Sizing a Subpanel

[wwhitney]

I don't see how, when the language clearly says "having a rating or setting not greater than".

I'm not saying that 430.62 requires you to use a 35A OCPD, I'm saying that when you do the computation, based on the information given so far, the possible outcomes are 30A max, 35A max, or 40A max. [I originally thought 30A max was not possible, but reviewing the algebra it is, although unlikely.]

Then I'm further saying that when larger than 30A is allowed, choosing to use 30A would be like using a 20A or 25A OCPD on the branch circuit for the 18.9 MCA / 30 MOCP unit. Which is allowed, but often here I see the attitude "just use the MOCP, it's allowed and is least likely to nuisance trip."

That's all I was saying about 430.62. I'm more interested in 430.63 and how to interpret it, and 430.63 would apply if either unit includes a non-motor load. 430.63 also includes the language "not less than" rather than "not greater than."

Cheers, Wayne

 

[don_resqcapt19]

I'm not saying that 430.62 requires you to use a 35A OCPD, I'm saying that when you do the computation, based on the information given so far, the possible outcomes are 30A max, 35A max, or 40A max. [I originally thought 30A max was not possible, but reviewing the algebra it is, although unlikely.]

Then I'm further saying that when larger than 30A is allowed, choosing to use 30A would be like using a 20A or 25A OCPD on the branch circuit for the 18.9 MCA / 30 MOCP unit. Which is allowed, but often here I see the attitude "just use the MOCP, it's allowed and is least likely to nuisance trip."

That's all I was saying about 430.62. I'm more interested in 430.63 and how to interpret it, and 430.63 would apply if either unit includes a non-motor load. 430.63 also includes the language "not less than" rather than "not greater than."

Cheers, Wayne

I think it is just that using the nameplate numbers is easier and you just do it that way.
Kind of like the 125% primary transformer protection...most make sure that the primary protection is at least 125%, but I have a PE friend who often uses less, especially where the 125% would require a larger disconnect rating or breaker frame size.

 

[Little Bill]

This is not my job, but I will pass on that the #10 feeder is ok for the sub.

 

[david]

Interesting discussion, I'm not going to add anything to it. Table 430.248.
How did you guys determine the horse power of the motors in this discussion?

 

[TX+ MASTER#4544]

Yes... Master--- mini splits to be exact

2023 NEC

Hey, thanks, just an 'ol master electrician and your mentor.

A feeder for a hermetic refrigerant motor compressor is found in Part IV of 440.43 ...."not less than the sum of each of the following"......
Item (1) use the rated load or B/C current...whichever is greater ....of motor-compressor(s). You will select the larger of the two, ie., either the rated load current or the B/C selection, whichever is greater.

See 440.32 (1) 125% of the rated load current
See 440.32 (2) 125 % of the B/C selection

Item (2) 440.33 ...sum of full load current of all motors. None in this example.

Item (3) .25 % of highest rated motor current (amps) in the group.

I will pause here for a moment; if these refrigerant motors are three phase then you might sketch them out on paper and balance them on each phase, or each line #1 or line #2 ( group) which is what is listed in item (3).

I think they are all single phase 120/240 or 240 single phase so therefore if they are all single phase (2 wire) then they are naturally balanced with a motor on line 1 and one motor on line 2 the (group).

I calculated this to be 38 amps. What type of wiring method is this installation?
If it's NM (romex) select from the 60 degree column.T.310.16 and chose a size 8 AWG with ground (EGC).
If its conduit and type THHN, select from 75 degree column and chose 2 size 8 AWG plus an EGC if required.
The EGC can be selected from T.250.122 but cannot be determined until OCPD is calculated. But never larger than the circuit conductors.
The feeder OCPD cannot be rounded up to the next size. See 440.22 (B) (1) and 430.62 (A)

I have blabbered too long on this subject so I will let someone else add to it.

Thanks for reading.
Comments accepted.
TX+MASTER#4544

 

[wwhitney]

A feeder for a hermetic refrigerant motor compressor is found in Part IV of 440.43

I assume you mean 440.34, as there is no 440.43. But 440.34 is in Part IV "Branch Circuit Conductors" so it does not apply to feeder conductors.

Cheers, Wayne

 

[david]

See 440.32 (1) 125% of the rated load current
See 440.32 (2) 125 % of the B/C selection

Item (2) 440.33 ...sum of full load current of all motors. None in this example.

430.62 (A) sends you to 440.22 (A) 175% than back to 430.62 (A) plus yhe sum of the full load currents of the other .motors of the group

 

[Eddie702]

The code is quite confusing. This is a simple problem and shouldn't be so difficult to figure out. The fact that you have to bounce all around the code to find an answer shows how poorly written it really is.

 

[david]

The code is quite confusing. This is a simple problem and shouldn't be so difficult to figure out. The fact that you have to bounce all around the code to find an answer shows how poorly written it really is.

McGraw Hill's commentary makes the comment that the compressor motors are cooled by the refrigerant in contrast to the motors in Article 430. They operate at otherwise overload conditions if they weren't being cooled.

So the commentary is saying you have article 440 modify some of the article 430 requirements.

When 440 says things like not to exceed and not to round up things like that

The motor is already operating in an overload condition however it's being cooled which makes that possible.

Yes you go back and forth a bit because as said article 440 is modifying article 430

 

[david]

But the language in 430.63 is "the feeder protective device shall have a rating not less than . . ." So whatever value we calculate, that's a minimum value, and if it's not a standard size, we have to go to the next higher standard size.

