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

 

[warmer]

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).

Wayne, thank you so much for your thoroughness. I have a question regarding the example and what’s “not spelled out” because that’s exactly where I’m getting stuck.

In other forums/threads I’ve seen the 125% applied to the max demand discussed in two notable ways; Some speculated that it was there to account for a .8 PF since max demand is in KW and others speculated that it was there because the continuous vs non continuous portion of max demand are unknown and applying 125% was to ensure continuous loads were accounted for just in case.

May I ask what your thoughts are on those opinions regarding the purpose of the 125% applied to max demand?

Also, would you find a max demand calculation that does not take PF into account acceptable? (Example : 530 KW max demand x 1.25 = 663 KVA).

Lastly, if you knew that half or more of the max demand was made up of continuous loads AND most of the new load was continuous as well - would your stance be the same on not considering the continuous portion of the new load? I’m imagining numbers in the thousands here, for context.

 

[wwhitney]

In other forums/threads I’ve seen the 125% applied to the max demand discussed in two notable ways; Some speculated that it was there to account for a .8 PF since max demand is in KW

If the demand data is from the utility, is it often in kW instead of kVA? I thought when utility have demand charges, they are based on kVA rather than kW. But I have no experience with that, so perhaps that is my misconception. Or perhaps when the tariffs doesn't have demand charges, you get demand data in kW.

If the demand data is from user installed monitoring, I would think that would be set up to monitor actual current as one of the variables, so you'd have actual kVA.

and others speculated that it was there because the continuous vs non continuous portion of max demand are unknown and applying 125% was to ensure continuous loads were accounted for just in case.

As regards the quoted portion of 220.87(2) in the OP, I would not agree. The rules in 210, 215, and 230 that are about actual ampacity (as defined in Article 100) do not use a 125% factor for continuous loads. Only the rules that are about termination considerations and misuse the term ampacity to refer to an uncorrected and unadjusted table value use the 125% factor.

There is a third possibility, namely that the demand data being used does not fully reflect the expected maximum current, either because a particular load combination that comes up rarely did not occur during the monitoring period, or because of time averaging (a 15 minute interval would not fully reflect a 5 minute peak current). So the 125% factor would be a correction for this.

I guess I'd say I don't know which of the 3 explanations are correct, but that the 125% factor is sufficient to cover them all in whatever proportions are likely to occur in the typical case.

Lastly, if you knew that half or more of the max demand was made up of continuous loads AND most of the new load was continuous as well - would your stance be the same on not considering the continuous portion of the new load? I’m imagining numbers in the thousands here, for context.

No experience with anything so large, but I would have thought that if you're dealing with currents in the 1000s of amps it would be cost effective to use 100% rated equipment to reduce the required wire sizes. In which case there is no 125% factor for continuous loads.

Cheers, Wayne