Optional calc for existing dwelling unit

[cdcengineer]

I usually avoid the optional calculation method, but on this project we have no choice.

In applying NEC 220.83(B), what do we do if we have more than four separately controlled space heating units? I'm going to take them at 100% because that's worst case scenario.

 

[charlie b]

You don't need to do that. You do count the first two items on the list at 100%. You also count the first three (i.e., less than four) separately controlled heaters at 100%. Then you make a list of everything else, including the fourth, fifth, and however many other separately controlled heaters there are. Within that list, you take 100% of the first 8KVA and 40% of the rest of that list.

 

[cdcengineer]

Thanks Charlie, that makes sense. Fortunately the calc justifies the services even when taking heaters at 100%. I just wanted to verify how to handle more than 3 heaters.

What's the consensus on optional calcs? Does this group feel good with them? I've avoided only because the high end stuff we're doing here in CO always expands during the construction phase and I like to have some breathing room while we add load after load before construction is complete.

Thx again