Dual Instant Water Heater

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tshephard

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Read all that traffic on wire size, and got me to questionning some things.

I'm to install 2, 120 volt, 3500 watt IWHeaters.
Seems like right at 30 amps. 125 % gets me 37.5 amps. #10 THHN copper cannot be protected above 30 amps (240.4D), so #8 THHN (55 amps) protected at 40 amps. #10 EGC (from 250.122)
By using a multiwire (Phase A, B, + one neutral)circuit to a junction box before splitting of to the 2 loads, I stay at 3 (or less) current carrying conductors.
Dual breaker with tied handles.
Am I working this out right?
Are these type of multiwire circuits more popular than I've seen (industrial, never)?

Thanks

[ December 05, 2003, 12:56 PM: Message edited by: tshephard ]
 
Re: Dual Instant Water Heater

The instant water heaters that I've seen adds for do not have a tank. Do these have a tank. If not
422.13 does not apply. I question if these heaters are considered continuous.
 
Re: Dual Instant Water Heater

No, they could almost not be continuous (without failure or sig. leak), but they may be at the same time.
The feeding equipment is SD QO, which I beleive is rated non-contin., therefore 125%.
Right, wrong?
 
Re: Dual Instant Water Heater

The rating for a circuit is 100% of the non-cont plus 125% of the cont. If there is no continuous
load then requirement for the 125% is not necessary. Continuous load is an load that is online for 3 hrs or more.You need to run this by the inspector.
 
Re: Dual Instant Water Heater

These are tankless heaters with an internal flow switch.
I use the 125 % because my understanding is:
Unless the entire circuit (including the supplying breaker) is marked "Continuous Use", I have to use the 125% over-safety factor. Since it is a home-type breaker (SD QO, and I have not bought it yet), I (maybe wrongly) assume the breaker will not say continuous use and I must do 125%. I believe I read here somewhere that the extra wire size help cools the breaker.
If I am totally misunderstanding this, straighten me out, with citations.
 
Re: Dual Instant Water Heater

Bob

Reading 422.13, I am not too sure that it says a tank is required for the section to be applied. It designates a capicity of 120 gal or less. Are these tankless units rated in capacity? What is your take on this, I am curious?

Pierre
 
Re: Dual Instant Water Heater

Pierre
The ads I have seen show the water flowing over the heating units. No water is stored in a tank.
422.13 uses the term "fixed storage" and I took that to mean it had some sort of tank to store the water. These units are said to be moe efficient because you do not have to store the water. There is no heat lost thru the tank during periods of no use. I do not know how much saving you would see.

TShepard
There are breakers rated continuous. The code defines continuous as load in use for 3 hours or more. There standard breakers are not rated for continuous duty but they can handle their rating for short periods. A 30 amp breaker can handle 30 amps for a short time and not trip. You would have to look at the curves to get the time. I would say that an hour or less would not trip the breaker. The increase wire size does act as heat sink and helps to cool the breaker.
 
Re: Dual Instant Water Heater

All circuit breakers are rated for 100% loading for periods not exceding 3 hours, after 3 hours they are rated for 80% loading. This should not be a surprise, as these are the same loading and time frames as used in branch circuit conductor sizing.

In my opinion, only if the heaters have been sized and are intended to run for a period greater than three hours will you need to use 125% of FLA in your calculations.
 
Re: Dual Instant Water Heater

Thanks for straightening me out. I have an industrial background, where everything was continuous.
These things are the size of a novel, have an internal flow switch, also come with a group of inserts to slow the water flow down. Basically a U tube with both sides passing through electric elements.
I like the 100% a lot better. Since they should only ever be momentary (hand washing) the smaller breakers should be able to trip for a major leak or the faucet left on.
 
Re: Dual Instant Water Heater

The tankless jobs i seen were 240 volt ,3 elements at 8 kw each 24 kw total.The poco hates them and it sure messes up the load calc on a home.So are we saying that #10 would actually be ok ? I think i would go over that to #8 and 40 amp breaker to avoid service calls.
 
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