Are you sure the tank isn't 2 x 3000W elements? As far as I know, all residential hot water tanks alternate the upper and lower elements. I just rewired a 50US gallon tank for a customer that had 2 X 3kW elements. You need 240V, as it won't work on 120V. You need a double pole 20A 240V breaker and 12/2 cable, and you need to protect the cable from the tank to the panel or a physically safe zone with 7/16" flex.

The heating elements would produce approximately 1/4 as much heat at 120 volts as at 240V. That probably isn't enough for reasonable performance.

The elements say 6000 watts @ 240 Volts and 4500 watts @ 208 Volts.
It was some kind of special heater the supply house gave me a deal on, perhaps it's for commercial use, I don't know I'm a plumbing contractor and we use mostly gas around here.
And they do alternate.
I have two 20 amp 120V breakers used together for a 240V hot tub pump. So as I understand it these should work, perhaps they should be pinned together ?
I assume I will be giving the elements less power than they can use @ 20 Amps ?
It will heat slower but just as efficiently correct ?

If they are 6000W elements, you will need a 40A 240V circuit on #8 wire.

Hound, you are all mixed up.

The amperage of a circuit does NOT limit what a load can draw. A load will draw the same regardless if it is fed by a 15A circuit or a 60A circuit. The only thing that can vary a load is the voltage applied.

Your 6000W heater will draw 25A @ 240v. This will overload the wiring and trip a 20A breaker in short order, albeit not immediately.
At 120v, which is ludicrous to even attempt, it will draw 50A and will trip a SP 20A breaker pretty much right away.

Your tub circuit WILL NOT work!

You need a 240v-40A circuit for this unit.
DO NOT Rube Goldberg this one and even consider anything else!

billhart is correct about the heat output at 120 volts. If the largest element is 6000 watts @ 240 volts, it will be 1500 watts @ 120 volts. That requires 12.5 amperes at 120 volts. The amperage limit for this usage for a 15 ampere circuit is 12 amperes, so this application needs a 20 ampere circuit. Powering the heater at 120 volts will do no harm at all nor pose any danger. Given enough time, the water probably will be at full temperature of the thermostat setting. But it will be slow to reach that temperature.

My typing is with two fingers, so any other response within 5 minutes of mine was posted while I was typing and was not seen by me until after I posted. The resistance of an electrical heating element is a fixed value. For an element rated 6000 watts at 240 volts, the resistance is 9.6 ohms.

Petey, how do you get 50 amps at 120? If it drew 50 amps at 120, it would produce the full 6000 watts and heat the water just as fast as at 240 volts would have.

At 9.6 ohms of resistance, it will only draw 1/2 as much amperage, or 12.5 amps, and produce 1500 watts of power.

I have my share of instances where the typing fingers get ahead of the mind. Petey is seldom wrong, so I do not chastise him in this instance.

O.K. now I'm totally confused.
I have two 120V 20A breakers that are used together to run an unused hot tub pump, they are joined together in one romex cable 12/2 w/ ground, there is no neutral used.
Can I use this to run these W/H elements ?
Or because they are rated @ 6000 Watts will they draw too much juice and trip the breaker ?

No...if the 120 circuit is fed from different legs it will deliver 240, but still most likely trip the breaker given the load of the water heater.

If they don't come from different legs, nothing will happen (no breaker blow, but no heat either).

Gentlemen, How can I make this work with the material at hand ?
I am getting conflicting answers here.
Do I need to change the elements to a lower wattage to make em work 240V 20A ?
Or can I use one 20A breaker w/ 120V ?
It does not matter if it takes 10 hours to heat the 50 gallons.

Joe, you are correct. My mistake. I was thinking of a comparable unit rated for 120v, which this one is not. Good catch.

Hound: If you've already got the 12/2 cable and two 20A breakers, I'd change the 2 elements to 3kW and be done with it. That way your $ outlay will be minimal, and you'll still have lots of quick hot water. Think of resale and future aggravation if you decide to sell the house.
Yes, the two separate 20A breakers must be joined together with a tie bar, and installed on opposite legs in the panel. If you don't know, that means one breaker must be on one side of a plastic divider on the right or left bus, the other breaker right above or below it but with the plastic divider between the two breakers.

