480VAC Blower Motor Starter Panel

[dcsva@aol.com]

I seem to be going in circles to find a code reference to determine the minimum size main breaker for a multi-starter control panel.
3-20HP 3-phase Blowers FLA 27amps
1-2KVA transformer 480-120 (2000/480=4.17amps)

Each motor has its own circuit breaker, contactor and overload device. The transformer has its own primary and secondary fusing.

I would always size it as:
27+27+27+4.17=85.17
85.17*1.25=106.43amp
therefore: 125amp main breaker minimum.

1. Does this seem right?
2. Is there a code reference that talks about the minimum main breaker size for this application?

 

[hillbilly]

Re: 480VAC Blower Motor Starter Panel

430-53(c)

 

[don_resqcapt19]

Re: 480VAC Blower Motor Starter Panel

The code does not specify a minimum breaker size, just a maximum breaker size. The minimum size is the smallest breaker that will permit the load to start without tripping.
The maximum breaker size rule is in 430.62. If you are using an inverse time trip breaker, the maximum size is 125 amps.
27(2.5)+27+27+4.17=125.67
I see no provision to round up to the next larger breaker in 430.62, so you must go down to 125.
Don

 

[jhines]

Re: 480VAC Blower Motor Starter Panel

You may be thinking of section 430.24 for conductors with multiple motors. It's the sum plus 125% of the largest motor.

 

[dcsva@aol.com]

Re: 480VAC Blower Motor Starter Panel

Don,
I also came up with the same value of 125.67amps.
With a 125amp inverse time breaker.

Thanks to all for the comments.

 

[don_resqcapt19]

Re: 480VAC Blower Motor Starter Panel

It's the sum plus 125% of the largest motor.

No, it is the sum of all the motors plus 25% of the largest motor.
27 + 27 + 27 + 4.17 + (.25)27 = 91.92
not
27 + 27 + 27 + 4.17 + (1.25)27 = 118.92
Don

 

[jhines]

Re: 480VAC Blower Motor Starter Panel

As long as the motors start one at a time, a 100 amp breaker should work. Checked breaker curve for a GE TED 100, works fine.

 

[stevebrown]

Re: 480VAC Blower Motor Starter Panel

So where did this wind up? I'm in agreement with an inverse time circuit breaker set at no more than the sum of the motors plus the control transformer, plus 25% of the largest motor.

I understand the error made in the first post of taking the entire load and adding 25% (it's a common mistake). However, I was confused by the earlier posts which were multiplying the largest motor by 2.5 to come up with 125 Amps.

Was this an error, or is there something more to the 2.5 multiplier?

 

[bennie]

Re: 480VAC Blower Motor Starter Panel

Will the three twenty horse power motors start automatically after a power outage?

 

[dcsva@aol.com]

Re: 480VAC Blower Motor Starter Panel

They are installing delay timers such that only on e motor will start at a time with 15 seconds between each start. That is if the system is calling for more than one motor to run.

 

[steve66]

Re: 480VAC Blower Motor Starter Panel

Steve:

I think the factor 2.5 comes from table 430.52. Article 430.62 says to use the value from this table for the largest motor, and then use the full load currents for the other motors.

Steve

 

[don_resqcapt19]

Re: 480VAC Blower Motor Starter Panel

Steve,

So where did this wind up? I'm in agreement with an inverse time circuit breaker set at no more than the sum of the motors plus the control transformer, plus 25% of the largest motor.

The nuber you get using the above is the minimum required ampacity of the feeder conductors. The other Steve is correct that the 2.5 comes from Table 250.52 for the maximum permitted size of an inverse time ciruit breaker used to protect the feeder conductors. See 430.62(A).
Don