Panel Overcurrent Protection Calculations

[dvcraven0522]

I have a panel that has a design of 370 amperes that is calculated per the NEC demand factors. This is a main lug only panel fed from a main distribution board. Is this circuit breaker required to a 100% rated. This is for a commercial store with a kitchen and bar areas.

I assume the thinking is most CB are not to be loaded more than 80% of its rating. Although I think that has been removed from the code. Your thoughts are most appreciated.

Dan Craven

 

[electrofelon]

I have a panel that has a design of 370 amperes that is calculated per the NEC demand factors. This is a main lug only panel fed from a main distribution board. Is this circuit breaker required to a 100% rated. This is for a commercial store with a kitchen and bar areas.

I assume the thinking is most CB are not to be loaded more than 80% of its rating. Although I think that has been removed from the code. Your thoughts are most appreciated.

Dan Craven

If you have done the article 220 calculations correctly, that already factored in any continuous loads so there is no need for any further adjustments.

 

[wwhitney]

For the OP, a 400A breaker is rated for 400A of non-continuous load. So you need to determine what fraction of your 370A load is continuous.

If you have done the article 220 calculations correctly, that already factored in any continuous loads so there is no need for any further adjustments.

How's that? Article 220 doesn't mention continuous loads (except for 220.82(C)(6)), so the computation gives you a single load value, 370A in the OP's case. You need to do a second calculation in order to determine what fraction of that load is continuous, and it seems to me some of the details of how to do that computation are unspecified.

Cheers, Wayne

 

[dvcraven0522]

The loads consist on lighting, power (receptacles & HVAC), and kitchen equipment where I used the demand factor of .65.

 

[infinity]

As electro stated you just use the load from the calculation. At 370 amps a 400 amp panel is code compliant. The 80% rule is for continuous loads but you've already accounted for that in the calculation.

 

[wwhitney]

. . . continuous loads but you've already accounted for that in the calculation.

Again, how's that?

If the actual lighting load (sum of fixture ratings) were 160A out of the 370A (obviously unlikely) and is considered a continuous load (seems plausible to me), then the minimum (non-100% rated) OCPD size for all of that load would be 410A.

Cheers, Wayne

 

[electrofelon]

How's that? Article 220 doesn't mention continuous loads (except for 220.82(C)(6)), so the computation gives you a single load value, 370A in the OP's case. You need to do a second calculation in order to determine what fraction of that load is continuous, and it seems to me some of the details of how to do that computation are unspecified.

Cheers, Wayne

I admit the code is rather unclear and convoluted on this. Article 220 doesn't specifically address continuous loads. For some reason they put those under the service, feeder, and branch circuit sections. But ultimately they will be figured in, and it seems most logical to figure it in as you do your branch circuits. Why or even how would you do it any other way?

 

[tortuga]

I think Wayne's question can be summed up as "can you apply a demand factor to a continuous load"

220.40 General. The calculated load of a feeder or service shall
not be less than the sum of the loads on the branch circuits sup-
plied, as determined by Part II of this article, after any applicable
demand factors permitted by Part III or IV or required by Part
V have been applied.

220.12 and 220.42 indicate yes for continuious lighting but establish a floor.

 

[wwhitney]

I admit the code is rather unclear and convoluted on this. Article 220 doesn't specifically address continuous loads. For some reason they put those under the service, feeder, and branch circuit sections. But ultimately they will be figured in, and it seems most logical to figure it in as you do your branch circuits. Why or even how would you do it any other way?

I don't follow you. Are you saying that for the load calculations, you use as inputs the minimum ampacity of the branch circuits? So that 25% extra for each continuous load is baked it at the beginning and percolates upwards to the final answer?

That's not the approach used in Annex D examples D3 and D3(a).

Cheers, Wayne

 

[tortuga]

That's not the approach used in Annex D examples D3 and D3(a).

Cheers, Wayne

If the OP calculation was done like example D3 i'd say he's fine:

Minimum Size Feeder (or Service) Overcurrent Protection​	VA​	​
(see 215.3 or 230.90)​	​	​
Subtotal noncontinuous loads​	12200​	​
Subtotal continuous loads not from table 220.12 at 125% (7200 VA × 1.25)​	9000​	​
Subtotal of continuous loads with 125% already included​	9120​	​
Total​	30320​	​
​	30320 VA ÷ 240 V = 126 A​	​
The next higher standard size is 150 A (see 240.6)​	​	​

 

[wwhitney]

If the OP calculation was done like example D3 i'd say he's fine:

Certainly if that's how the OP got 370A, there's no problem. But the way the OP is written, I would assume that 370A doesn't include any 125% factor for the continuous portion of the load.

Example D3 makes the distinction between the load calculation and the minimum OCPD calculation. Only the latter includes the 125% factor for continuous loads.

Cheers, Wayne

 

[tortuga]

Example D3 makes the distinction between the load calculation and the minimum OCPD calculation. Only the latter includes the 125% factor for continuous loads.

Cheers, Wayne

I see it as building up to two conclusions
1) Minimum Size Feeder (or Service) Overcurrent Protection
and
2) Minimum Size Feeders (or Service Conductors) Required (including panel busbar)