I would like to connect an analog electric clock to the upper and lower elements of my electric water heater so I can see how much time they are actively heating water. My objective is to evaluate the power use before and after adding a solar pre-heating system.
Each heating element has two 110v wires connected. No common wire.
I understand how U.S. 220v is 2 x 110v and can run on just two wires.
But 110v needs a common wire to complete the circuit.
As a test, I connected a clock to one of the 110v leads of each element and system ground (the thermostat cover screw).
Now the clocks run when each element is hot and I can tell how much time is spent heating water by each element.
I have since removed the clocks because I read that drawing 110v from one leg of a 220v and system ground is "against code, illegal and extremely dangerous".
But is it really dangerous? An analog desk clock draws very little power.
Georgia, USA

 

Sorry you just flunked residential electric 101. No 110 in the US and the two wires connected to the element are 240. You could do this with two relays with 240 coils. Connect one across each element and use them to control the 120 to the clock.

P.S. No common either. The term you want is neutral, the center tap on the transformer that provides 240v to your house.

 

Rather than using two hour meters you would be better off using a single watthour meter to measure the total power used by the water heater. Here are a couple of inexpensive models currently available from Ebay.

5 in 1 AC264V 100A Digital Combo Panel Meter Volt Amp kWh Watt Working Time Ct | eBay

5 in 1 100A AC Digital Power Meter Volt Amp kWh Watt Energy Time Clock 110V 220V | eBay

They appear to be the same unit but one has a shipping charge that adds to the cost. Of course you will need to build an enclosure for the meter but that could be as simple as a four-inch square deep box with a blank cover cut out to fit the display.

I have one of these and it is a pretty nice meter. Using a watthour meter makes if far easier to calculate the cost of operation than using a running time meter. Simply multiply the kWh reading by the cost of the electricity in kWh to find the cost of operation. This is far easier than having to first calculate the number of hours of operation and multiplying by the wattage of the elements, which is probably not an exact figure, and then multiplying by the cost of the electricity. Plus, it is wired at the electrical input of the heater, not to the individual elements.

 

Doing what you did put neutral current on the grounding conductor and created a shock hazard.

 

But it was very little current.
So touching my volt meter to the tank and one lead of the heating element creates a shock hazard?
An electric clock draws very little power. How does 3 watts present a shock hazard?

 

A "ground path" is not a viable return no matter how low the current. You could use a 2:1 step-down transformer connected to both element terminals on the primary and the secondary to the clock and it would be acceptable. Doing so would require TWO transformers and TWO clocks.

Why are you against using the proper instrument such as I described in my previous post?

 

Because heart rhythms can be disrupted by as little as 6 milliamps (0.006 amps) when someone is wet, sitting in the bathtub for example. If the grounding path of the plumbing path is disrupted, current can flow through unintended paths. Your 3 watt clock puts about 25 mA on the ground, 4 times the lower lethal threshold. I'll acknowledge it's a low probability of occurring, but the code bans current on the ground for a real reason.

The voltmeter by comparison sits behind a very large resistor which limits current to generally much less than 1mA.

 

Thank you for your informative and patient replies despite my ignorance of electrical terminology and concepts!
My project is a temporary curiosity in hopes of assessing the value of a solar pre-heater for my electric water heater. We pull ice cold well water into the heater all summer, even while its 95 degrees and blazing sun outside. So my project is not worthy of any significant expense. A pre-heater will undoubtedly be worthwhile, especially since such an investment is tax deductible.
If you will indulge me, I have one more question...
If I meter either side of a heating element connection and ground, I always get 120 volts, even when the element is not hot. Yet if I meter across the inactive element, I read 0 volts. Can anyone help me understand why?

 

You are the only one that can determine where the threshhold of "significant expense" lies but to me the fifteen dollar cost of the meter means giving up one trip to the premium burger joint. The additional expense of the enclosure is a few dollars so maybe two trips to the burger joint or taco place. After your little experiment of collecting data on the water heater has passed you might find other uses for the meter as well.

This one is easy if you understand some basic concepts of electricity. I'll try to explain.

Electricity from the utility comes to your neighborhood at a voltage of somewhere between 2400 and 26,000 depending upon the utility and the age of the system. Near your house there is a transformer, either pole mounted (overhead distribution) or a pad mounted (ground level) that that steps down this higher voltage to 240 volts. The transformer winding (the secondary) that outputs this 240 volts has a "tap" in the middle of the winding called a center tap. When measuring the voltage if you check between the two ends of the secondary winding you get 240 volts nominal (+/- 10%) but between either end and the center tap you will get 120 volts. This is how you get the two voltages with only three wires. Additionally, this center tap is also connected to the earth (grounded) for safety purposes that take volumes to describe and are often difficult to understand.

The center-tapped conductor is called the "neutral" conductor in common usage although the official designation is the "grounded conductor" with the two end conductors being commonly know as "hot conductors" (in relationship to the earth) and the official name of the hot conductors is "ungrounded conductors".

