WIre Size for subpanel as tankless WH disconnect

[DSLTech]

Hi!

Looking for help on a the correct wire size I need.

I'm installing this tankless water heater in the basement with a 70 foot run from the 200amp breaker box in the garage. I want to put in a 100 amp subpanel as a disconnect. This is all indoors through wood framing/ no conduit. The feed will pass next to ceiling insulation in the basement.

What Size/Type of feed should I get. Electrical supply store said #2/#2/#2/#4 aluminum is fine. Should I go with that?

Thanks!
DSLTech

 

[Movover]

Someone with the actual code might step in but I beleive 2/2/2/4 is only allowed 90 amps, but as a sub feed to another panel its 100 amps so you should be fine, you will need 3 double pole 40A breakers and 8ga copper for the runs to the unit from the sub panel.. Thats alot of power

 

[Norcal]

As a subfeed #2 AL is 90A.

 

[alfredeneuman]

Due to the Voltage Drop, you'd need a 1/0 Aluminum conductor to feed 100 Amps at a 70' distance

....as opposed to #1 if the sub was right next to the panel that feeds it

 

[Aceman]

I think electric tankless is a mistake, anything I've heard about them hasn't been good. Gas on the other hand, might be okay.

It wouldn't surprise me at all if that tankless blinks your lights every time it kicks on.

 

[Charles (in GA)]

Something in the oven, wife cooking on the stove, dishwasher running, A/C cranking out cold air, and you decide to take a shower. Suddenly you are in the dark.

 

[barnjunkie]

I personally think that tankless hot water heaters are a bunch of hype. My wife and I built a house a couple of years ago and the new hot water heaters are so well insulated, you can put your hand on top of them and can't feel a temperature difference from ambient.

The new faucets and appliances run such a small amount of water, we've never ran out and we only have any 40 gallon tank.

 

[barnjunkie]

If you are sure that's what you want, you probably need to check the specifications of what you are installing. Some of the newer electric tankless heaters are two stage, and require 2 separate runs of 6 awg copper wire (60amps each).

 

[Charles (in GA)]

If you are sure that's what you want, you probably need to check the specifications of what you are installing. Some of the newer electric tankless heaters are two stage, and require 2 separate runs of 6 awg copper wire (60amps each).

The OP's unit is basically three stage, as it requires three separate 40 amp double pole breakers and three pairs of #8 wire to supply it. This is due to NEC requirements that no heating device have supply circuits of more than 60 amps per circuit, and thus are limited to max loads of 48 amps per circuit (heating of water or air is considered a continuous load).

The OP's question concerned the sub feed from his main panel to his subpanel that will hold these three DP breakers to supply the water heater.

 

[jvitez]

So......

--the water heater the OP wants is a 24kW load
--water heating is considered a continuous load
--continuous loads must have the supply cable and breaker upsized at 125% of the maximum load

then doesn't he need a 125 amp feed from his service panel to a 125 amp or higher rated subpanel, as the whole subpanel is dedicated to a continuous load?

 

[DSLTech]

Re: Wire Size for subpanel as tankless WH disconnect

Obviously, the #2 AL feed is a no go...

Thanks all for the concern about my household, but I'm just a single guy in a 1100sqft 1ba/3bdr rancher and I want continuous hot shower water. Love long showers.

Now the question is: Due to code does the sub need to be 125 amp or 100?

And what gauge for that 70' run? So far we have...

Due to the Voltage Drop, you'd need a 1/0 Aluminum conductor to feed 100 Amps at a 70' distance

....as opposed to #1 if the sub was right next to the panel that feeds it

 

[alfredeneuman]

So......

--the water heater the OP wants is a 24kW load
--water heating is considered a continuous load
--continuous loads must have the supply cable and breaker upsized at 125% of the maximum load

422.13 Storage-Type Water Heaters. A fixed storage-type
water heater that has a capacity of 450 L (120 gal) or less shall
be considered a continuous load for the purposes of sizing
branch circuits.

The WH is a tankless on demand type, not a storage type with a tank, so then the circuit doesn't need to be calculated at 125%


(I used the Southwire Voltage Drop Calculator to come up with the 1/0 figure)

 

[barnjunkie]

This is not a storage type HWH, so this nec section does not apply.

 

[jvitez]

Storage type, not on-demand type. Got it! Thanks gents.