demand water heater

[JDB3]

I am wiring an 840 or so square foot garage apartment. Trying to figure the load, I know, go to Mike's free stuff for help. BUT the thing that is throwing me is that they plan to use a "demand water heater", from their existing living living quarters that has a rating of 75 Amps. New apartment has 5 Kw heat with 2 ton heat pump, washer, dryer, range, shop area with 3 circuits plus 2 20 amp 220 volt saw & vacuum system. They also want to feed their existing quarters from this garage apartment. Main thing that is throwing me is the 75 AMP demand water heater! Does this need to be figured @ 100%? :? I had just originally planed on a 200 amp service. Thanks.

 

[renosteinke]

I am a little confused ...

They want to supply the garage with hot water from the main house, but they also want to 'feed' the house from the garage?

Maybe you mean the water heater will be in the house, while the service is at the garage? If that's the case, I suggest you treat it as two services at the drop.

The PoCo will have to size for the larger combined load, but after the meter you can split it into two panels. This will let you use a diversity factor for the service drop. This also works if you actually set up two meters.

You'll have to do separate load calcs for the house and for the garage, then add the totals and apply the diversity factor.

You can't really avoid 100% for the heater, since the thing will need all of that when it's running. It's not as if that load is spread over time - the thing is either 'on' or 'off.' Just like the air conditioner - and it's quite possible that both will run at the same time.

 

[JDB3]

Sorry for my wording, they plan to take the demand water heater from existing living quarters & use it in the new dwelling. Living quarters above garage/work shop. Also feed power from new dwelling to their existing quarters.

 

[Dennis Alwon]

IMO the demand water heater is figured at 100% for the initial calculation and then you use the demand factor for the totals. I have a similar situation with a steam generator that is rated 24kw. The darn thing will rarely be used but I have to figure it in

 

[renosteinke]

Your solution is, I repeat, to split the services.

That way, your calculations might show - just inventing numbers here - 140A for the main house and 65A for the garage .... meaning, most likely, a 200-A service and a 100-A service. The service feed would be sized for 200 amps, once you allow for 'diversity.'

Sounds a lot easier than jumping up to a 320/400-A service.

Feed it all from one panel, and your calcs would be 205A, which leads to lots of up-sizing. It also means that the 'main' will kill both buildings. Maybe not such a good design.