Sizing a Subpanel

[wwhitney]

Now I'm trying to compute the feeder size based on 430.62

430.62 says to start with the "largest rating or setting of the branch circuit short-circuit and ground-fault protective device for any motor supplied by the feeder" and further clarifies "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".

440.22 says to use the branch-circuit selection current, which will be on the nameplate (and is equal to the Rated Load Current if not separately specified on the nameplate), so we don't need to use the tables in 430 for hermetic refrigerant motor-compressors. But you are correct that if the largest motor is not a hermetic refrigerant motor-compressor, then 430.6 tells us to use the table FLC based on HP, rather than the nameplate FLC, for this part of the calculation.

One thing I'm not certain about is that 430.62 then says to add "the sum of the full-load currents of the other motors of the group;" is that the nameplate FLCs or the tables FLCs? 430.6 says to use the table values for determining "the ampacity of conductors or ampere ratings of switches, (and) branch-circuit short-circuit and ground-fault protection," but 430.62 is about determining the feeder SCGF protection, not the branch-circuit SCGF. So I think for this part of the computation we do use the nameplate values, rather than the table values.

Cheers, Wayne

 

[david]

430.62 says to start with the "largest rating or setting of the branch circuit short-circuit and ground-fault protective device for any motor supplied by the feeder" and further clarifies "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".

The Op presented two compressor article 440 motors

The above does not seem to give any guidance, for that.

The above seems to address one article 440 motor along with other article 430 motors

Article 440 motors 440.4 (A) do not provide any fla values in that list

But still reading

 

[david]

says to use the table values for determining "the ampacity of conductors or

To me I read that to mean use the table values

 

[david]

430.62 says to start with the "largest rating or setting of the branch circuit short-circuit and ground-fault protective device for any motor supplied by the feeder" and further clarifies "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".

I don't want to make you mad but so far the only code section I found giving guidance to supply conductors with two or more article 440 motors is 440.33

But I'm still looking and reading what your saying

 

[Little Bill]

I'm almost sorry I asked!

 

[augie47]

I'm almost sorry I asked!

I think you may be speaking for many

 

[david]

O

I don't want to make you mad but so far the only code section I found giving guidance to supply conductors with two or more article 440 motors is 440.33

But I'm still looking and reading what your saying

I'm almost sorry I asked!

To bad you didn't take pictures of both units data plates

 

[david]

The Op presented two compressor article 440 motors

The above does not seem to give any guidance, for that.

The above seems to address one article 440 motor along with other article 430 motors

Article 440 motors 440.4 (A) do not provide any fla values in that list

But still reading

I guess you take the lock Rotor current, convert that to horse power, than covert horse power to fla if the feeder only supplies compressor motors if you work from 430.62

 

[Little Bill]

O


To bad you didn't take pictures of both units data plates

 

[Little Bill]

 

[wwhitney]

Getting the exact minimum feeder ampacity would require the nameplate of the new unit, but adding the MCAs is conservative, so 9A + 18.9A = 27.9A is more than enough. #10 Cu is 30A or 35A depending on wiring method and termination limits.

For the feeder OCPD, again without the 2nd unit's nameplate data, assuming it is all motor is conservative. So 14.1 * 225% + (18.9 - 14.1*125%) + (9/125%) = 40.2. If the 30A OCPD doesn't hold on startup of the existing unit when the new unit is running, you can bump the OCPD up to 35A or 40A.

Cheers, Wayne

 

[david]

Getting the exact minimum feeder ampacity would require the nameplate of the new unit, but adding the MCAs is conservative, so 9A + 18.9A = 27.9A is more than enough. #10 Cu is 30A or 35A depending on wiring method and termination limits.

For the feeder OCPD, again without the 2nd unit's nameplate data, assuming it is all motor is conservative. So 14.1 * 225% + (18.9 - 14.1*125%) + (9/125%) = 40.2. If the 30A OCPD doesn't hold on startup of the existing unit when the new unit is running, you can bump the OCPD up to 35A or 40A.

Cheers, Wayne

9 x 230= 2070 va
18.9 x230 = 4347 va

60417 ÷ 230 = 27.9 amps

Since this feeder is for hvac eguipment I think I'll calculate the feeder ampacity based on a va load according to article 220

 

[TX+ MASTER#4544]

I hate to say this McGraw-Hill's National Electrical Code 2014 Handbook
Frederic P. Hartwell, Joseph F. Mc Partland, Brian J. McPartland

Say on page 1100 that he is right. Referring to the feeder conductors

In part "Breakers in the subfeeder panel are rated using 430.62 and the conductors by 440.33

I know it is a commentary and you may not be interested in there comments

david
2023
RE: 440.33
So don't say ..... "I hate to say this...."
But I understand.

Refer to your 2014 NEC Code book the following: (If you have one)

440.32 Single Motor Compressor
440.33 Motor-Compressor(s) With or Without Additional Motor Loads
440.34 Combination Load

All 2014 Code sections are almost verbatim as the 2023 NEC.

