Water Heater 150amp Maximum circuit code

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anthem

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Does anyone know if this is addressed in the current or new code ? I have someone wanting to put in a high recovery water heater (electric). Anyhow, apparently their is a code that limits 150amps of service into a water heater (residential code). And since water heaters must be overrated by 25% that means the max ampacity water heater that can be installed is 120amps.

This does not apply for a commercial electric water heater which is what the customer wants to put in. Apparently, we can find a water heater that takes multiple circuits for multiple groups of elements (haven't found one yet), go gas (which is not available here), or figure out if its been addressed in the new electrical code. Anyone else hit this kind of problem before ?
 
anthem said:
Does anyone know if this is addressed in the current or new code ? I have someone wanting to put in a high recovery water heater (electric). Anyhow, apparently their is a code that limits 150amps of service into a water heater (residential code). And since water heaters must be overrated by 25% that means the max ampacity water heater that can be installed is 120amps.
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That is one huge current draw. I don't know about the code on that but you may want to consider 2 or 3 standard 60 or 80 gal water heaters hooked up in series, ie hook the first water heater with the cold water into cold tap then come out of the hot tap to the next heaters cold tap then out of that one to the third one. The last water heater feeds the hot water to the faucets
 
Take a look at 422.11(F)(1). This will require that the heating elements be subdivided into groups of not more than 48-amps and fused at not more than 60-amps. Commercial (422.11 D) is even more restrictive with the maximum overcurrent rating at 50-amps. This is fairly common on electric space heating or heat pump electric heat backup units.
 
Yes, definitely quite large. The multiple water heater scenario is the one they are trying to avoid (minimizing real estate). They are willing to go to a high recovery electric but apparently there is a code restriction that places a maximum on residential water heaters which doesn't make a lot of sense. A commercial electric one at about 54kw can do about 200gph vs about 50gph for residential.

The 120amps is about a 30kw unit, which is about 2x to 2.5x what the regular residential water heaters run. Commercial electric ones on 80-120gallon ones can go to about 54kw or more. But yes, what they want to do is put a commercial electric unit in, but the limitation is not on max amp delivered to a particular panel or load, but max allowable load into a water heater (no matter what the water heater can accept).
 
haskindm IMO the code section to look at is 422.11(F)(3).

That section allows certain ASME rated water heaters and steam boilers to have elements that draw up to 120 amps and they shall not be protected at more than 150 amps.

This is commercial or dwelling unit.

I can't imagine that any listed water heater will not meet the NEC.

If they need more than 120 amps of resistance heating they can add more elements on other internal circuits.

There is no maximum total current limit. Only a limit of 120 amps to one heating circuit.
 
Bob,
Good call, I guess I didn't read far enough. This section of the code looks like such an installation would be OK. I sure wouldn't want it on my electric bill!
I guess I can go home now as I have learned something for today!
Thanks,
 
haskindm said:
I sure wouldn't want it on my electric bill!
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I agree!

I once worked on an office building that had forced hot water heating.

The original boiler was all electric, it had three 200 amp 480 3 phase feeders supplying it.

It had been abandon and replaced with a Natural gas unit......I wonder why? :D
 
Exactly. I think thats the limit. However, all the major water heater manufacturers (AO Smith, State, Bradford White) have commercial electric water heaters that can take 150, 183, 200, 240 amps. They are then broken down into heating 'elements' of usually 18kw each. So multiples of 18, 36, 54kw, etc. However, all the water heaters are all fed with one circuit and not multiple circuits. It would be a moot issue if we could feed 3 circuits (one into each group of heating elements), but that doesn't appear to be the case. It appears that they all accept one input which is fused and then internally its broken up to feed each group of heating elements. Of course, no one is willing to change that configuration as they all have their UL ratings already on them.

Obviously it would be less amperage on three phase or on 480v (1 or 3), but that isn't available. The homeowner has 800amp service and wants to reduce real estate - so we're tied to trying to do a higher recovery unit. What is hurting is the 150amp limit along with the 25% overrating required to deliver the circuit. I doubt I'll be able to get a code mod unless its addressed in an upcoming code release, but that is the limitation I'm now working with.
 
