Sizing a Subpanel

[Little Bill]

Say you have an existing circuit ran with #10 conductors. It is a disconnect for an HVAC unit. Now you are adding a mini split right next to the unit. The existing unit has an MCA of 18.9A on a 30A breaker. The mini split has an MCA of 9A on a 20A breaker. Can the disconnect be replaced with a subpanel with breakers for each unit without upsizing the #10 conductors? I know the MCA will be under 30A, but the breaker sizes go over the 30A. Not sure what you go by in sizing the subpanel feeders, the breakers or the MCA?

 

[Eddie702]

I don' believe you go by the breakers or the mca. I think you take the actual running load of what is on the feeder x 1.25. But I am not sure and don't have my code book handy.

What are the voltages of the equipment. I assume the existing is 240 volt single phase. is the mini split 120v or 240 volt?

 

[infinity]

If you sized the larger load at 125% and the smaller loads at 100% wouldn't you still be under 30 amps?

 

[Little Bill]

If you sized the larger load at 125% and the smaller loads at 100% wouldn't you still be under 30 amps?

No, 18A x 125% = 22.5A
9A + 22.5 = 31.5A

 

[Dennis Alwon]

No, 18A x 125% = 22.5A
9A + 22.5 = 31.5A

The mca has 125% built in so you don't use it twice.

18.9 amps /1.25 = 15

15 x 1.25 = 18.9

9 amp / 1.25 = 7 amps

18.9 + 7 = 26 amps.

 

[Little Bill]

The mca has 125% built in so you don't use it twice.

18.9 amps /1.25 = 15

15 x 1.25 = 18.9

9 amp / 1.25 = 7 amps

18.9 + 7 = 26 amps.

I was replying to Rob saying to add 125% to the larger load.

 

[Dennis Alwon]

I was replying to Rob saying to add 125% to the larger load.

Yes but the load isn't 18.9 it is 15 amps after you remove the 125% that was added to the mca. So you end up with 18.9 on the larger unit

 

[augie47]

I agree with Dennis, Post #5

 

[wwhitney]

If you read the nameplate for each of the two units, they should specify what the largest motor is (typically compressor RLA). That's information you need to calculate the feeder MCA / MOCP, but whatever the values, the feeder MCA will come out no more than the sum of the MCAs = 18.9A + 9A = 27.9A.

For example, to make up some numbers, say the 18.9 MCA / 30 MOCP unit has an RLA of 8A; that means the other loads in it are 8.9A, as 18.9 = 125% * 8A + 8.9A. And the MOCP calculation works out as 250% * 8A + 8.9A = 28.9A, round up to the next standard size.

Likewise, say the 9 MCA / 20 MOCP unit has an RLA of 6A; that means the other loads in it are 1.5A, as 9 = 125% * 6 + 1.5. And the MOCP works out as 250% * 6 + 1.5 = 16.5A, round up to the next standard size.

So with those numbers, the feeder MCA would be 125% * 8 (largest motor) + 8.9 + 6 + 1.5 = 26.4A.

The language in 430.63 on the feeder OCPD is confusing to me, but it says to start by determining the MOCP for the largest motor, or 250% * 8A = 20A, already a standard size, otherwise you'd round up to a standard. Then to add to that all other loads, or 20 + 8.9 + 6 + 1.5 = 36.4A. And 430.63 says that the OCPD shall have a rating not less than that (the confusing part, I think it should be not greater than that), so 40A would be the minimum feeder OCPD size.

Cheers, Wayne

 

[Dennis Alwon]

If you read the nameplate for each of the two units, they should specify what the largest motor is (typically compressor RLA). That's information you need to calculate the feeder MCA / MOCP, but whatever the values, the feeder MCA will come out no more than the sum of the MCAs = 18.9A + 9A = 27.9A.

For example, to make up some numbers, say the 18.9 MCA / 30 MOCP unit has an RLA of 8A; that means the other loads in it are 8.9A, as 18.9 = 125% * 8A + 8.9A. And the MOCP calculation works out as 250% * 8A + 8.9A = 28.9A, round up to the next standard size.

Likewise, say the 9 MCA / 20 MOCP unit has an RLA of 6A; that means the other loads in it are 1.5A, as 9 = 125% * 6 + 1.5. And the MOCP works out as 250% * 6 + 1.5 = 16.5A, round up to the next standard size.

So with those numbers, the feeder MCA would be 125% * 8 (largest motor) + 8.9 + 6 + 1.5 = 26.4A.

The language in 430.63 on the feeder OCPD is confusing to me, but it says to start by determining the MOCP for the largest motor, or 250% * 8A = 20A, already a standard size, otherwise you'd round up to a standard. Then to add to that all other loads, or 20 + 8.9 + 6 + 1.5 = 36.4A. And 430.63 says that the OCPD shall have a rating not less than that (the confusing part, I think it should be not greater than that), so 40A would be the minimum feeder OCPD size.

Cheers, Wayne

I get what you are saying but it doesn't allow us to go to the next size breaker. We should be looking at 440.22- NO?

 

[wwhitney]

I get what you are saying but it doesn't allow us to go to the next size breaker.

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 really think this is a long-standing error in 430.63. And what it really should say is "where the feeder conductors are sized in accordance with 430.24, the feeder protective device shall have a rating not more than . . ."

