Motor Feeder Calculation

[Richard S]

We have a site where they are installing a new chiller, cooling tower and associated pumps and we are questioning the size of the distribution board and feeder conductors feeding this equipment. I believe I've attached a partial single line diagram.

Loads
(1) 800 Ton Chiller (Primary RLA = 548A, MCA = 694A)
(1) 30 HP Cooling Tower Fan (FLA = 40A)
(1) 50 HP Condensor Water Pump (FLA = 65A)
(1) 100 HP Chilled Water Pump (FLA = 124A)

Per 430.24 ampacity of conductors = 125% of Largest motor + sum of FLA of other motors.

Using Chiller RLA as largest motor I come up with 548A x 125% = 685A + 40A + 65A + 124A for a total of 914A required for motor feeder conductor size.
Based on that calculation, can these loads be fed from an 800A 100% rated circuit breaker that is feeding an 800A 100% Rated distribution board?
215.3 allows for sum of continuous + non-continuous loads for 100% rated breakers, but how do motors transpose in that? Example D3(a) in Annex D uses the 125% of largest motor + sum of other motors to determine ampacity of motor loads and then adds that to the non-continuous loads for the total feeder calculation.

Scratching my head on this one.

 

[petersonra]

ampacity tells you how big the conductors have to be.

OCPD is something else. see exception 1 to 215.3.

215.3 Overcurrent Protection. Feeders shall be protected
against overcurrent in accordance with the provisions of
Part I of Article 240. Where a feeder supplies continuous
loads or any combination of continuous and noncontinuous
loads, the rating of the overcurrent device shall not be less
than the noncontinuous load plus 125 percent of the continuous
load.
Exception No. 1: Where the assembly, including the overcurrent
devices protecting the feeder(s), is listed for operation
at 100 percent of its rating, the ampere rating of the
overcurrent device shall be permitted to be not less than the
sum of the continuous load plus the noncontinuous load.

It seems to me the minimum size of the CB feeding this stuff is 777A. You can't get a 777A CB so you have to go up to 800A. Personally, I would never put a 777A load on an 800A CB unless I was pretty desperate, but I think it is code legal.

 

[Richard S]

ampacity tells you how big the conductors have to be.

OCPD is something else. see exception 1 to 215.3.

It seems to me the minimum size of the CB feeding this stuff is 777A. You can't get a 777A CB so you have to go up to 800A. Personally, I would never put a 777A load on an 800A CB unless I was pretty desperate, but I think it is code legal.

Thanks for your reply Bob,
I looked at that as well and came up with that same possibility. Then I looked at the example in annex D that I referenced [D3(a)] where they also looked at sizing a 100% rated breaker based on the sum of continuous & non-continuous loads, but the non-continuous loads were based on motor loads calculated using 125% of largest motor.

It doesn't make sense to have 914A rated conductors with a 800A distribution board. I fully agree with you about this being a desperate attempt to make what seems to be a mistake work.

 

[petersonra]

Thanks for your reply Bob,
I looked at that as well and came up with that same possibility. Then I looked at the example in annex D that I referenced [D3(a)] where they also looked at sizing a 100% rated breaker based on the sum of continuous & non-continuous loads, but the non-continuous loads were based on motor loads calculated using 125% of largest motor.

It doesn't make sense to have 914A rated conductors with a 800A distribution board. I fully agree with you about this being a desperate attempt to make what seems to be a mistake work.

Especially with this kind of equipment. The suppliers tend to undersize the motors a hair if they think they can get away with it so everything tends to run at or just above FLC. You could easily have an actual load of 825 amps. Probably won't trip the breakers but it is just too close for my comfort.

 

[petersonra]

Thanks for your reply Bob,
I looked at that as well and came up with that same possibility. Then I looked at the example in annex D that I referenced [D3(a)] where they also looked at sizing a 100% rated breaker based on the sum of continuous & non-continuous loads, but the non-continuous loads were based on motor loads calculated using 125% of largest motor.

It doesn't make sense to have 914A rated conductors with a 800A distribution board. I fully agree with you about this being a desperate attempt to make what seems to be a mistake work.

Take a close look at what it actually says. It came up with a 150A OCPD because 100% devices are not available at 125 Amps, not because it is not permitted.

Conversion to amperes using three significant figures:
113,200 VA / (480V ? ?3) = 136 A
Minimum size overcurrent protective device: 136 A
Minimum standard size overcurrent protective device (see 240.6): 150
amperes
Where the overcurrent protective device and its assembly are listed for
operation at 100 percent of its rating, a 125 ampere overcurrent protective
device would be permitted. However, overcurrent protective device assemblies
listed for 100 percent of their rating are typically not available at the
125-ampere rating. (See 215.3 Exception.)