I almost installed a 28Kw tankless water heater in a home. Why "almost"?

Because I warned the homeowners that, with a single 200a residential panel with no less than two 100a sub-panel feeders (a 4-car garage/shop and a complete basement apartment), there is a risk of tripping the main breaker.

I went on to explain that the only sure cure was a second panel, complete with new meter and drop, and even though they had the only house on the transformer (rural) with a short POPC-supplied drop, they changed their minds.

Despite the distributor's assurance that the four elements would 'rarely' be on simultaneously, and because of my inability to predict the problem with 100% certainty, they chose to forgo the installation, and stick with LP for hot water.

I felt that it would have been grossly negligent on my part had I not expressed my concern about the service's capability to supply such an additional load with the existing equipment, and recommended the service upgrade with panel.
 
Try a different approach if there is not multi branch circuit kit for the water heaters you're looking at. How about a regular water heater with an insta hot in front of it. The key design question is if they have a tremendous demand of hot water at once (high GPM, 3 showers at once), or a normal flow but long lasting (one shower, but 3 people one after the other)? If the flow is not high but long lasting, an instant hot water heater will heat the water by itself continuously. These typically have two or three 40A circuits that feed them. The tank type will handle short bursts of high demand for hot water (washing machine, dish washer) as the warm (instead of hot) water from the insta hot mixes with the already hot water in the normal tank.

You'll need to do some thinking to determine the size of the tank type and the size of the insta hot. I they want to fill a hot tub and have 2 showers at once, this will be very expensive. I hope this house has at least a 400A service...
 
That might be an option if we can't figure out a better way. The multi branch kit is exactly what we want and I'm waiting to hear back from state and bradford white to see if they will do it.

The house has 800amp service so power delivery isn't the issue - its the restriction of how much power delivery to a particular water heater in one circuit that is the issue. You can deliver 6 circuits at 150amp/circuit into 120amp rated elements into one heater, but you can't deliver 151 amps. So, yeah, you can 900amps broken out into individually, but one circuit over 150 and its a no-no.
 
What kind of panelboard do you put in a house like that (an industrial one, or something normal like QO)? Is it multiple 200's, or a giant 800A panel? How big can the feeder/branch circuit breakers in the panel be?

I can envision a future home owner going to Home Depot trying to find a replacement 150 amp branch circuit breaker for his water heater.
 
I think its going into an transfer switch that has a service entrance rated cut-off. That then feeds any subpanels that you may have (like 4 x 200amp, and others)
 
That was my initial thought, why would anyone need so much hot water ???? To do it electrically is such a waste of energy LP or natural gas would save so much in the long run.

I knew a guy years ago that built his own home in the Hamptons in NY, He had 2 systems for potable water and baseboard heating both went thru a solar system that fed water heaters (electric) That where fed from a fireplace that fed the applicable items IE: faucets and baseboard heaters.His total annual cost at the time 1979 was $450.00 annually.I was impressed since the 4 family I owned was costing me almost $3,000.00 a year to heat and produce hot water.

Guess I have met allot of ingenious people over the years,I also met a guy that was an EE.He devised a system in upstate ny near Niagra falls power plant that devised a system to pick up the eddy currents from the lines that ran through his property that he then used for his own power.He refused to disclose the way he did it but there was no POCO hook up and he had all the electric he needed.They tried to get him for theft of service but he was not in contact with the power lines so that didn`t fly.All I saw was a probe on a tower leading into a shed and he didn`t let anyone in there.

Ingenuity is a great thing :)
 
I forgot to mention in my prior post that this 28Kw heater, which had four elements, was meant to be wired with either two or four circuits. There were removable jumpers between the lugs for circuits 1 and 2, and between those for circuits 3 and 4.

My intention was to run a pair of 60a feeders from the main panel to a pair of 4-space sub-panels next to the heater, and use four 30a circuits to supply the heater. I would have simply used four 30a circuits, but the main panel didn't have the space.

Added: I felt that a single 120a feeder wasn't economical, nor was trying to find such a breaker for a residential panel.
 
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