220.87(2) - Intent of “…Plus new load..”

[warmer]

220.87(2):
“The maximum demand at 125 percent plus the new load does not exceed the ampacity of the feeder or rating of the service.”

This section does not specifically say anything about application of demand factors to the new load.

Do you believe the intent is to NOT require demand factors be applied to the new loads? So no consideration for new continuous loads, for example?

EDIT: I worded that horribly. All new load at 100% or 125% for continuous new loads, etc?

 

[Gregg Harris]

Existing load study plus new load times 125% should not exceed feeder ampacity

 

[warmer]

Existing load study plus new load times 125% should not exceed feeder ampacity

I appreciate the response Greg. I think it’s the existing load time 125% though. It doesn’t say that for the new load, which is what I’m trying to figure out.

 

[wwhitney]

I appreciate the response Greg. I think it’s the existing load time 125% though. It doesn’t say that for the new load, which is what I’m trying to figure out.

Article 220 (in 2026 NEC, it's Article 120) basically never discusses continuous vs non-continuous. The output of the article is just a number called "load." However, other articles refer to the continuous load vs the non-continuous load, so you need a procedure to determine how the load value from Article 220 is broken down into continuous vs non-continuous components. I would say you have to trace the individual inputs to the computation, grouping them into continuous vs non-continuous, and then see how much of the final load value is attributable to the continuous input load values. [And in some case it's not clear how to do that, e.g. 220.82].

In particular if a feeder is protected by a non-100% rated breaker, it needs to be sized at a minimum termination ampacity (not wire ampacity) of 125% times the continuous portion of the load plus 100% times the non-continuous portion of the load. So as an example, suppose your 220.87 maximum demand comes out to 110A. You are going to add another 80A of load, of which 60A is continuous and 20A is non-continuous. Your new load is 125% * 110 + 80A = 218A, of which 60A is continuous. I'm going to say that since the maximum demand already gets a 125% factor to be converted into a load, you don't have to consider any of that load to be continuous (although that is not actually spelled out).

So if your feeder is protected by a non-100% rated breaker, that breaker would need to be at least 125% * 60A + 158A = 233A, or 250A in practice. 233A is also the minimum termination ampacity (that's the column entry in the ampacity table for your termination temperature rating, without any adjustment or correction). Your minimum wire ampacity (after adjustment and correction, starting with the column for the insulation temperature rating) would be 218A per 215.2, but would be 226A per 240.4(B) since you must use a 250A breaker.

Cheers, Wayne