dwelling Service entrance calculations

flatlander · Jul 27, 2015

Hi, I have a customer who is building a new home that will be all electric. They are empty nested with the occasional visitors
He plans on installing tankless water heaters 1 for each bath , kitchen and washer outdoor bar. So 6 water heaters. 46,000w

How do we do the load calc. It is unlikely all will be used at the same time?

However it could happen.

augie47 · Jul 27, 2015

flatlander said:
Hi, I have a customer who is building a new home that will be all electric. They are empty nested with the occasional visitors
He plans on installing tankless water heaters 1 for each bath , kitchen and washer outdoor bar. So 6 water heaters. 46,000w

How do we do the load calc. It is unlikely all will be used at the same time?

However it could happen.

220.60 gives you a bit of wiggle room but since they are not electrically interlocked it would be a local AHJ question, IMO.
Considering most inspectors don't take into account the customers lifestyle as the occupancy may well change, I feel like you will need to build a service based on full load, but that My opinion.

luckylerado · Jul 27, 2015

I have to agree, I think you are stuck using the nameplate and treating them as fixed appliances in you service calc. I am unaware of a demand factor specifically for tankless water heater. I could absolutely realistically conceive of a situation where all of these are on at the same time.

Incidentally, I have not ever heard someone say they are satisfied with these. I have heard some good comments about the gas type tankless but never anything good about the electric type

iwire · Jul 27, 2015

augie47 said:
220.60 gives you a bit of wiggle room but since they are not electrically interlocked it would be a local AHJ question,

Which makes me think that if you did electrically interlock them the NEC would allow the calculation to be done using the highest load the interlock scheme would allow.

Smart $ · Jul 27, 2015

iwire said:
Which makes me think that if you did electrically interlock them the NEC would allow the calculation to be done using the highest load the interlock scheme would allow.

Seems the more prudent option would be installing fewer heaters and just piping their outputs to the common locations.

iwire · Jul 27, 2015

Smart $ said:
Seems the more prudent option would be installing fewer heaters and just piping their outputs to the common locations.

Kind of takes away from the instant part of the deal but mostly it takes work away from the electricians so I would not suggest it.

Smart $ · Jul 27, 2015

iwire said:
Kind of takes away from the instant part of the deal but mostly it takes work away from the electricians so I would not suggest it.

Not suggesting fewer as preferred solution, but interlocking would absolutely kill the instant part for one that is interlocked out from concurrent use, too.

As far as work goes, just build as desired without little or no concern for reducing demand. If owner wants 6 heaters, give him 6 heaters and let the rest fall where it may... :angel:

flatlander · Jul 27, 2015

Thanks everyone.
Looks like 100% nameplate it is.
That's what I thought. I thought I'd double check before I sold a 400 amp service to the new home owner.
Looks like lots of #8 and #6 on this one

kwired · Jul 27, 2015

iwire said:
Kind of takes away from the instant part of the deal but mostly it takes work away from the electricians so I would not suggest it.

And may require higher capacity unit anyhow. It takes a specific amount of heat to raise the water a specific temperature at a specific rate of flow. Put same rate through one heater that was previously flowing through 4 different heaters and raise water same temp, you will need about 4 times larger heater. If the four individual units are oversized some then maybe you can get away with some lesser value on a single heater though.

I think you also may have more issues with regulating water temp when there is low flow rate on a large unit.

iwire · Jul 28, 2015

kwired said:
And may require higher capacity unit anyhow. It takes a specific amount of heat to raise the water a specific temperature at a specific rate of flow. Put same rate through one heater that was previously flowing through 4 different heaters and raise water same temp, you will need about 4 times larger heater. If the four individual units are oversized some then maybe you can get away with some lesser value on a single heater though.

I think you also may have more issues with regulating water temp when there is low flow rate on a large unit.

All true if they were going to be used at the same time. The premise was they would not be used at the same time.

romex jockey · Jul 28, 2015

220.82 B 3 D allows for H2O heat under general loads via the optional route

~RJ~

Smart $ · Jul 28, 2015

romex jockey said:
220.82 B 3 D allows for H2O heat under general loads via the optional route

That's likely the calculation route which will provide the most reduced demand load without any special consideration by the AHJ or add-on interlocking equipment.

cpinetree · Jul 28, 2015

luckylerado said:
Incidentally, I have not ever heard someone say they are satisfied with these. I have heard some good comments about the gas type tankless but never anything good about the electric type

This X2!!

kwired · Jul 28, 2015

flatlander said:
Hi, I have a customer who is building a new home that will be all electric. They are empty nested with the occasional visitors
He plans on installing tankless water heaters 1 for each bath , kitchen and washer outdoor bar. So 6 water heaters. 46,000w

How do we do the load calc. It is unlikely all will be used at the same time?

However it could happen.

iwire said:
All true if they were going to be used at the same time. The premise was they would not be used at the same time.

If those occasional visitors are the kids/grandkids - they will be used at same time, just not on a daily basis. That is also the time the HVAC and the kitchen loads will be higher then usual. Heating load maybe a little reduced because of excess heat created elsewhere though.

I often have wondered if a storage type heater set at a low temperature is the way to go, then you can use smaller instant water heaters to raise the water just 10, 15 maybe 20 degrees instead of 50, 60 maybe even 70 degrees .

iwire · Jul 28, 2015

kwired said:
If those occasional visitors are the kids/grandkids - they will be used at same time, just not on a daily basis.

And if those visitors are dirty transients the showers won't be used at all.

See we can make stuff up all day ..........

augie47 · Jul 28, 2015

romex jockey said:
220.82 B 3 D allows for H2O heat under general loads via the optional route

~RJ~

Good point !

220.82(B)(3)(d) for "optional" and 220.53 for "standard" will help on your calculations.
Looks like one of those areas where the "optional method" might be beneficial.

kwired · Jul 28, 2015

iwire said:
And if those visitors are dirty transients the showers won't be used at all.

See we can make stuff up all day ..........

Maybe they host a family or class reunion and several show up with their RV and need a place to plug it in and all want a 50 amp supply outlet.

OP better just install a capabilities to handle at least a megawatt just in case.

We are trying to make this job more profitable for OP

flatlander · Jul 28, 2015

Yea.....what if.

dwelling Service entrance calculations

flatlander

Senior Member

augie47

Moderator

luckylerado

Senior Member

iwire

Moderator

Smart $

Esteemed Member

iwire

Moderator

Smart $

Esteemed Member

flatlander

Senior Member

kwired

Electron manager

iwire

Moderator

romex jockey

Senior Member

Smart $

Esteemed Member

cpinetree

Senior Member

kwired

Electron manager

iwire

Moderator

augie47

Moderator

kwired

Electron manager

flatlander

Senior Member