Feeder Calculation Largest Motor Multi Branch Circuit

[fifty60]

Trying to size a feeder per NEC 430. I am deeling with hermetic compressors so I am using a combination of 430 and 440. I have a mixed load that consists of 3 branches. 1 branch contains my largest motor and a condensor fan. The next branch contains a smaller compressor. And the 3rd branch has a heater.

When calculating the MINIMUM feeder OCPD, I am taking the largest motor (compressor) OCPD plus the sum of all the other concurrent loads. My question is this: Since my largest motor is on a branch that has another motor load, do I take the value of the OCPD only in my feeder calculation, or do I take the value of the OCPD plus the FLA of the condenser fan on the same branch as my largest motor(compressor)?

The OCPD on the branch that contains the largest compressor contains another motor. I am using this branches OCPD, and then also add again the smaller motor on the same branch, I will be counting this smaller motor 2 times and that seems wrong.

 

[petersonra]

I am confused about why the BC OCPD is in the feeder calculation, or how you figure it is in there twice.

 

[david luchini]

Per 430.62, you don't use the Branch OCPD when figuring the Feeder OCPD, you use the largest rating or setting for any motor supplied by the feeder.

If your largest motor was on a 20A branch circuit, but the largest motor alone had a max OCPD of 15, you would use 15 in the 430.62 calculation, plus the full load currents of the other motors.

And this would give you the MAXIMUM feeder OCPD, not the MINIMUM as you suggest.

 

[petersonra]

Per 430.62, you don't use the Branch OCPD when figuring the Feeder OCPD, you use the largest rating or setting for any motor supplied by the feeder.

If your largest motor was on a 20A branch circuit, but the largest motor alone had a max OCPD of 15, you would use 15 in the 430.62 calculation, plus the full load currents of the other motors.

And this would give you the MAXIMUM feeder OCPD, not the MINIMUM as you suggest.

I think this gives you the maximum feeder OCPD for the wire size that is the minimum wire size allowed for the feeder.

 

[GoldDigger]

I think this gives you the maximum feeder OCPD for the wire size that is the minimum wire size allowed for the feeder.

But remember that, for a single motor with overload protection, the maximum OCPD for the branch circuit may be larger than the value corresponding to the minimum wire size required to feed the motor's internal overload current limit. And it is all compliant. At least according to MH and other authorities.
Once you add a second motor it gets harder to justify that relationship, and it may end up giving you the same results anyway.

 

[fifty60]

I am calculating the feeder OCPD for a machine that has 2 motor branches and a heater branch. If it just had motors I would be calculating the MAXIMUM OCPD, but since it is a mixed load (which I believe is covered by 430.65, sorry I do not have my codebook at this moment) instructs you to use NO LESS THAN the FLA's of all the non motor loads and add that to the result of 430.62. To me, this is giving you the minimum for the mixed load.

If I have a branch circuit that consists of more than 1 motor, I will have a single Branch OCPD for the entire branch. So I should not use the rounded up result of 430.62 in this calculation, but should calculate the OCPD for the largest motor and then add the fla's of everything else (including the FLA of the smaller motor on the same branch?)

 

[kwired]

Just use the value necessary for the largest motor and add 100% of all other load. If other load is a motor add the corresponding full load current from 430 Part XIV tables. If other than a motor add rated current of the load. Remember this gives you a maximum value, if a lower setting will hold it is not a violation.

 

[fifty60]

I agree that is the interpretation of 430.62. My situation falls under the guidance of 430.63. I have motor loads and heater loads (on different branches) off my feeder. Here is the wording of 430.63 "...the feeder protective device shall have a rating NOT LESS THAN that required for the sum of the other loads pluss...(3)the rating permitted by 430.62." To me, this is clearly giving me a minimum OCPD.

 

[fifty60]

Also, when looking at 430.24 it appears that I can get a higher rating for my conductors than what I will get for my OCPD. 430.24 requires that I use the sum of: 125% of the largest motor, 100% of other motors, and 125% continuous non motor loads. 430.63 sizes the OCPD for the feeder at the sum (which I interpret as 100%) and the rating permitted by 430.62. If I am using dual element time delay fuses and can size my motor at 125% FLA, the I would be sizing my feeder OCPD at 125% FLA largest motor pluss 100% of all other loads. Am I interpreting this correctly?

