Service calculations with continuous loads

[kmh]

In all the service calculation examples I've seen, including Mike Holt's articles and videos, none ever applied 125% to continuous loads for purposes of calculating the service size. Fixed electric space heating, for example, is defined in article 424 as a continuous load, yet that is never taken into account when calculating service size. I generally use 220.80 optional method for sizing services.
I've done a lot of research to justify this practice, but have found very little that gets to the point. Can anyone explain why continuous loads don't matter in sizing services?

 

[david luchini]

In all the service calculation examples I've seen, including Mike Holt's articles and videos, none ever applied 125% to continuous loads for purposes of calculating the service size. Fixed electric space heating, for example, is defined in article 424 as a continuous load, yet that is never taken into account when calculating service size. I generally use 220.80 optional method for sizing services.
I've done a lot of research to justify this practice, but have found very little that gets to the point. Can anyone explain why continuous loads don't matter in sizing services?

125% of continuous loads is used to determine the minimum ampacity of service entrance conductors before the application of any adjustment or correction factor. See 230.42(A).

Fixed electric space heating is not generally considered a continuous load, as it is thermostatically controlled. Article 424 says fixed electric space heating shall be considered continuous load for the purpose of "branch circuit sizing."

 

[kmh]

125% of continuous loads is used to determine the minimum ampacity of service entrance conductors before the application of any adjustment or correction factor. See 230.42(A).

Fixed electric space heating is not generally considered a continuous load, as it is thermostatically controlled. Article 424 says fixed electric space heating shall be considered continuous load for the purpose of "branch circuit sizing."

Thanks. But I still need to provide some backup to an electrician as to why the continuous portion of the load doesn't affect the service disconnect breaker sizing.

 

[electrofelon]

Article 424 says fixed electric space heating shall be considered continuous load for the purpose of "branch circuit sizing."

Interesting, I never really noticed that. So does that mean we treat it is continuous for branch circuit sizing but not feeder or service sizing?

 

[david luchini]

Thanks. But I still need to provide some backup to an electrician as to why the continuous portion of the load doesn't affect the service disconnect breaker sizing.

Can't help you with that one, other than to say that the example D3 in Annex D shows the Service Overcurrent Protection calculation using 125% of the continuous loads. But there's nothing in the text of the Code that spells that out. Maybe they just expect you to use some common sense. (E.g., in example D3 what would be the logic of requiring #1/0 service conductors but allowing a 125A service OCPD?)

 

[kmh]

Service calculations with continuous loads

Interesting, I never really noticed that. So does that mean we treat it is continuous for branch circuit sizing but not feeder or service sizing?

It applies to branch and feeder circuits, and to service entrance conductors (230.42).

 

[david luchini]

Interesting, I never really noticed that. So does that mean we treat it is continuous for branch circuit sizing but not feeder or service sizing?

Yes. Of course nothing prohibits you from treating it as continuous load for feeder or service sizing.

 

[kmh]

Can't help you with that one, other than to say that the example D3 in Annex D shows the Service Overcurrent Protection calculation using 125% of the continuous loads. But there's nothing in the text of the Code that spells that out. Maybe they just expect you to use some common sense. (E.g., in example D3 what would be the logic of requiring #1/0 service conductors but allowing a 125A service OCPD?)

Actually, you did help me with that one. Example D3 states that 125% does apply to the Service OCPD. So the electrician is right, and I'm wrong.
The examples I referred to originally never specifically mentioned any continuous load, but they did include fixed electric space heating. 424.3(B) says that is a continuous load. So I thought it odd that they didn't add 25% of the continuous load to size the service OCPD. Someone pointed out earlier that it's not continuous because it's thermostatically controlled. I'll have to find where that's supported in the code, and why it would override 424.3(B)

 

[david luchini]

Actually, you did help me with that one. Example D3 states that 125% does apply to the Service OCPD. So the electrician is right, and I'm wrong.

Glad I could help, but they Annex is not part of the Code, just informational material.

The examples I referred to originally never specifically mentioned any continuous load, but they did include fixed electric space heating. 424.3(B) says that is a continuous load. So I thought it odd that they didn't add 25% of the continuous load to size the service OCPD. Someone pointed out earlier that it's not continuous because it's thermostatically controlled. I'll have to find where that's supported in the code, and why it would override 424.3(B)

424.3(B) does NOT say that fixed electric space heat is a continuous load. 424.3(B) says that fixed electric space heating shall be CONSIDERED a continuous load for the purpose of BRANCH CIRCUIT sizing.