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Old 12-01-2010, 04:42 PM   #1
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wiring question


I've been thinking of installing a solar water heater on my house. Before doing that I wanted to determine exactly how much electricity I was using at my electric water heater, to determine if it would be worth it, and ultimately determine how much power I would be saving..

So, I bought an hour meter http://us.element-14.com/jsp/search/productdetail.jsp?SKU=78H0980&CMP=AFC-GBE14 to monitor my hot water heater electric usage.

The meter has two wire terminals on it. The wiring directions that came with it say to "connect one terminal to the power wire and the other terminal to the neutral wire". Problem is, that on a 240v water heater there is no neutral wire.

What do I do?

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Old 12-01-2010, 06:12 PM   #2
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Is the unit rated for use on 240 volt circuits?

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Old 12-01-2010, 06:24 PM   #3
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I looked at the specs for that unit, and it's 240v safe, but there's one fundamental gotcha with your idea.

That hour meter will count the hours that it has power applied to it. If you simply wire it to the hot wires feeding your water heater, it'll just keep counting up all the time. You would have to figure out a way to wire it such that it only gets power when the thermostat turns on the heating elements. Further, assuming your planning on simply calculating kilowatt-hours based on the number of hours running, and the wattage of the water heater, you have to keep in mind that a water heater has two elements. If I recall correctly, an electric water heater sometimes only has one element on, sometimes both.

If you can afford it, you might want to look into something like The Energy Detective instead. It provides realtime monitoring of the power used by the house as a whole, or an individual circuit. It uses current transformers around the wires, instead of a direct connection. It would be more elegant, and quite possibly safer as well.

Just my humble opinion, of course.
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Old 12-01-2010, 06:55 PM   #4
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Originally Posted by McSteve View Post
I looked at the specs for that unit, and it's 240v safe, but there's one fundamental gotcha with your idea.

That hour meter will count the hours that it has power applied to it. If you simply wire it to the hot wires feeding your water heater, it'll just keep counting up all the time. You would have to figure out a way to wire it such that it only gets power when the thermostat turns on the heating elements. Further, assuming your planning on simply calculating kilowatt-hours based on the number of hours running, and the wattage of the water heater, you have to keep in mind that a water heater has two elements. If I recall correctly, an electric water heater sometimes only has one element on, sometimes both.

If you can afford it, you might want to look into something like The Energy Detective instead. It provides realtime monitoring of the power used by the house as a whole, or an individual circuit. It uses current transformers around the wires, instead of a direct connection. It would be more elegant, and quite possibly safer as well.

Just my humble opinion, of course.
Yes I did consider all these things. And I've already worked out all those obstacles.

The hour meter cost $25 bucks. Nothing else even comes close in price for the purpose I need.

A friend of mine who is familiar with electric said to just connect the neutral to the water heater ground.

makes sense.
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Old 12-01-2010, 06:57 PM   #5
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Yes I did consider all these things. And I've already worked out all those obstacles.
How?
...
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Old 12-01-2010, 06:59 PM   #6
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A friend of mine who is familiar with electric said to just connect the neutral to the water heater ground.
Negative, do not do that. Ground is not meant to be a current carrying conductor. Since the meter is okay for 240v, you would just wire it to both hot wires instead.
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Old 12-01-2010, 08:29 PM   #7
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Negative, do not do that. Ground is not meant to be a current carrying conductor. Since the meter is okay for 240v, you would just wire it to both hot wires instead.
That was my first inclination, but now I'm not so sure. Since the meter only uses 1 watt of power, would the grounding wire option really be that bad?

Now I'm not sure which way to go.

McSteve, you're saying with the 240 rating I'm ok with no neutral?

Versa, I'm just going to connect the power to the top heating element, record the hours it is energized for a week, multiply that by 4 to get a monthly average for the top element. Then do the same thing for the lower element.

Undoubtedly the lower element will be on less, but I won't know exactly until I do my experiment.

Just need to resolve the wiring issue.

thanks guys
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Old 12-01-2010, 09:37 PM   #8
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Quote:
Originally Posted by bmancanfly View Post
Yes I did consider all these things. And I've already worked out all those obstacles.

The hour meter cost $25 bucks. Nothing else even comes close in price for the purpose I need.

A friend of mine who is familiar with electric said to just connect the neutral to the water heater ground.

makes sense.
For first thing ., whoever that person gave you a advice about connect the netural to the water heater ground that is a major no no .,,

Second thing about the hour meter that do not count kilowatts hours at all it only count time as long the power is on and the way the water heater conferation it almost impossible to read it unless you add a CT { current transfomer } then it will able tell when the water heater is on or not.

Merci.
Marc
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Old 12-02-2010, 08:52 AM   #9
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french electrician, I addressed those concerns in my previous post.

Still trying to determine if connecting both "hot" wires to the meter, with no neutral, would be ok.
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Old 12-02-2010, 09:07 AM   #10
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Taking a bit different tack, but are sure that you don't already have this information, on your poco bill? I don't generally look at other people's electric bills, so don't know how common this is, and haven't had a bill with hot water on it since we switched to propane several years ago, but prior to that, the electricity for our water heater was calculated separately, and was itemized as such on our bill.
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Old 12-02-2010, 09:18 AM   #11
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Taking a bit different tack, but are sure that you don't already have this information, on your poco bill? I don't generally look at other people's electric bills, so don't know how common this is, and haven't had a bill with hot water on it since we switched to propane several years ago, but prior to that, the electricity for our water heater was calculated separately, and was itemized as such on our bill.
To do this would mean another meter or load switch just for the water heater.
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Old 12-02-2010, 10:03 AM   #12
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To do this would mean another meter or load switch just for the water heater.
That's true; exactly what we had, and like I have seen on other houses. Power came into the main meter, where it, of course, fed the main panel in the house, but it also split off two wires that went to a considerably smaller meter, mounted to the side, and the wires from that meter went through the wall, and into a disconnect, which fed only the water heater. I have seen that configuration on many houses, but, again, I don't know how common it is across the country. The reason for it, as explained to me quite a long time ago, was that the poco would give you a reduced rate for electricity to run the water heater, but it also granted them the authority to shut off one of the elements during periods of peak power consumption. Anyway, it may be a mute point, but thought that I would mention it.
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Old 12-02-2010, 01:31 PM   #13
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So no one wants to weigh in on how to actually wire this thing?
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Old 12-02-2010, 06:20 PM   #14
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So no one wants to weigh in on how to actually wire this thing?
I thought everyone already told you that it won't work.
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Old 12-03-2010, 01:38 PM   #15
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I thought everyone already told you that it won't work.
Really? I don't recall everyone saying that, not even you.

As it turns out I connected both terminals of the meter to the each of the hot wires at the upper element. The meter only runs when the upper element is energized.

It works perfectly.

I'll do this for a week or so to get a daily, and then monthly, average usage for the upper element. Then I'll do the same thing with the lower element for a week.

If I had two meters I could do both elements at the same time, but I don't.

This should allow me to make a very accurate projection of the amount of time my water heater is actually "on". Multiply that by the 4500 watts per element and I'll know how many kilowatts I'm using to to heat water.

Simple - once I got past the wiring question.


Last edited by bmancanfly; 12-03-2010 at 01:40 PM.
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