Sizing OCPD for more than one motor

[muhasarr]

If I have three 25hp motors ( two running and one standby) plus a 5hp motor located in one control panel which is fed from an MCC, how do I size the main breaker and conductor from the MCC to the control panel?

Below are my calculations but I have a feeling it is not correct. Help needed.

1- FLA for largest pump(25HP) is 34A
2-FLA for 5HP pump is 7.6A
3- So my main circuit breaker= 2.5(3.4A) + 7.6A = 92.6A (So my breaker size is 100A)
4- Solving for conductor: 125%(34A) + 7.6A= 50.1 (According to NEC 310.15(b)(16), the conductor should be a #AWG)

Am I missing anything in my calculations?

 

[augie47]

You would need top size both based on ALL motors that could run at the same time.
If I understand you situation at least two of the 25 HP can rum simultaneously so you need to add the other pump Table FLA,

 

[petersonra]

If I have three 25hp motors ( two running and one standby) plus a 5hp motor located in one control panel which is fed from an MCC, how do I size the main breaker and conductor from the MCC to the control panel?

Below are my calculations but I have a feeling it is not correct. Help needed.

1- FLA for largest pump(25HP) is 34A
2-FLA for 5HP pump is 7.6A
3- So my main circuit breaker= 2.5(3.4A) + 7.6A = 92.6A (So my breaker size is 100A)
4- Solving for conductor: 125%(34A) + 7.6A= 50.1 (According to NEC 310.15(b)(16), the conductor should be a #AWG)

Am I missing anything in my calculations?

yes - you are missing something. two 25Hp motors.

you have to figure the minimum ampacity of the wire first. add up all the FLA and tack on 25% of the largest motor. that is the smallest conductor ampacity.

3X 34
1x 7.6
.25x 34

119 A.


unless the system is interlocked so there is no way to run all three motors you have to account for them in the ampacity calculation.

the convoluted calculation for maximum OCPD for this size conductor is next.

2.5X 34
2X 34
7.6

161A

 

[Smart $]

....

the convoluted calculation for maximum OCPD for this size conductor is next.

2.5X 34
2X 34
7.6

161A

With emphasis...!!!

 

[petersonra]

I would probably use a 1/0 with a 150A CB.

Than I would not have to argue with people over the calculations.

But I am not running a lot of wire.

I can never quite recall if you have to use the 2.5X FLA, or if it is the size of the CB that you add in. I think it is actually the size of the CB that the 2.5X calculation comes up with. I can't recall off the top of my head if you round up or down to the std Cb size. I always end up looking it up.

 

[Smart $]

...

I can never quite recall if you have to use the 2.5X FLA, or if it is the size of the CB that you add in. I think it is actually the size of the CB that the 2.5X calculation comes up with. I can't recall off the top of my head if you round up or down to the std Cb size. I always end up looking it up.

OCDP* + FLA's

*Largest branch OCPD rating of group, not OCPD of largest motor.

See exceptions if a branch instantaneous-trip device is used or sizing OCPD for an MCC.

 

[petersonra]

OCDP* + FLA's

*Largest branch OCPD rating of group, not OCPD of largest motor.

See exceptions if a branch instantaneous-trip device is used or sizing OCPD for an MCC.

This is what the code actually says.

V. Motor Feeder Short-Circuit and Ground-Fault
Protection
...
430.62 Rating or Setting ? Motor Load.
(A) Specific Load. A feeder supplying a specific fixed motor
load(s) and consisting of conductor sizes based on
430.24 shall be provided with a protective device having a
rating or setting not greater than the largest rating or setting
of the branch-circuit short-circuit and ground-fault protective
device for any motor supplied by the feeder [based on
the maximum permitted value for the specific type of a
protective device in accordance with 430.52, or 440.22(A)
for hermetic refrigerant motor-compressors], plus the sum
of the full-load currents of the other motors of the group.

I suppose your distinction makes more sense if you had a mixture of CBs and fuses on the downstream motors.

430.52 allows you to round up to the next standard sized OCPD.

However, exception #2 in 430.52 allows you to increase the size of the OCPD on an individual motor if you have issues with tripping, so I don't know if that affects the sizing of the feeder breaker or not. taken literally as written i could see that you could use 3X or 4X (if a CB) of the largest motor for the maximum OCPD allowed in the feeder. however, exception #2 does not allow you to round up.

 

[Smart $]

This is what the code actually says.

V. Motor Feeder Short-Circuit and Ground-Fault
Protection
...
430.62 Rating or Setting ? Motor Load.
(A) Specific Load. A feeder supplying a specific fixed motor
load(s) and consisting of conductor sizes based on
430.24 shall be provided with a protective device having a
rating or setting not greater than the largest rating or setting
of the branch-circuit short-circuit and ground-fault protective
device for any motor supplied by the feeder [based on
the maximum permitted value for the specific type of a
protective device in accordance with 430.52, or 440.22(A)
for hermetic refrigerant motor-compressors], plus the sum
of the full-load currents of the other motors of the group.

I suppose your distinction makes more sense if you had a mixture of CBs and fuses on the downstream motors.

430.52 allows you to round up to the next standard sized OCPD.

However, exception #2 in 430.52 allows you to increase the size of the OCPD on an individual motor if you have issues with tripping, so I don't know if that affects the sizing of the feeder breaker or not. taken literally as written i could see that you could use 3X or 4X (if a CB) of the largest motor for the maximum OCPD allowed in the feeder. however, exception #2 does not allow you to round up.

