the ease of Kill a Watt is that it has a plug and receptacle integrated with it, making it marketable.
if a person needed to make regular measurements of 220V device current draw they would make a plug/receptacle set with separated conductors, frequently making a slit in a flat cord. the flat cord (like lamp cord/zip cord on steroids) and carefully with a razor slit just the thin part between two of the conductors. this can be done without any damage to the electrical character of the cord. make the slit large enough to fit the arm of the meter through. you can tape the cord around the cord at the ends of the slit to preserve integrity and tape over the whole area when done with the measurements.
if you didn’t want to invest in plug/cord/receptacle for multiple time uses, where just measuring on one device then you could do this in the cord to that single device. unplug before cutting.
I would like to point out: Many dryers have the power cord attached to a strain-relief plate. This plate can be removed with one or two screws, and then the individual conductors are available for clamping.
Yes, sure. People use them for travelling. I agree that making up a short extension cord and slitting for a clamp ammeter is probably the better solution
A couple other options which will give ballpark estimates with no screwdrivers required:
(1) Measure with a clamp ammeter at the service mast. This means you’re measuring the current of everything in the house, of course, so you have to subtract off the rest (increasing the measurement error). For a high-power appliance this will give you a ballpark figure as long as you have a reasonable idea of what else is on. (For this to work, you obviously have to know when thermostat-controlled appliances like refrigerators and air conditioners are on.) This probably requires a ladder, but the wires are separated so you can clamp on one phase at a time.
(2) Watch the meter. (The meter measures actual power, so it won’t help if you’re trying to estimate power factors.) Then convert from kWh to amps; again you have to subtract off the rest of the house.
Note that some appliances have fairly constant current draws when they’re on, but some vary a lot depending on where in their cycles they are.
Here is a method that requires no cutting of the cord or opening panel or appalliance. Purchase a 220 VAC plug and cord cap (female end of cord), some wire THHN or TW. Make up a short cord with the wires exposed. Plug the dryer into the cord and plug the cord into the wall outlet. Now you can clamp on each wire and take a reading.
Dryer’s heaters are 240 VAC, the blower motors are normally 120 VAC.
OK OK now hang on a minute. We’ve got the cart rolling down the hill ahead of the horse.
Firstly - why do you want to know the amp draw of these appliances?
1 - You are trying to determine the proper size breakers for a replacement breaker panel.
2 - You believe that the appliance may be drawing more current than it should, and want to check the actual AMP draw against the appliance’s rated AMP draw.
3 - You’re curious.
OK, so if it’s #1, then don’t bother with all the willy nilly testing of the current draw during operation. You need to size your breakers according to the maximum anticipated amp draw, not the amp draw during normal low load operation. This information can be found within the appliances manual, or you could look directly at the motor (or whatever else the 220V line is feeding) and there will invariably be a legend plate listing the specifications…one of which will be the Maximum amp draw for the motor/device. Generally (definitely with motors) any device will draw more current during startup than during operation. Motors can draw up to double the running amps during startup.
If it’s #2 - well then go ahead with all the willy nilly testing nonsense. I am a service technician for industrial equipment and I handle quite a bit of electrical diagnostics on a daily basis. The guy upthread who said open up the drier and clamp the ammeter right onto the single hot wire at the pigtail had the right idea. No need to go cutting into wires with your level of experience.
I didn’t comprehend that part at first. I guess you can disregard most of that prior post.
Well then your options are to read the manuals…or open up the units…and read the legend plates on the individual components. That’ll give you what the manufacturer considers the maximum amp draw. And you need *that *info…and it is quite irrelevant what the amp draw is at any given time for the purposes of almost any homeowner. The maximum is what you should be looking for. And it may not be drawing maximum when/if you decide to test it.
Very true…but if someone is competent enough to properly assemble a tester cord and to get a legitimate reading from it…then they can probably open up the unit.
I have contemplated having a new breaker box on hand and waiting for an extended outage and making the swap my self. Once the power is off, I lose all fear.
Having changed more than a couple panel boxes, that would terrify me. Without pulling the meter from the meter socket that looks like Russian Roulette.
What happens if some lineman at the substation says, “Vern, we’re good. Go back online”, while you’re [in]conveniently holding the business end of the SE cable?
I want to thank very much everyone who attempted to or answered my question factually; it was a great help. Upon consideration I’ve decided to go with the option of modifying an extension cord for the dryer, and for the case of the air conditioner and oven…I’m still thinking on that. If I can get a good experiment together (which Cecil has asked me to do) you may read about the results later this year.
Keep in mind that current is just… current. It is not power. Furthermore, you can’t calculate real power from a simple current measurement with any degree of accuracy in most cases. To do that, you need a power analyzer or modern oscilloscope.
Does your A/C have a fusible disconnect next to the compressor, or any at all? It means getting your hand close to some live contacts, but may work.
There are times when you know it is going to be a while. The power isn’t going to come back on while the wires are still on the ground over there where they are setting the new pole. Nor while the climber is still up in the tree cutting. I used a scheduled outage last summer to hack back the tree branches from my service line.
FYI, when we need to measure power here in the lab, we use a Voltech PMiv 4-channel power analyzer. We have two of them. If you were nearby I could probably lend you one.
Doing a quick Google search, I see that electric dryers tend to have around 5000 watts in heating elements and 250 watts in the motor (and surely some negligible amount for the electronics). The resistive component of the power is characterized by a simple P=VI, and while that’s not accurate for the inductive load of the motor, the difference can only throw off the total by ~5%, and probably less than that in practice.
That said, the PowerAngel meter (and almost certainly the Kill-a-Watt since it uses the same internals) does do a correct integral of V*I (instead of multiplying by the RMS of each). A while back I was curious, so I wrote the company and they actually sent me the formulas they use. Pretty surprising for a $30 consumer product, actually.
I have no idea what the PF of a dryer is, but you’re probably correct that the reactive component of the impedance is small. But it appears Una is doing some kind of study, and I just wanted to point out an error component when converting a simple current measurement into real power.
It’s pretty simple, actually: the voltage and current are simultaneously sampled using a couple S&H’s, and then multiplexed into an ADC. The processor multiplies the two readings and performs a moving average.