From this configuration you can see that even though the earth is not a very good conductor of electricity the very low current drawn by a voltmeter will register when the probes are connected between a "hot" conductor and the earth, either directly or by some metallic source that is in contact with the earth. Normally the ONLY time current is passed through the earth or a safety ground conductor is during fault conditions and the purpose of this is to cause a high amperage flow that will trip a circuit breaker or blow a fuse.

Now with that background let us look at your question. In order to measure voltage there must be a difference in potential between the two measuring points. If you have a length of wire that has one end connected to either a hot or neutral point and then you attempt to measure the voltage between the two ends of this piece of wire you will get a reading of zero BECAUSE there is no difference in potential. To further explain, connect one end of a wire to the negative side of a battery and connect the other end of this wire to a light bulb. Nothing happens because you do not have a "complete circuit" but only a supply TO the lamp. Now connect another piece of wire from the other light bulb contact to the positive side of the battery and the bulb will light. Take the voltmeter and connect one probe to the battery negative terminal and the other probe to the light contact that connects to the battery negative and you will read zero volts (a very sensitive meter will read some fraction of a volt and this is the "voltage drop" through the piece of wire). You can do this on the positive side and get the same results.

How does all of this relate to the water heater element? Very simple. When the element is NOT energized (the thermostat is "open") one side of the element is connected to one of the secondary conductors from the transformer serving the house. When you connect one probe of the voltmeter between either terminal of the element and connect the other voltmeter probe to the earth or a grounded conductor you are measuring the voltage between one end of the transformer secondary to the center tap...120 volts. It works on either terminal of the heating element BECAUSE the element itself is simply acting as another length of wire in the circuit, a wire with a very low resistance that has almost no effect upon the very low current used by the voltmeter. Placing the two voltmeter probes, one each on each of the element terminals offers no differential voltage to measure when the element is NOT energized.

When the element IS energized, the thermostat is closed, the element is connected to each end of the transformer secondary and therefore receives the full 240 volts which can be measured with the voltmeter connected to only the element terminals. Still, connecting the voltmeter with one probe to either element terminal and the other to ground will measure from one transformer secondary terminal to the center tap, the secondary end changing as you change the voltmeter probe between the two element terminals.

Read this through a few times and then draw some pictures and trace the current flow. If you still don't understand then post back.

 

Don, I'm going to make a wild guess that a solar preheater is probably a good investment. Many years ago my dad made a solar pool heater out of nothing more than a fanned out roll of black plastic irrigation tubing tossed up on the roof of the garage with a few hold down straps. On a summer day you could run the garden hose in one end and the water would be so hot you couldn't touch it coming out the other side...and you guys get a lot more sun in Georgia than we do in Michigan. The biggest issue for your particular household would be if you all take showers before sunrise or not. The preheater wouldn't do any good if the tank is getting filled when it's dark out.

 

Fantastic explanation, Furd! I read your reply hanging on every word.
I just ordered 2 step-down 240v to 120v European travel converters for $11 each. I'll connect those to the heater elements and plug my clocks into them.
As an added bonus, I plan to calculate how long we would need to be away from home to save money by turning the water heater off. I'll compare the KWH of running the heater while we're away vs. turning it off and heating from stone cold upon return. I'll also put a clock on my well pump behind the pressure switch. That way, I'll be able to tell how many minutes the well pump actually runs each day. If the minutes per day suddenly jumps, I'll know there's a leak in the underground water line somewhere. Ever wonder what it costs to take a hot shower or run a load of whites? I'll soon be able to tell you!
Should I report back with my findings?
Best regards,
Don

 

With the two European adapters you are now seven dollars over the cost of the kilowatt-hour meter. To do it properly you will need to snake two #12 conductors of THHN wire to each element and then to receptacles to plug in the adapters. This wire really needs to be in conduit of some sort outside of the water heater casing. You will, of course, do whatever you will.

Adding an hourmeter to the well pump is not a bad idea but rather than using an analog clock I would rather see a running time meter, resettable if you wish or maybe even two, one non-resettable for total run time and a resettable one to easily measure usage time on a daily, weekly or whatever basis. Keeping log readings along with date and time of reading will make the resettable meter only a convenience as the periodic running time can be calculated from the logged readings.

Whether or not it will be more economical to turn off the power to the water heater while away will depend upon several factors, mostly how long you are away. Standby losses from electric water heaters are already low and with the new energy standards being enacted today they will be even lower in the future. Depending on where your heater is located it may be that the standby losses are actually contributing to heating the house, albeit not to any significant degree. Something else to consider is if you are comfortable in waiting anywhere from a couple of hours to perhaps several hours before being able to use any significant amount of hot water upon returning home.

Sure, keep us informed as to your progress and results. We enjoy all news of projects here no matter how they turn out. If you have great success others will want to follow your lead and if you end up with a money pit that may keep others from making the same mistakes.

Best of luck in your endeavors.