Electric Hot Water Heater Operation?

CoaLen · By: **CoaLen** On: Sun Feb 13, 2011 6:23 pm

I'm beginning the process of trying to size a standby generator for my house. I will have the experts come out and offer their opinion as to what I need, but to protect myself, and because I like to learn about these things, I'm trying to do some homework upfront.
My water heater draw has me puzzled and I'm sure there's a simple explanation but I haven't been able to find it yet.
Here's the situation: I installed an electric water tank using a 2 pole 30A circuit breaker, 10 gauge 3 conductor wire. It's been running fine since November. Now that I'm trying to figure the wattage of all my appliances, I'm noticing that both the upper and lower elements are rated at 4500W. That's 9000W/240V= 37.5A .
Why aren't I tripping my circuit breaker sometimes?

For instance, when I filled the tank for the first time and had 50 gallons of cold water to heat. Could it be that both elements of a water heater are never on at the same time? In which case all I would have to account for in my generator numbers would be 4500W. But I need to understand what's really going on first.
Any of you guys know the answer to this?
-Len

lsayre · By: **lsayre** On: Sun Feb 13, 2011 6:34 pm

There are some good answers at this forum site:

http://forums.mikeholt.com/archive/inde ... 98091.html

WNY · By: **WNY** On: Sun Feb 13, 2011 6:39 pm

That's total at 240V, you have to divide it in 1/2, since you are pulling from 2 breakers. So you are pulling 18.75 amps from each breaker. that is why you do not blow it. Then I think you only use 1 element at a time once up to temperature.Since the top of the tank is warmer then the bottom, the bottom one probably is used more. I think.

CoaLen · By: **CoaLen** On: Sun Feb 13, 2011 7:08 pm

Thank you Dave, that makes sense. The 9000W load is being shared by the two 30A breakers.
Thanks for the quick answer.
-Len

Freddy · By: **Freddy** On: Sun Feb 13, 2011 7:42 pm

When a water heater calls for heat, the top element comes on. It heats the top quickly so you have a small amount of hot water fast. After the top is up to temp the top element kicks out and the bottom element kicks in. Both elements never are on at the same time.

mozz · By: **mozz** On: Sun Feb 13, 2011 8:03 pm

Somebody is getting the 240-120 voltages and currents mixed up. It's one breaker, 30 amps total at 240v. The correct wire should be 10 gauge, 2 conductor/with ground.

lsayre · By: **lsayre** On: Sun Feb 13, 2011 8:08 pm

But didn't the originator of this thread calculate the amps correctly? The formula is Watts/Volts = Amps. Therefore 9,000/240 = 37.5 Amps should be correct. It seems that the answer that both elements are never on at the same time is the best answer.

CoaLen · By: **CoaLen** On: Sun Feb 13, 2011 9:19 pm

Having a better idea of what to look for I went back over the information available on the water tank and found the following :

BRADFORD WHITE M250T6DS2 50 gal tall electric water heater:

Non-Simultaneous and Simultaneous Operation
Double element electric water heaters are designed to operate in a Non-Simultaneous or Simultaneous mode.

Non-Simultaneous Mode: Allows only one heating element to operate at a time. For example, when the tank is cold, the upper element is energized first, heating the top of the tank. Only when the upper thermostat is satisfied, the upper element is de-energized and power is directed to the lower thermostat, energizing the lower element and heating the bottom portion of the tank until the lower thermostat is satisfied. As hot water is drawn off the tank, it is replaced with cold water delivered through the dip tube to the bottom of the tank. The bottom of the tank cools, the lower thermostat will call for heat energizing the lower element. If enough hot water is drawn from the tank, the top portion of the tank cools and the upper thermostat will call for heat, de-energizing the lower element and allowing only the top element to energize until the upper thermostat is satisfied.

Simultaneous mode: allows both heating elements to operate at the same time. That is, if either thermostat (upper or lower) is calling for heat, the corresponding heating element is energized independent of the other.

General:
All models UL and CUL listed. These heaters are wired inter-locking
(Non-Simultaneous, Single Phase) 240V with two 4500W elements, unless otherwise specified.

So it appears I will only have to account for 4500W draw due to the method of element operation.

Wood'nCoal · By: **Wood'nCoal** On: Sun Feb 13, 2011 11:00 pm

Most residential electric water heaters are Non-Simultaneous.

I'm On Fire · By: **I'm On Fire** On: Sun Feb 20, 2011 9:35 pm

This is why I never tied my hot water heater to my generator. I couldn't figure out the draw either.

Now that it's explained here I'll reconsider tying it to the generator.

Electric Hot Water Heater Operation?

Electric Hot Water Heater Operation?

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