a few questions on motors/continuous load

[JohnDS]

hey guys,just wondering if someone can refresh my memory on these two subjects and help me apply the calculations, it has been a while.

motor nameplate rating 11a.

.80x15a ciruit=12a therefore 11a does not exceed 80% of the circuit ampacity, so is this correct a 15a circuit would be fine?

now if the motor will be running for 3 hours or more, would this be correct?

11ax1.25=13.75a So since this now exceeds the 12a(80%of15a), would i go with a 20amp?

i am basically confused on whether to use the 80% rule or the 125% rule. from what i understand, the 80% rule is for motors, and the 125% rule is for continuous load?

 

[Dennis Alwon]

I think you are double dipping. First off I am assuming this is not an a/c type motor where the nameplate has the 125% built in. All other motors should use T.430.250 so you need to know the HP.

Once you get the amp. from the Table then you would multiply by 1.25 or 125% for he branch circuit selection.

I don't believe you need to do it again fora motor running 3 hours or more.

 

[david luchini]

motor nameplate rating 11a.

.80x15a ciruit=12a therefore 11a does not exceed 80% of the circuit ampacity, so is this correct a 15a circuit would be fine?

You are missing an important factor in your motor circuit. Dennis is correct that you should be using the horsepower of the motor and the tables in Art 430, rather than the nameplate current.

But there are two circuit determinations to be made for a motor circuit: the conductor size and the overcurrent protection device size. Assuming the 11Amps is based on a motor such as a 3HP, 200V motor (table 430.250) then the conductors size must not have an ampacity of less than 125% of that full-load current. 11x1.25=13.75. #14awg conductors would meet this ampacity.

For the overcurrent device, an inverse time c/b can be sized to 250% of the full load per table 430.52. 11x1.25=27.5A. The exception no. 1 to 430.52(C)(1) allows the next standard amperage rating, so you can use a 30A c/b.

So for your 11A motor, #14awg conductors from a 30A c/b would be allowed. There is no need to consider 3 hour running time.