 

[kwired]

I agree that is the interpretation of 430.62. My situation falls under the guidance of 430.63. I have motor loads and heater loads (on different branches) off my feeder. Here is the wording of 430.63 "...the feeder protective device shall have a rating NOT LESS THAN that required for the sum of the other loads pluss...(3)the rating permitted by 430.62." To me, this is clearly giving me a minimum OCPD.

And the rating permitted by 430.62 comes from values that 430.62 sent you to 430.52 to obtain. 430.52 values are maximum values, but are allowed to be less.

 

[fifty60]

So I see that the Motor part of the Feeder OCPD calculation sets a maximum, but the "Not Less Than" part for the non motor loads opens the door up for interpretation quite a bit. You could take it to be 100%...but really it does not say 100%. 430.24 does get specific saying 125%. Does this allow you to size the feeder (using 430.63) at the value you obtain in 430.62 plus 100% of the other loads...and then have the feeder conductors oversized a little bit so that the feeder OCPD can be adjusted in the field if need be?

 

[petersonra]

I am getting a head ache.


430.62(A) refers to feeder conductor sizes based on 430.24.

430.62 (B) allows you to use an OCPD based on the feeder conductor ampacity if the ampacity is greater than that determined by 430.24.

As an example:

A 20 HP motor and two 5 HP motors on a feeder.

125% of the 20 HP motor is 33.8 A, and 100% of the two 5 HP motors is 15.2 A, for a total ampacity required by 430.24 of 49 A. That gives me a minimum conductor size of #8. The conductors are good for 50A which is more than the calculated value from 430.24. So I can invoke 430.62(B) and just put in a 50A CB.

Or

I can invoke 430.62A and put in a bigger CB on the feeder.

A typical 20Hp motor might have a 60A CB on it.

60 + 7.6 + 7.6 = 75.2. So I could put a 75A CB on the feeder.

Is the perceived benefit of the larger breaker that it is less likely to trip?

I would probably put 100A breaker on it and run #3 if I was worried about tripping. But my distances are very short so the cost of the extra copper is limited.

 

[petersonra]

I am getting a head ache.


430.62(A) refers to feeder conductor sizes based on 430.24.

430.62 (B) allows you to use an OCPD based on the feeder conductor ampacity if the ampacity is greater than that determined by 430.24.

As an example:

A 20 HP motor and two 5 HP motors on a feeder.

125% of the 20 HP motor is 33.8 A, and 100% of the two 5 HP motors is 15.2 A, for a total ampacity required by 430.24 of 49 A. That gives me a minimum conductor size of #8. The conductors are good for 50A which is more than the calculated value from 430.24. So I can invoke 430.62(B) and just put in a 50A CB.

Or

I can invoke 430.62A and put in a bigger CB on the feeder.

A typical 20Hp motor might have a 60A CB on it.

60 + 7.6 + 7.6 = 75.2. So I could put a 75A CB on the feeder.

Is the perceived benefit of the larger breaker that it is less likely to trip?

I would probably put 100A breaker on it and run #3 if I was worried about tripping. But my distances are very short so the cost of the extra copper is limited.

I also have issues with people who would look at a 75A CB on #8 and say the wire is too small. There is no difference in cost to me for the bigger CB and the extra $5 in wire is worth the fewer explanation calls.

 

[kwired]

I am getting a head ache.


430.62(A) refers to feeder conductor sizes based on 430.24.

430.62 (B) allows you to use an OCPD based on the feeder conductor ampacity if the ampacity is greater than that determined by 430.24.

As an example:

A 20 HP motor and two 5 HP motors on a feeder.

125% of the 20 HP motor is 33.8 A, and 100% of the two 5 HP motors is 15.2 A, for a total ampacity required by 430.24 of 49 A. That gives me a minimum conductor size of #8. The conductors are good for 50A which is more than the calculated value from 430.24. So I can invoke 430.62(B) and just put in a 50A CB.

Or

I can invoke 430.62A and put in a bigger CB on the feeder.

A typical 20Hp motor might have a 60A CB on it.

60 + 7.6 + 7.6 = 75.2. So I could put a 75A CB on the feeder.

Is the perceived benefit of the larger breaker that it is less likely to trip?

I would probably put 100A breaker on it and run #3 if I was worried about tripping. But my distances are very short so the cost of the extra copper is limited.

You are on the right track. Two things need calculated.

conductor ampacity

overcurrent device setting.

Conductor ampacity is 125% largest motor plus 100% all other load, which (I didn't double check your numbers) would mean your minimum ampacity is 49 amps in your example.

