Whole house load calculation -

[jcbartlow]

So I have a question on an electric instant hot water heater that has 3 - 40a, 240 volt feeds to it for a total of 120 amps. Do you have to use the full 120 amps on the whole house load calculation? Any help would be appreciated.

 

[suemarkp]

Yes and no. If each breaker is 40A, I doubt the actual load on each circuit is 40A (probably 32A or less). But you do need to take all of the separate heater loads and sum them. I've never seen the nameplate on one of these to know if they give an overall total (e.g. 17 KW) or they just list each stage separately.

This will not be a continuous load though, so at least you don't need to take this huge number and multiply by 1.25... However, usually each branch breaker will be sized 125% (e.g. a 40A breaker will serve at most a 32A load).

Finally, if this is a dwelling and you're using the optional calculation, a demand factor will be applied just like all the other loads (40% which will cut this down a lot).

 

[Dennis Alwon]

So I have a question on an electric instant hot water heater that has 3 - 40a, 240 volt feeds to it for a total of 120 amps. Do you have to use the full 120 amps on the whole house load calculation? Any help would be appreciated.

As stated above just because the breakers are rated 40 amps does not mean the load is rated that high. I suspect that the 40 amp breaker may be only seeing 32 amps or so however you need the nameplate of the water heater and use it at 100%.

The load calculation also stated above will diminish the load a great deal. If you are using a feeder to the unit with 3 breakers at the unit then you will have to use the 100% of the nameplate for those conductors.