Motor Question

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A/A Fuel GTX

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WI & AZ
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Electrician
I need a little clarification here. I have a 3 phase 5hp 208v motor. According to 430.250, the FLA is 16.7a. The conductors shall have an ampacity of 20.88a per 430.22 which would put me up to #10awg. The OCPD could be as high as 40a per 430.52. I am assuming from this that it would be legal to put #10's on a 40 a breaker for this motor circuit. I realize that 430.52 is a maximum value so would it be ok to use a 30a breaker if I chose to in this situation?
 

stars13bars2

Senior Member
Yes you are correct in that motor applications come from the motor tables, and are not limited to other breaker tables such as the ones associated with table 310.16. You can install a 40 amp breaker on the #10 wire, and without knowing any of the other particulars I think that you have already upsized the #10 where a #12 would work just fine, with the 40amp breaker.
With that said I have just noticed that you do have one more post than I, so I may not know so much after all.
 

haskindm

Senior Member
Location
Maryland
Actually since 250% of the 16.7 amps is 41.75 amps, you may round up to the next standard size breaker which is 50 amps. If that is not sufficient for starting the motor, you may then go up to 400% (exception C) which would allow a 60 amp breaker. Keep in mind that on a motor the circuit breaker only provides protection against short circuits and ground faults. Overload protection is provided by the "heaters" or other devices.
 

templdl

Senior Member
Location
Wisconsin
Then, if the motor is located let's say 15' from the starter and the breaker, which may be located in a panel 30' from the starter you can use as small as #12 wire between the motor and starter and starter to breaker and apply up to a 60a breaker.
 

A/A Fuel GTX

Senior Member
Location
WI & AZ
Occupation
Electrician
jtester said:
Doesn't 240.4 allow 25 amps on #12 in this application?

Jim T

I would say no to 240.4. Article 430 states that generally, motor branch circuit conductors be rated at 125%of the FLA in the tables. The OCPD is then sized from 430.52 and that is where you can get branch circuits that seem to be violation based on 310.16.
 

templdl

Senior Member
Location
Wisconsin
m73214,
I agree unless I'm missing the point.
An OCPD as part of a motor starter I can agree with 43.52 but as a branch circuit where the OCPD may be located in a panel apart from the starter how does one justify the cable sizing and its protection?
When the OCPD is part of a combination motor starter that is where you have the OLR for OL protection, and since a thermal magnetic breaker is allowed to be sized so darned high the only thing that it brings to the party is short circuit protection anyway.
Dave
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
The branch circuit OCPD for a motor circuit only provides the short circuit and ground fault protection for that circuit no matter where the branch circuit OCPD is located. The motor overload protection provides the overload protection for both the branch circuit conductors and the motor. The branch circuit conductors are sized at 125% of the full load current as shown in Tables 430.247 through 430.251 and the bracn circuit OCPD is sized per 430.52.
Don
 
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