UL 508 vs NEC 430 OCPD calculation

[dbeasley488]

Just designed a feeder for a industrial control panel with starters and circuit breakers within the panel. I sized the feeder(125% of largest motor plus remaining motors) and feeder OCPD(175% of largest motor plus remaining motors) per NEC 430. The panel supplier cited UL 508 in that the feeder should be sized by taking 115% of all motor loads plus the full load currents of all other loads and dividing by .8 so that the breaker shall carry more than 80% of its nominal ampere rating. To say the least I'm a bit confused in regards to their interpretation. I have my opinion of their interpretation but what do you guys think?

 

[Smart $]

Ooops... misread the question on my first reply...

Feeder is under purview of the NEC, not UL 508. However, if the nameplate indicates minimum circuit ampacity and/or maximum ocpd then your feeder and ocpd must meet these requirements. Yet, this would only be appropriate for a manufactured assembly or system.

 

[Smart $]

Addendum:

A manufactured assembly or system would include the control panel, the motors, all other associated loads, and wiring between them. Wiring that interfaces with not-included equipment falls under the NEC.

 

[SG-1]

I think placing more than 80% continuous load on most breakers would violate their UL Listing.

The breaker would have to be listed for 100% of continuous load to place more than 80% continuous load on it.

 

[dbeasley488]

Ul 508

Ul 508

I think placing more than 80% continuous load on most breakers would violate their UL Listing.

The breaker would have to be listed for 100% of continuous load to place more than 80% continuous load on it.

Oops, meant to say the breaker shall not carry more than 80% of FLC. I just finished reading what UL508 says. In section 18.1.3, it simply says "breaker shall have rating of 115% of full load current of any assigned horsepower rating". With that and conversations with other engineers, I'm going with the more conservative UL feeder calculation. Thanks for you comments.

 

[Smart $]

Oops, meant to say the breaker shall not carry more than 80% of FLC. I just finished reading what UL508 says. In section 18.1.3, it simply says "breaker shall have rating of 115% of full load current of any assigned horsepower rating". With that and conversations with other engineers, I'm going with the more conservative UL feeder calculation. Thanks for you comments.

Need I remind you the UL method has no bearing on feeder sizing and OCPD ratings unless the NEC permits such, as is the case for many manufactured assemblies with requirements on the nameplate.

You can't just choose to use the UL method because it is more conservative.

 

[Hameedulla-Ekhlas]

Just designed a feeder for a industrial control panel with starters and circuit breakers within the panel. I sized the feeder(125% of largest motor plus remaining motors) and feeder OCPD(175% of largest motor plus remaining motors) per NEC 430. The panel supplier cited UL 508 in that the feeder should be sized by taking 115% of all motor loads plus the full load currents of all other loads and dividing by .8 so that the breaker shall carry more than 80% of its nominal ampere rating. To say the least I'm a bit confused in regards to their interpretation. I have my opinion of their interpretation but what do you guys think?

I have not read UL 508 what it says and according to NEC you have done correctly the feeder size but 175% for OCPD is not correct.


Example : For what ampacity must you size the feeder conductor if it supplies two motors? The terminals are rated for 75?C.


One 7.5-hp, 230V (40A), single-phase motor

One 5-hp, 230V (28A), single-phase motor



Let's walk through the solution.


The largest motor is 40A.

40AЧ1.25+28A=78A.

80A is the closest selection that's at least 78A.

Per Table 310.16, a 6 AWG conductor rated at 75?C provides 65A of ampacity, so it's too small. However, a 4 AWG conductor provides 85A of ampacity, which will accommodate the necessary 78A. Therefore, you need to size this feeder conductor at 4 AWG.






For breaker Size:

What size feeder protection (inverse-time breaker) do you need for the following two motors?


5-hp, 230V, single-phase motor
3-hp, 230V, single-phase motor

Step 1: Get the motor FLC from Table 430.148.


A 5-hp motor FLC is 28A.
A 3-hp motor FLC is 17A.

Step 2: Size the branch-circuit protection per the requirements of 430.52(C)(1), Table 430.52, and 240.6(A)


5-hp: 28AЧ2.5=70A



3-hp: 17AЧ2.5=42.5A (Next size up is 45A.)


Step 3: Size the feeder conductor per 430.24(A).


The largest motor is 28A.

(28AЧ1.25)+17A=52A

Table 310.16 shows 6 AWG rated 55A at 60?C as the smallest conductor with sufficient ampacity.


Step 4: Size the feeder protection per 430.62.


It must not be greater than the 70A protection of the branch circuit plus the 17A of the other motor, which is the total of all loads on that feeder.

70A+17A=87A


Choose the next size down, which is 80A.

The panel supplier cited UL 508 in that the feeder should be sized by taking 115% of all motor

The service factor - SF - is a measure of periodically overload capacity at which a motor can operate without overload or damage. The NEMA (National Electrical Manufacturers Association) standard service factor for totally enclosed motors is 1.0.

A motor operating continuously at a service factor greater than 1 will have a reduced life expectancy compared to operating at at its rated nameplate horsepower.

Example - Service Factor
A 1 HP motor with a Service Factor - SF = 1.15 can operate at

1 HP x 1.15

= 1.15 HP

without overheating or otherwise damaging the motor if rated voltage and frequency are supplied to the motor.

Insulation life and bearings life are reduced by the service factor load.

 

[benaround]

What does sizing the feeder have to do with the % of ocpd ?