I'm looking at an example in a commentary explaining this.
50 hp motor 65 fla(+25% [83 amps] )

120 amp continuous lighting load =150 amp

Combined not less than 230 amps

Using 200 amps as the max overcurrent for the 50 hp motor + 150 amp lighting load = 350 amp max overcurrent for the feeder

Same example with additional motor loads

(30 hp, 10 hp, 10 hp) + 60 hp
(40fla, 14fla, 14,fla) +83 amps

Motors 149 amps + 150 lighting combined = not less than 299 amps

50hp overcurrent 200
200 + additional motor fla=

268 max for motors + 150 amp lighting load =418 max

400 amp standard over current protection without exceeding the 418 max

I'm not making any statement on this just presenting you an example to consider

 

[wwhitney]

Using 200 amps as the max overcurrent for the 50 hp motor + 150 amp lighting load = 350 amp max overcurrent for the feeder

But 430.63 doesn't say "not more than," it says "not less than." So 350A would be the minimum OCPD for the feeder, not the maximum.

Although I'm not sure how the example got 200A as the max OCPD for a 65A FLA motor, 250% * 65A = 162.5A, next standard size up is 175A.

Cheers, Wayne

 

[david]

But 430.63 doesn't say "not more than," it says "not less than." So 350A would be the minimum OCPD for the feeder, not the maximum.

Although I'm not sure how the example got 200A as the max OCPD for a 65A FLA motor, 250% * 65A = 162.5A, next standard size up is 175A.

Cheers, Wayne

Since 350 is also a standard size it's both kind of

That's why the example presents the additional motor flas to take it to 418 max and a 400 amp standard

I think he is saying the not less than is the 200 max for the 50hp plus the 150 amp lighting load

With the 418 max

 

[wwhitney]

Since 350 is also a standard size it's both kind of

Where is the language in 430.63 imposing any maximum? I see only a minimum imposed, not a maximum.

Cheers, Wayne

 

[david]

Although I'm not sure how the example got 200A as the max OCPD for a 65A FLA motor, 250% * 65A = 162.5A, next standard size up is 175A.

He must be using the non time delay fuse 195amp 430.52 (C) (1) exception no 1

 

[david]

Where is the language in 430.63 imposing any maximum? I see only a minimum imposed, not a maximum.

Cheers, Wayne

His answer 430.63 says in effect, that the protective device for a feeder supplying a combined motor and lighting load may have a rating not greater than the sum of the maximum rating of the motor feeder protective device and the lighting load.

Because your using the max rating of the motor feeder protective devise

 

[wwhitney]

His answer 430.63 says in effect, that the protective device for a feeder supplying a combined motor and lighting load may have a rating not greater than the sum of the maximum rating of the motor feeder protective device and the lighting load.

But 430.63 doesn't say that. It says "not less than." Seems like an error in 430.63.

Cheers, Wayne

 

[david]

table 430.
52 when you do the computation using the max rating than it is a max is what he is saying

Anyhow this is your and Don's discussion so I'm backing out now

 

[david]

But 430.63 doesn't say that. It says "not less than." Seems like an error in 430.63.

 

[david]

" protection for a feeder to both motor loads and a lighting/ or appliance or other load mustbe rated on the bases of both of those loads. The rating or setting of the overcurrent device must be sufficient to carry the lighting and / or appliance load plus the rating or setting of the motor branch circuit protective device."

The NEC shall not be less than

 

[TX+ MASTER#4544]

2023 NEC

Hey, thanks, just an 'ol master electrician and your mentor.

A feeder for a hermetic refrigerant motor compressor is found in Part IV of 440.43 ...."not less than the sum of each of the following"......
Item (1) use the rated load or B/C current...whichever is greater ....of motor-compressor(s). You will select the larger of the two, ie., either the rated load current or the B/C selection, whichever is greater.

See 440.32 (1) 125% of the rated load current
See 440.32 (2) 125 % of the B/C selection

Item (2) 440.33 ...sum of full load current of all motors. None in this example.

Item (3) .25 % of highest rated motor current (amps) in the group.

I will pause here for a moment; if these refrigerant motors are three phase then you might sketch them out on paper and balance them on each phase, or each line #1 or line #2 ( group) which is what is listed in item (3).

I think they are all single phase 120/240 or 240 single phase so therefore if they are all single phase (2 wire) then they are naturally balanced with a motor on line 1 and one motor on line 2 the (group).

I calculated this to be 38 amps. What type of wiring method is this installation?
If it's NM (romex) select from the 60 degree column.T.310.16 and chose a size 8 AWG with ground (EGC).
If its conduit and type THHN, select from 75 degree column and chose 2 size 8 AWG plus an EGC if required.
The EGC can be selected from T.250.122 but cannot be determined until OCPD is calculated. But never larger than the circuit conductors.
The feeder OCPD cannot be rounded up to the next size. See 440.22 (B) (1) and 430.62 (A)

I have blabbered too long on this subject so I will let someone else add to it.

Thanks for reading.
Comments accepted.
TX+MASTER#4544

.....in Part IV of 440.43 is incorrect.... It should be 440.33 Motor-Compressor(s) With or Without Additional Motor Loads.
There is no Section 440.43. My mistake. Least some are reading.
TX+MASTER#4544