But the max breaker can be 250% of the largest motor, plus all other loads. I believe since 75 amps is not a standard size you can go to 80 amps. BTW a single 20 HP motor could have up to a 70 amp breaker supplying it, though most cases a 60 is pretty common. The only reason for this higher value is to allow for starting, overload protection for each motor is covered by a separate device, and therefore if the motor can't be overloaded then the conductor can't be either.

ADD: the breaker provides short circuit and ground fault protection and 2.5 times FLA setting still provides this protection.

 

[david luchini]

But the max breaker can be 250% of the largest motor, plus all other loads. I believe since 75 amps is not a standard size you can go to 80 amps.

You can't use the next size up when sizing the feeder sc&gf protective device. 430.62(A) says "not greater than."

BTW a single 20 HP motor could have up to a 70 amp breaker supplying it, though most cases a 60 is pretty common.

You are correct about the 70A breaker for the largest motor. This would give you 70+7.6+7.6=85.2, which would then round down to and 80A breaker for the feeder.

 

[petersonra]

You are on the right track. Two things need calculated.

conductor ampacity

overcurrent device setting.

Conductor ampacity is 125% largest motor plus 100% all other load, which (I didn't double check your numbers) would mean your minimum ampacity is 49 amps in your example.

But the max breaker can be 250% of the largest motor, plus all other loads. I believe since 75 amps is not a standard size you can go to 80 amps. BTW a single 20 HP motor could have up to a 70 amp breaker supplying it, though most cases a 60 is pretty common. The only reason for this higher value is to allow for starting, overload protection for each motor is covered by a separate device, and therefore if the motor can't be overloaded then the conductor can't be either.

ADD: the breaker provides short circuit and ground fault protection and 2.5 times FLA setting still provides this protection.

I think you are mixing up feeders and group motor rules.

The rating of the feeder CB in 430.62 (A) is based on the largest rated downstream CB. What is actually there, not what might be there if they made it the largest possible value.

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],

Note it says the maximum size of the actual Cb on the motor itself is based on 430.52 or 440.22(A), but not that you can use that number for calculating the feeder CB size. you have to use the number that is actually there.

it does not say you can round up to the next std size CB here so I don't think you can.

 

[david luchini]

I am calculating the feeder OCPD for a machine that has 2 motor branches and a heater branch. If it just had motors I would be calculating the MAXIMUM OCPD, but since it is a mixed load (which I believe is covered by 430.65, sorry I do not have my codebook at this moment) instructs you to use NO LESS THAN the FLA's of all the non motor loads and add that to the result of 430.62. To me, this is giving you the minimum for the mixed load.

I think 430.63 is a bit confusing, but it is still giving you a maximum size OCPD for the minimum size feeder conductor. In other words, it is letting you use an OCPD which is higher than the feeder ampacity for mixed motor and appliance/lighting loads.

The MINIMUM size required OCPD for a feeder is already covered in 215.3

 

[david luchini]

Note it says the maximum size of the actual Cb on the motor itself is based on 430.52 or 440.22(A), but not that you can use that number for calculating the feeder CB size. you have to use the number that is actually there.

it does not say you can round up to the next std size CB here so I don't think you can.

Interesting, that's not the way it reads to me. To me it reads the maximum permitted value for the largest motor, plus the sum of the full load currents, and round down to the next standard device.

So even if your three motor had branch c/b's of 60, 20 and 20, the feeder OCPD could be 80A based on 70+7.6+7.6=85.2.

 

[petersonra]

I think 430.63 is a bit confusing, but it is still giving you a maximum size OCPD for the minimum size feeder conductor. In other words, it is letting you use an OCPD which is higher than the feeder ampacity for mixed motor and appliance/lighting loads.

The MINIMUM size required OCPD for a feeder is already covered in 215.3

430.63 refers you back to 430.62 for more than 1 motor.

I agree that 215.3 covers the minimum size of the OCPD on the feeder.

 

[petersonra]

Interesting, that's not the way it reads to me. To me it reads the maximum permitted value for the largest motor, plus the sum of the full load currents, and round down to the next standard device.

So even if your three motor had branch c/b's of 60, 20 and 20, the feeder OCPD could be 80A based on 70+7.6+7.6=85.2.

read this

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

what does it actually say.

the stuff after it in brackets is explanatory - it just tells you where the maximum value of the motor OCPD can be found.

it also does not require you to use a standard size CB. If the number came out to 85.2, you could use an 85.2A CB if you could find one.