We should be looking at 440.22- NO?

440.22 is under "Part III. Branch-Circuit Short-Circuit and Ground-Fault Protection," so I believe it has no bearing on the feeder OCPD sizing. Article 440 has no discussion of feeder OCPD sizing that I see, which leaves us to look to Article 430 for this application.

Cheers, Wayne

 

[don_resqcapt19]

Say you have an existing circuit ran with #10 conductors. It is a disconnect for an HVAC unit. Now you are adding a mini split right next to the unit. The existing unit has an MCA of 18.9A on a 30A breaker. The mini split has an MCA of 9A on a 20A breaker. Can the disconnect be replaced with a subpanel with breakers for each unit without upsizing the #10 conductors? I know the MCA will be under 30A, but the breaker sizes go over the 30A. Not sure what you go by in sizing the subpanel feeders, the breakers or the MCA?

Assuming that there is a 30 amp OCPD at the supply end of the 10s, I don't see any issues.

 

[LarryFine]

Say you have an existing circuit ran with #10 conductors.

NM or something else?

 

[TX+ MASTER#4544]

Say you have an existing circuit ran with #10 conductors. It is a disconnect for an HVAC unit. Now you are adding a mini split right next to the unit. The existing unit has an MCA of 18.9A on a 30A breaker. The mini split has an MCA of 9A on a 20A breaker. Can the disconnect be replaced with a subpanel with breakers for each unit without upsizing the #10 conductors? I know the MCA will be under 30A, but the breaker sizes go over the 30A. Not sure what you go by in sizing the subpanel feeders, the breakers or the MCA?

2023 NEC
So you want to make the existing 10AWG a feeder for the 2 refrigerant motor compressor?
TX+MASTER

 

[Dennis Alwon]

2023 NEC
So you want to make the existing 10AWG a feeder for the 2 refrigerant motor compressor?
TX+MASTER

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

 

[wwhitney]

Assuming that there is a 30 amp OCPD at the supply end of the 10s, I don't see any issues.

How do you read 430.63? The literal wording says that the minimum OCPD rating is given by a computation that will come out to more than 30A.

In contrast, if each of the HVAC units contains only motor loads, then 430.62 applies. 430.62(A) provides a maximum OCPD size allowed when the conductors are sized via 430.24 as we are doing. The exact value of that calculation requires knowing the maximum motor size in each unit, but we can already infer given the data in the OP that the calculation is going to come out to at least 35A. So in this case a 35A OCPD would definitely be allowed, possibly higher.

Cheers, Wayne

 

[don_resqcapt19]

NM or something else?

NM is 30 amp wire and a 30 amp feeder is fine for that load

 

[don_resqcapt19]

How do you read 430.63? The literal wording says that the minimum OCPD rating is given by a computation that will come out to more than 30A.

In contrast, if each of the HVAC units contains only motor loads, then 430.62 applies. 430.62(A) provides a maximum OCPD size allowed when the conductors are sized via 430.24 as we are doing. The exact value of that calculation requires knowing the maximum motor size in each unit, but we can already infer given the data in the OP that the calculation is going to come out to at least 35A. So in this case a 35A OCPD would definitely be allowed, possibly higher.

Cheers, Wayne

The language in 430.62 is "not greater than". I see no code issue or real world issue using the 30 OCPD for two MCAs that total 27.9 amps.

 

[wwhitney]

The language in 430.62 is "not greater than". I see no code issue or real world issue using the 30 OCPD for two MCAs that total 27.9 amps.

Well, the 9 MCA / 20 MOCP unit could easily be drawing 5A when the 18.9 MCA / 30 MOCP unit starts up. So if one is inclined to use the full 30A branch OCPD for such a 18.9 MCA / 30 MOCP unit, then I would think it would be logical to use a 35A OCPD on the feeder. Conversely, if you'd be happy with a 25A branch OCPD on the 18.9 MCA / 30 MOCP unit, then I agree there's little reason to go beyond 30A OCPD on the feeder.

As to the 430.62 computation, I get that the largest motor in each unit must be at least 5A, otherwise the calculated MOCP values would be smaller than given in the OP. Based on that, I get that the computation required by 430.62 will give you at least 35A.

Cheers, Wayne

 

[don_resqcapt19]

Well, the 9 MCA / 20 MOCP unit could easily be drawing 5A when the 18.9 MCA / 30 MOCP unit starts up. So if one is inclined to use the full 30A branch OCPD for such a 18.9 MCA / 30 MOCP unit, then I would think it would be logical to use a 35A OCPD on the feeder. Conversely, if you'd be happy with a 25A branch OCPD on the 18.9 MCA / 30 MOCP unit, then I agree there's little reason to go beyond 30A OCPD on the feeder.

As to the 430.62 computation, I get that the largest motor in each unit must be at least 5A, otherwise the calculated MOCP values would be smaller than given in the OP. Based on that, I get that the computation required by 430.62 will give you at least 35A.

Cheers, Wayne

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

Yes, it might be logical, but not required by the code. It is common to see the breaker smaller than the maximum permitted OCDP.

It is my opinion that the 30 amp breaker is not likely to trip under the starting load of the larger unit with the smaller unit already running.

I don't see either a code issue or a real world issue using a 30 amp breaker for the feeder OCDP.