It may be commentary but it's the truth.
I've never thought that a double" E" (an electrical engineer) would be the author of some false commentary in the NEC.
Sometimes the commentary actually sheds more light on the subject than the actual Code reference does.
Finally, anyone may submit this particular question to the NFPA Standards for their official interpretation.
Thanks for reading.
Comments accepted.
TX+MASTER#4544

 

[david]

It may be commentary but it's the truth.
I've never thought that a double" E" (an electrical engineer) would be the author of some false commentary in the NEC.
Sometimes the commentary actually sheds more light on the subject than the actual Code reference

on page 1095 of the comments. Here he says article 440 does not address feeders. Feeders are addressed in article 430

i think what the OP asked about could have been answered from article 220. We calculate the size conductors and overcurrent for sub -panels all the time. I do remember in my earlier years the min Circuit amps and max overcurrent was not included on name plates for hvac as it is today. IAEI monthly magazine had articles on doing these calculations back then

 

[david]

Getting the exact minimum feeder ampacity would require the nameplate of the new unit, but adding the MCAs is conservative, so 9A + 18.9A = 27.9A is more than enough. #10 Cu is 30A or 35A depending on wiring method and termination limits.

For the feeder OCPD, again without the 2nd unit's nameplate data, assuming it is all motor is conservative. So 14.1 * 225% + (18.9 - 14.1*125%) + (9/125%) = 40.2. If the 30A OCPD doesn't hold on startup of the existing unit when the new unit is running, you can bump the OCPD up to 35A or 40A.

Cheers, Wayne

A couple observations on the branch circuits in this thread. When we talk about motors in 430 we are referring to name plates that have FLa on them along with other information.
When we refer to article 440 those have rated load currents or branch circuit selection currents on there name plates.
The branch circuit for the hvac with 14.1 rlc was sized by 440.33
When I read 430.62 the first thing I blew right past is the branch circuits would have had to been sized by 430.24 and that's not the case in this thread. I don't think 430.62 can be used

I'm not even sure 430.63 can be used for more than one compressor is in the group. 430.63 tells you what to do for one motor and tells you what to do if there are two or more motors, but dosnt tell you what to do if there are two or more compressors.
I'm thinking when you have hvac units being supplied by a feeder we go to article 220 and 215

 

[wwhitney]

When I read 430.62 the first thing I blew right past is the branch circuits would have had to been sized by 430.24 and that's not the case in this thread.

The sizing methodology in 430.24 is in accordance with the sizing methodology in Article 440 Part IV, so there is no conflict here.

Cheers, Wayne

 

[david]

The sizing methodology in 430.24 is in accordance with the sizing methodology in Article 440 Part IV, so there is no conflict here.

Cheers, wayne

If you don't mind me asking the Op presented an hvac unit with 14.1 rated load amps and lock rotor current of 73.

How is working with fla the same as working with rla

To get close to the manufactures 18.9 mca you would have to interpolate the horse power to around 21/2 hp

So how are these two methods the same?

 

[wwhitney]

So how are these two methods the same?

Notice the scope in 430.1 does not exclude Article 440 equipment; Article 440 equipment just has to comply with both Article 430 and Article 440.

430.7(D) on the markings of "Multimotor and Combination-Load Equipment" says the marked conductor ampacity "shall be calculated in accordance with 430.24." So and there's no conflict applying 430.62 when the conductors are sized using the nameplate information, the conductors have been sized in accordance with 430.24 as required by 430.62.

There's no conflict between Article 440 Part IV Branch Circuit Conductors and Article 430 Part II Motor Circuit Conductors; the computations specified are identical. There is a conflict between the computations in Article 440 Part III Branch-Circuit SCGF and Article 430 Part IV Motor Branch SCGF (175%/225% vs 250%/300%-400%), which is why 430.62/430.63 explicitly reference both 430.52 and 440.22(A) as applicable.

Cheers, Wayne

 

[david]

Notice the scope in 430.1 does not exclude Article 440 equipment; Article 440 equipment just has to comply with both Article 430 and Article 440.

430.7(D) on the markings of "Multimotor and Combination-Load Equipment" says the marked conductor ampacity "shall be calculated in accordance with 430.24." So and there's no conflict applying 430.62 when the conductors are sized using the nameplate information, the conductors have been sized in accordance with 430.24 as required by 430.62.

There's no conflict between Article 440 Part IV Branch Circuit Conductors and Article 430 Part II Motor Circuit Conductors; the computations specified are identical. There is a conflict between the computations in Article 440 Part III Branch-Circuit SCGF and Article 430 Part IV Motor Branch SCGF (175%/225% vs 250%/300%-400%), which is why 430.62/430.63 explicitly reference both 430.52 and 440.22(A) as applicable.

Cheers, Wayne

Ok this all has been interesting a lot to think about 175%/225% I did notice the manufacture used the 225% to compute the 18.9 mca, and the commentary used the 175% in that example

 

[david]

Ok this all has been interesting a lot to think about 175%/225% I did notice the manufacture used the 225% to compute the 18.9 mca, and the commentary used the 175% in that example

Excuse me max overcurrent protection try do do this atca red light