OCPD

[shockrocker68]

I currently ran power to a blower motor control panel, { a manual disconnect switch, a starter}.The motor FLA was 6.0 amps 3 phase, 480 Volt motor, I pulled #10 awg for a voltage drop THHN,I needed a 30 Amp CCt BKR in the panel, But my boss had told me that I needed A 70 amp bkr for the application, I thought that the Bkr protected the wire, if that is the case would A 70 Bkr be to much, Also does the bkr have to rated for start up current, Please respond.

 

[bob]

Re: OCPD

Look at table 430.52. You are allowd to go to 250% of the FLA of the motor. 2.5 x 6 amps = 15 amps when using an inverse time breaker. The breaker will only protect for short circuit. The thermal protection in the motor will protect for overloads.

 

[websparky]

Re: OCPD

Hi Bob,

I agree with your calcs. What has me concerned is the boss wants a 70A breaker??

Doesn't that seem a tad high in relation to the FLA?

 

[bob]

Re: OCPD

70 amps breaker is not correct. Boss may be thinking of another motor. Using table 450.150 the closest motor to the 6 amps listed is a 3 or 5 hp. Even at 208 volts neither the 3 or 5 hp would require a 70 amp breaker. You would have to goto a 20 hp to need a 70 amps breaker.
I think the Boss must be using the 30 amp ampacity for the #10 conductor. 30 x 2.5 = 75.

[ April 07, 2004, 12:58 PM: Message edited by: bob ]