Before I'd start modifying the water heater and trying to make the appliance fit the wiring on hand, I think I'd bite the bullet and run a 30A 240V circuit (#10-2 AWG w/g) using a proper 30A double-pole breaker.

Rube Goldberg might not like that approach, but you'd have what amounts to "standard" hot water wiring in place when that hot water heater gives up the ghost and, in the meantime, it should cover the 6000W demand.

(Yeah, I know there's no such thing as "standard hot water wiring", but most replacement heaters in this size range are likely to draw 5300-5600W or so.)

Hi, PETEY:

I think we're in agreement about the general approach, but I'm curious about why you recommended a 40A circuit. Wouldn't 30A do the trick here, or am I wrong about that?

Putting smaller wattage elements in the heater will not save significant money, it will just make it slow to re-warm after you draw from it (unless they are so small it never gets up to the thermostat temperature).

As heat is either carried off in the water or lost into the world, the heating elements will try to make up the loss until the thermostat is happy. This heat can be supplied by high wattage elements running for short duty cycles or smaller wattage elements running more of the time. It amounts to about the same watt-hours so the cost of the electricity is about the same.

If you want to save money, you have to insulate better or lower the thermostat.

terrible, 6000W ÷ 240v = 25 amps.
A 30A circuit can have a continuous load of 24A.
NEC 422.13 tells us that a storage type water heater shall be considered a continuous load.

A small difference I agree, but in my world code is code.

I really don't want to go to higher amp circuts because it's only a 100 amp panel and pretty maxed out with A/C, dryer and a subpanel for an addition. And running a new wire is a problem as well.
Today I was at Homecheapo and I decided to peruse the W/H isle, I bought two 240V 3500Watt elements figuring these should work w/ the 20A breakers. 8 bucks each.
Assuming I'm o.k. with the 3500 Watt elements running alternating the only part I don't get is getting them in the right phase, both breakers are above each other w/o a plastic divider ? I assume each leg of the 240V has to come from opposite sides of the panel ? Why does it run the pump now ?

If there is no plastic divider separating the phases then look at the lugs where the breakers push into the bus. One lug should be on the left side of the panel, the adjacent lug should be on the other side. If the 240V pump runs OK now, then the breakers must be correctly installed on opposite legs in the panel.

In the typical panel with 2 columns of breakers, getting opposite legs doesn't require the breakers to be in opposite columns. The bus bars are arranged so that every other breaker in a column is on opposite hot legs.

Petey wrote:

NEC 422.13 tells us that a storage type water heater shall be considered a continuous load.

Thank you for your reply and explanation.

[Sorry for the digression, houndhandler.]

It sounds to me that the person asking the questions does not understand enough about wiring to do this safely and should get someone that knows what they are doing to help.
Even if one does not get a shock or worst setting it up, there is always the fire hazard.

Based on what I read so far, if he switches the elements to 3500's and they only run one at a time, then the 12/2 wire would be just fine and this is the way I have seen many a water heater installed. The thing I did not care for is the use of two single 20 amp breakers for a 240v load. Code requires the breakers to be tied together so that both trip if one trips. If he gets the correct breaker, this should be safe.

Static

"The thing I did not care for is the use of two single 20 amp breakers for a 240v load. Code requires the breakers to be tied together so that both trip if one trips. "

Already addressed 9 posts up.

Got er figgered out now, thanks.

"It sounds to me that the person asking the questions does not understand enough about wiring to do this safely and should get someone that knows what they are doing to help."

That's what I got from this thread as well.

FWIW, I'm a plumbing HVAC contractor not an electrician.
I've rewired my whole house and everything works fine, it's just been a while and I get a little confused when we go to 240V.
I could look it up in a book or bug my electrician friend but it's easier to ask here.
I know how to run 15A and 20A 120V circuts, 3-ways, ect... don't worry I'm not gonna burn down my house.