Residential Electric Load – Which multiplier w/Tankless & EV Chargers? 0.75 or 0.4 or…?

[borisj]

I know how to perform a residential load calculation using either:
-(a) the Standard Method, which includes a 0.75 multiplier for fixed appliances (so, a 4500W water heater would be treated as adding 3375W to the residence's demand), or
-(b) the Optional Method, which includes a 0.4 multiplier.
Which multiplier should be used with electric tankless water heaters or EV chargers? The I've read that some AHJs require including 100% of the nameplate wattage. One Mike Holt thread reported an AHJ treating these things as continuous loads, with a 125% multiplier. Others have arbitrary additors (in one CA county, you just add 15k... regardless of nameplate wattage!). My nearby AHJs have shrugged.
Stiebel Eltron's electrical dept said: "A tankless load is no different than any other load. It falls into the 10k+40% category. It’s used very very occasionally."
Is there an NEC code section on point, or a general rule here? The choice makes extreme differences in load calculations. Thanks!

 

[wwhitney]

I would say the NEC allows either your (a) or (b), and that absent any amendment, an AHJ requiring 100% is close to overstepping.

However, obviously the tankless could draw full nameplate, and the EVSE could draw its full rating, and those could happen simultaneously. So I would say that as a reality check, you better check that the sum of the nameplates is smaller than the service size. Because with a 40% factor you could get a calculated service size smaller than the sum of the nameplates, which really could trip the main breaker.

This means that the NEC load calcs need to be updated to better handle these loads.

Cheers, Wayne

 

[borisj]

Hi Wayne, thanks for the post, I've learned a lot from you lurking over the years. It looks like NEC 625 requires EVSE to be treated as continuous / 125%. But the tankless thing remains shrouded in mystery. I totally agree that 40% seems inadequate. But what's adequate? I could just say 80% for the heck of it, but I'm still not sure if that's conservative or unsafe... it feels that arbitrary.

 

[JoeStillman]

The best tankless water heater for your electric service is a gas-fired one.

 

[borisj]

The best tankless water heater for your electric service is a gas-fired one.

Very true, but doesn't override the question of how best to include an electric tankless in an overall residential load calculation, which is something thousands of people have to do every day -- and something that, if they're doing it with the 40% multiplier, I'd say they're likely doing incorrectly.

 

[JoeStillman]

Very true, but doesn't override the question of how best to include an electric tankless in an overall residential load calculation, which is something thousands of people have to do every day -- and something that, if they're doing it with the 40% multiplier, I'd say they're likely doing incorrectly.

I'd be willing to bet that tankless water heaters and EV chargers weren't on the radar when article 220.82 was written. I'm not comfortable with the 40% factor for those either, even though the code does not forbid it. Nobody could argue that the tankless heater is a continuous load. EV chargers are another matter.