125% of new continuous loads on existing service

[dummycheck]

I'm working on a project where we are adding loads to an existing service and am getting some conflicting opinions on whether we need to have a 125% multiplier at the service entrance for new loads we're adding, all of which are considered continuous.

Topology is straight forward: New feeder breaker in the service entrance feeds a step-down transformer that feeds a panel that feeds the loads.

All the branch circuit conductors and feeder circuit conductors are sized for 125% of the load they serve per NEC 210.19(A)(1)(a) and NEC 215.2(A)(1)(a).

The question comes in at the service entrance. Considering NEC 220.87, we have taken the actual demand over a 1-year period, found the maximum demand at 125% plus the new load, and made sure this does not exceed the ampacity of the service entrance. Some of the opinions I have heard is that the new load also needs to be taken at 125% because it is continuous. Others have said that this section doesn't say anything about a multiplier so you take the new load as-is.

I tend to believe that we just take the new load at 100% because the NEC is typically very specific on multipliers or references to other sections of the code that apply in a given situation. But I want to make sure I'm not missing anything here.

 

[charlie b]

I agree with you. Circuit conductors and circuit breakers are sized at 125% of the continuous load in order to reduce the potential of overheating. That has nothing to do with how much power any given load will draw from the electrical system. I don’t see anything in 220 that tells us to add 25% for continuous loads or for any other reason.

 

[wwhitney]

I'm working on a project where we are adding loads to an existing service and am getting some conflicting opinions on whether we need to have a 125% multiplier at the service entrance for new loads we're adding, all of which are considered continuous.

Where OCPD is 100% rated, you don't need a 125% continuous use factor for conductor sizing or OCPD sizing. So assuming non-100% rated OCPD, then:

230.23 and 230.31 for Overhead and Underground Service Conductors, respectively, have no 125% factor for continuous loads. Everywhere else you will encounter a 125% factor. That include Service Entrance Conductors (230.42 for conductors) and Feeders (215.2 for conductors and 215.3 for OCPD). Note that there is not an explicit 125% continuous use factor for OCPD on Service Entrance Conductors, but such OCPD is the OCPD for the Feeder on its load side, so 215.3 applies.

Thus if you are in the situation of existing Service Conductors (Overhead or Underground) with multiple sets of Service Entrance Conductors supplied by them (perhaps you are even adding some Service Entrance Conductors), then you don't need a 125% continuous use factor for evaluating the adequacy of the existing Service Conductors. But starting at the Service Entrance Conductors, you do need a 125% continuous use factor (absent 100% rated OCPD).

Cheers, Wayne

 

[jaggedben]

In my opinion you satisfy 220.87 using the single 125% factor required by that section. Adding another 125% factor because the loads are continuous is not required, because 220.87 is what you're going by.

In my opinion there is a giant conflict between Article 220 as a whole and 215.2, when it comes to feeders. But as Wayne pointed out, there is no such conflict, or less of one, for service conductors.

 

[wwhitney]

In my opinion you satisfy 220.87 using the single 125% factor required by that section. Adding another 125% factor because the loads are continuous is not required, because 220.87 is what you're going by.

If you mean another 125% factor (for continuous use) on the 220.87 calculation, I agree. But new added continuous loads would still get a 125% factor when applying Articles 210/215/230.

As to the 125% factor in 220.87, I don't necessarily see it as a continuous use factor. It could also be a scale factor to account for extrapolating from the worst case 15 minute usage observed over a fixed time period to a worst case usage estimate for the entire lifetime of the installation. Of course, in the event that worst case 15 minute usage is in fact a continuous usage (and therefore presumably the data log would show that same usage for at least 12 intervals in a row), the 125% factor would take care of that as well.

As to your comments about a conflict between 220 and other articles, it is weird that nothing in Article 220 mentions continuous loads (well, except 220.82(C)), while articles 210, 215, and 230 care about continuous loads. The only conclusion I can draw is that we need to go through Article 220 twice, or in parallel pathways, once to get total load and once to get continuous load.

That works fine for the calculation pathways (although is ambiguous with respect to the Optional pathways that use 100% of the first 10 (or 8) kW, and 40% of the rest), but leaves the question of what sort of output 220.87 gives you. The answer both of us are suggesting is that after applying the 220.87 requisite 125% factor, you treat that result as all non-continuous load.

Cheers, Wayne