Installing Overwattage Element To Water Heater Legal?

Electric_Light

Many water heaters are designed for 240v and would have the right capacity if it's on 240v, but in multi-dwelling units, many are fed 208 due to service feed being coming from 208Y/120v 3phase service and cause it to operate at 75% output.

If the heater is 3kW 240v, would it be legal to install a 4kW 240v element, so that it would actually provide 3kW on 208v service?

480sparky

No.

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Electric_Light

What would you do to get the rated BTU out of a 240v water heater on a 208v installation? Would using the same kW rated heating element meant for 208v be legal?

480sparky

Try ordering a 208v heater.

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Anti-wingnut

Huh?

208/240= 86%

Speedy Petey

HUH?

A 4500W 240v element = 12.8 ohms
A 12.8 ohm element at 208v = 3380 watts

4500 x 75% = 3375 watts.
Close enough.

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Speedy Petey

Agree.

Just replace the elements if the heater is existing.

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480sparky

Uh, try 208²/240²=75.11111

Or, you could do it the long way, using Ohms law.

First, determine the resistance of the element.
R=E²/P
R=240²/4000
R=57,600/4000
R=14.4 Ohms

Now, calcuate watts using 208 volts and 14.4 ohms.

P=E²/R
P=208²/14.4
P=43,264/14.4
P=3004.4444 watts.

Now, compare 4000 watts to 3004.4444 watts. Purty close to 75% if you divide 3004.4444 by 4000.



In short, it's not a linear relationship.



Not if the heater is rated for 240v only. If it has a 4000w element for 240v and it is rated for 208v, it will be 3004.44w at 208. That will be on the label. Installing a higher wattage element will void the listing.

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Anti-wingnut

Pete and 480,

Thanks for the lesson

DuMass

Well… if the WH nameplate specifies 240V and you connect it to a 208V supply… haven’t you already violated the UL listing of the equipment?
Even if you consider plus or minus 10% for supply voltage, 208V looks like it would still be too low.
So does this open up the floodgates for other field modifications?
That will be up to you.

For some reason, these heaters get installed all the time in condos and apt buildings around here. I guess the people specifying and purchasing the equipment for these projects just don’t know any better, or care.

Electric_Light

It is rated at 240/208, but, lower output at 208v. I was basically wondering if it is acceptable to install overwattage 240v heater so that it would achieve the same output on 208v supply.

If the heater is rated at 3kW @ 240v and 2.25kW @ 208v, then installing a "4kW @ 240v" element would simply enable the heater to operate at 3kW. Though the controls would see higher current, I would guess there is enough overhead to accommodate this..?

480sparky

There is going to be a nameplate on the unit. If it states "3Kw @ 240v", then installing it on a 208v system already violates the listing.

If it states 3Kw @ 240v and 2.25Kw @ 208v, then you're stuck with a 2.25 Kw water heater.

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Speedy Petey

I meant replace the 240v 4kW elements with a 208V 4kW element. A clear and permanent label would be mandatory.
What is you take on this as far as a violation goes?

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480sparky

If the label says it can take a 4Kw/208v element, fine. If there's no 208v on the label, you don't have the correct heater.

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Williams Ex Co

Not trying to hijack this but can someone explain where the 208V comes from? I get the single leg =110v and 2 legs =220V stuff but would like to know more about the 208V.... Just wonderin'? 480? Anyone?

480sparky

It's not just simple addition. It's electrical math. It has to do with the 120° electrical angle between the phases.

Sin(120°)=0.866. 240X.866=207.84, or 208 volts.

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Williams Ex Co

480.... Uh, ok, thanks.... Doesnt sound like anything I will ever have to deal with. My electrician is on speed dial anyway! Thanks anyways!

Inner10

I'l give you a much more simplified version. In lots of commercial buildings the service is supplied by a 3 phase system instead of a 2 phase system. It allows the use of big motors and high current systems to be more efficeient then a 2 phase system. 3 phase motors are far more efficient and do not require starter windings (although some use star-delta starters to reduce initial draw).

If you look at the voltage differential between one phase and neutral in a 3 phase system you get 120 volts, if you look at phase to phase you get 208 volts. Where in a 2 phase system you would get 120 to neutral and 240 phase to phase.

A 3 phase is root3 times the line to neutral voltage.

So... the issue here is that heaters intended for a 2 phase system are being connected to a 3 phase power system that results in inadequate voltage for the elements.

Making sense? Better yet am I correct?

480sparky

Don't confuse single-phase with 2-phase. Two totally different animals.

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Inner10

It may be a canadian thing but we commonly incorrectly call split phase 2 phase.

I know its wrong but its force of habit...All i can remember from school is that 2 phase was a 4 wire system that was used many moons ago...

This is what happens when I stay up drinking and reading forums on my laptop....