430.52(C)(1) Exception No. 1 allows you to upsize to next standard size. You can use that value in the 430.62 determination... but you cannot round up the result of 430.62.

The "based on" size is what's permitted in 430.52(C)... including exceptions.

And as noted, largest ocpd rating of any motor, not necessarily the largest motor.


If you upsize under 430.52(C)(1) Exception No. 2, the upsized value carries to 430.62... except in cases of branch instantaneous-trip devices, where you would have to use their non-instantaneous counterpart values if the feeder SC/GFPD is a non-instantaneous-trip device.

 

[petersonra]

what is it you are trying to get at here? I sense you are trying to make a point but I am not getting it.

are you trying to say that the feeder OCPD has to be based on the size of the largest actual installed OCPD for the motors, and not the theoretical largest OCPD that could be code compliant?

maybe an example is in order.

say you have 2 motors.

one is 100 FLA and the other 42 FLA.

The max CB on the 100 FLA motor is 250A, which just happens to be a std size.

250 + 42 = 292A.

you can't round up the feeder CB so you are left with a 250A CB.

Are you trying to say that if you actually installed a 200A breaker on the largest motor that you would have to use that value instead for the feeder OCPD calculation and thus would have to use a maximum 225A CB for the feeder?

But if you used the 4X exception you could have a 400A Cb on the largest motor and would be able to have a 400A CB on the feeder?

By the way, this interpretation is the way UL508a is explicitly stated. It does not have the confusing "based on" language.

32.3.1 The size of the overcurrent protection shall not exceed:
a) The rating of the largest branch circuit protective device in the circuit plus the full-load
currents of all other motors or other loads in the group;

 

[Smart $]

Not trying to make any particular point... just discussing the finer points. UL508a is cut and dry... NEC is not. :happyno:

Typical calculations are as you exhibited... but the NEC actually permits one to use the upsized-by-exception values for determination. The feeder breaker in your example could easily be 400A as noted... and compliant under the NEC. 430.62 does not limit the determination to the largest actually-used OCPD rating or setting.

As for the largest OCPD rating of any motor, and not necessarily the largest motor, take for example a 10HP 230V 1? motor with FLA of 50A on same feeder as a 15HP 230V 3? motor with an FLA of 42A. The largest OCPD rating or setting would be for the 10HP motor and not the 15 HP motor.

 

[petersonra]

Not trying to make any particular point... just discussing the finer points. UL508a is cut and dry... NEC is not. :happyno:

Typical calculations are as you exhibited... but the NEC actually permits one to use the upsized-by-exception values for determination. The feeder breaker in your example could easily be 400A as noted... and compliant under the NEC. 430.62 does not limit the determination to the largest actually-used OCPD rating or setting.

As for the largest OCPD rating of any motor, and not necessarily the largest motor, take for example a 10HP 230V 1? motor with FLA of 50A on same feeder as a 15HP 230V 3? motor with an FLA of 42A. The largest OCPD rating or setting would be for the 10HP motor and not the 15 HP motor.

We had this discussion once before. I am not convinced we ever really settled it.

It seems like the vast majority of people just use the 2.5X rule on the largest motor. If one could use 4X, why wouldn't one? It is a curiously phrased provision that I am not sure has a good answer short of asking for an official interpretation.

 

[Smart $]

We had this discussion once before. I am not convinced we ever really settled it.

It seems like the vast majority of people just use the 2.5X rule on the largest motor. If one could use 4X, why wouldn't one? It is a curiously phrased provision that I am not sure has a good answer short of asking for an official interpretation.

It was changed in 2002 cycle. Here's the text from ROP...

(Log #3365)
11- 48 - (430-62): Accept
SUBMITTER: Paul Dobrowsky, Holley, NY
RECOMMENDATION: Revise to read as follows:
A feeder supplying a specific fixed motor load(s) and
consisting of conductor sizes based on Section 430-24 shall be
provided with a protective device having a rating or setting not
greater than the largest rating or setting of the branch-circuit
short-circuit and ground-fault protective device for any motor
supplied by the feeder [based on the maximum permitted
value for the specific type of a protective device in accordance
with Section 430-52 and shown in* Table 430-152, or Section
440-22(a) for hermetic refrigerant motor-compressors],.plus
the sum of the full-load currents of the other motors of the
group.
SUBSTANTIATION: The rating determined for the branch
circuit is already determined by rounding up of the overcurrent
device, in accordance with Section 430-52. The existing text
within the brackets directly conflicts with text in the first
sentence that requires you to use the sizing of the branch-
circuit short-circuit and ground-fault protective device, that has
already been sized in accordance with Section 430-52(c),
Exception No. 1 or 2. This clears up a conflict in calculating
the feeder overcurrent protective device size.
PANEL ACTION: Accept.
NUMBER OF PANEL MEMBERS ELIGIBLE TO VOTE: 15
VOTE ON PANEL ACTION:
AFFIRMATIVE: 15

*italic red text substituted for stricken-through text

The proposed text removed the reference to Table 430.52 used previously and allowed upsized OCPD under Exceptions No. 1 or 2. I believe the intent was to permit the determination with an actual up-sized rating, not the potential upper limit... but the revised text does not state this.