Sizing OCPD

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Designer69

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hey guys when sizing an OCPD you do it with regards to the branch load not the power conductors right? (I may have this backwards).

for instance, I have a 3ph 25 horse @480 motor which draws 34A and I want to use a NEMA 2 starter with 50A thermal magnetic ckt breaker for protection. However I want to use #8 wire @90 deg C which is good for 55A.

that doesn't mean I now need to upsize my breaker to be over 55A does it? or that I would have to downsize my wire to #10 which is good for around 38A.

Thanks
 
Motor branch circuit conductors need to be sized to carry the full load currents from the motor tables in accordance with 430.22 through 25. The motor short circuit and ground fault protection needs to be sized in accordance with 430.52.

So for the typical inverse time breaker used for short circuit and ground fault protection this device can be sized up to 250% of the full load current from the motor tables.

Chris
 
had you not said motor, I'd say my basic... determine the load, choose
your conductor, choose your overcurrent device..
then you will have to forgive me, but for "designer" the balance of your question somewhat leads me aghast.
First, overcurrent protection" might be a bit of a misnomer..are we speaking overload protection or short-circuit/ground fault protection.
You state you are using a starter with a breaker. Normally the overload protection is handled by the starter overload device and the breaker is the short-circuit/ground fault potective device.
Your conductors would need to have a 42.5 amp amapcity, which most #8 conductors would handle, but the 90? rating probably should even be considered at this point.
If your breaker is being used a ground-fault/short circuit protection, then a 50 might be too small to handle in-rush. NEC would allow an 80 amp ( + or - ) SC/GF protetcive device, but in the case of motors, your #8 would still be complliant under normal conditions..
 
Yes, you are thinking backwards.

A 50A breaker can feed a 100A wire, but not vice versa.

Some people dig though the properties of the wire to minimize it and I believe you can for motors (the wire is rated LESS than the breaker). I try to keep it simple and the same.

Longer story, motor is 34A full load, * 1.25 = 42.5A, upgrade to nearest standard, 50A over current protection is fine. I typically use nameplate or manufacturer's recommendations, but if not available, I use 125% of FLA rounded up.
 
Guys,
I like to check 430.22, 430.52, and 430.72 each time.
That's my standard method.
Write down the nameplate HP, FLA, LRA, etc., then refer to these sections.
Figure the OL, figure the conductor size, figure the OCPD size, figure the fuse size.
Then review what I've done.
 
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