago. He put 3 outlet boxes in the garage and has the wiring of these new outlets going to the 15 amp ceiling outlet that the garage door opener is plugged into. I have checked what else is on the ceiling outlet's circuit and I found that there is 795 watts of indoor and outdoor lights, and the ceiling outlet which just has the .5HP garage door opener. I come up with 373 watts for the garage door opener which makes for a grand total of 1168 watts.

My question is if I should put these 3 new outlets on this circuit? If I plug my 7 amp (840 watts) shop-vac into a new outlet and all the lights are on I will be over the safe capacity of the circuit. Am I correct in my thinking? I would put the new outlets on a new circuit but I would have to run a new cable from the crawlspace in the attic down to the basement.

Any help or comments would be greatly appreciated. Thanks.

Jason
 
  #2  
Old 07-15-05, 10:46 PM
R
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My question is if I should put these 3 new outlets on this circuit? If I plug my 7 amp (840 watts) shop-vac into a new outlet and all the lights are on I will be over the safe capacity of the circuit. Am I correct in my thinking?
The short answer is .....yes. Your thinking is mostly right with a few misconceptions. In the physics world 1 hp = 746 watts, one half of that is 373 watts...in the motor world this aint the case. But thats a long story.

There is an article section in the National Electrical code that says in a nutshell that one motor on a 15 or 20 amp circuit cannot exceed in amps 50% of that 15 or 20 amp branch circuit and still have other loads on that branch circuit.

This .5 hp garage door opener is really more in the neighborhood of 9 amps FLC. The NEC table 430.148 says its 9.8 amps FLC (full load current) and that is the figure we would use to size the circuit for this 1/2 hp garage door motor. That has to be multiplied by 125% to size the ampacity of the conductors serving the motor, but lets dont go off in that direction too far.

Simply 9.8 x 1.25 = 12.25 amps and it is on a circuit rated at 15 amps. So you can see that we are well past 50% just with the motor on that circuit. So technically and according to code that is all that should be on that circuit. Just that one motor.

So lets look at the loads that may be on that circuit as you described them.
1.) 9.8 amp motor
2.) 795 watts of lights (roughly 7 amps)
3.) 7 amp vacuum motor (high start torque too boot)

Thats 9.8 + 7 + 7 = 23.8 amps without including the 125% of the required increase on the garage door opener motor which is the largest motor.

So thats 23.8 amps on a 15 amp rated circuit. The deal here IMO is that the vacuum and lights and garage door motor usually wont be all running at the same time and more than likely most of the time things would be ok. It takes a while for a circuit breaker to trip sometimes on overload and the garage door opener motor may stop running before that occurs...the tripping of the CB, and take that load off the circuit. But as you asked if they were all on at the same time you get into problems with overloading the circuit.

So in my opinion you should find another circuit to put these three outlets on.

Hope this makes sense to you.
 

Last edited by Roger; 07-15-05 at 11:46 PM.
  #3  
Old 07-17-05, 05:30 PM
lordsoth0
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Roger,

Thanks for the very informative response. I will find another circuit for these outlets or run a new line. Thanks again.

Jason
 
 

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