Blower motor - inrush current

[NMCB13]

I have a blower motor that draws 49 amps on start up, and 11.2 amps running.
I am concerned about the supply conductors I installed which are No. 12 AWG.
What is the correct way to determine the supply conductor size?

Also this motor is thermally (internal) protected, is any other protection required other than the circuit breaker?


Motor spec's:
Phase: Single (1)
V 115/208-230
A 22/12.5-11.0
Duty: CONT.
max. amb. 40
SF 1.0
RPM 3450
code H/M

 

[augie47]

motor

motor

conductor size-- 430.22
I didn't see where you listed HP, which is the "key" but from your data one would assume 1-1/2 or 2 hp in which case your #12 is code compliant.

You can look at 430 V to check your breaker size and 430 IX to make sure you have satisfied the disconnect rule.

 

[charlie b]

The reason there is a concern over conductor ampacity, and a concern over sending too much current through a conductor, is related to overheating of the insulation system. Although the starting current of a motor is very high, the amount of heat generated during the short starting period will be very small. It will not present any undue risk of damage to the insulation. Therefore, the ampacity of the conductors supplying the motor is not based on the starting current, but rather on the other factors that are addressed in the code section to which Augie47 alluded.

 

[bob]

I have a blower motor that draws 49 amps on start up, and 11.2 amps running.
I am concerned about the supply conductors I installed which are No. 12 AWG. What is the correct way to determine the supply conductor size?
Also this motor is thermally (internal) protected, is any other protection required other than the circuit breaker?

The code will allow you to size the breaker at 250% of the FLA of the motor.
This amperage should be taken from the tables in section 430 as augie said.
Assuming the 11.2 is correct, 11.2 x 2.5 = 28 amps. In you case you may want to use 2 time the FLA = 22.4 amps. a 20 or 25 amp breaker would
likley work. The problem comes from the starting amp, 49 amps, tripping the
breaker if it is sized too small.

 

[boater bill]

Won't the #12 wires create a ampacity/protection conflict if used on the 25 amp breaker?
The trip curve of the 20 amp magnetic breaker should allow the 49 amp inrush for enough time to get the fan up to speed, but I would check the curve of the breaker being used.

 

[tony_psuee]

Won't the #12 wires create a ampacity/protection conflict if used on the 25 amp breaker?

Bill,

No it won't, 240.4G allows specific conductor applications for 14, 12, and 10AWG wire to exceed the limitation of 240.4D.

tony