[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.

 

[Electric_Light]

No.

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.

 

[Anti-wingnut]

cause it to operate at 75% output.

Huh?

208/240= 86%

 

[Speedy Petey]

Huh?

208/240= 86%

HUH?

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

4500 x 75% = 3375 watts.
Close enough.

 

[Speedy Petey]

Try ordering a 208v heater.

Agree. :thumbsup:

Just replace the elements if the heater is existing.

 

[480sparky]

Huh?

208/240= 86%

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.

...........Just replace the elements if the heater is existing.

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.

 

[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. :shifty:

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.

 

[Speedy Petey]

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.

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?

 

[480sparky]

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?

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.

 

[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]

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?

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.

 

[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]

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?

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]

.......high current systems to be more efficeient then a 2 phase system. .........

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

 

[Inner10]

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

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....