How to measure wattage of 240V devices?

[Swamp_Yankee]

I'm trying to get a handle on how much startup/continuous load my generator (6250W continuous-8500W surge) can take without really lugging it. I have a Kill-A-Watt so I know the startup and continuous loads of things like the refrigerator, washer/dryer, and various other household appliances. However, I would really like to know the startup and continuous wattage of my well pump and 1.5 ton minisplit, which are both 240V and obviously both hardwired. I know that there are estimates (especially for well pumps) that you can look up online, but there are so many variables beyond the HP of the pump (depth, total length of power wire including horizontal run to the box, age of pump, type of pump, etc...) that I don't put much stock into them. I believe there are devices you can clamp on over the casing of a wire that will measure current, which I think you could then put into an equation with the voltage to find wattage?

 

[Highbeam]

Running wattage is easy. You go to Home Depot and buy a clamp on ammeter. You put one of the hot wires through the jaws and it measures flow. Double it since you’re on a double pole circuit.

Startup surge is impossible to know without ridiculous equipment. The spike is fast and you need the peak. Also, the duration of the surge is just as important as the peak wattage. Some generators have enough of a flywheel to bang through.

The published guidance on surge loads is almost always ridiculously conservative.

 

[Highbeam]

I'm trying to get a handle on how much startup/continuous load my generator (6250W continuous-8500W surge) can take without really lugging it. I have a Kill-A-Watt so I know the startup and continuous loads of things like the refrigerator, washer/dryer, and various other household appliances. However, I would really like to know the startup and continuous wattage of my well pump and 1.5 ton minisplit, which are both 240V and obviously both hardwired. I know that there are estimates (especially for well pumps) that you can look up online, but there are so many variables beyond the HP of the pump (depth, total length of power wire including horizontal run to the box, age of pump, type of pump, etc...) that I don't put much stock into them. I believe there are devices you can clamp on over the casing of a wire that will measure current, which I think you could then put into an equation with the voltage to find wattage?

Modern well pumps and mini splits have much much lower surge loads than old school equipment.

 

[Ashful]

Ammeter on one leg, times 240 volts. But don’t double it, Highbeam, what’s flowing in on one leg is flowing out on the other.

This is ignoring Power Factor, but let’s not get academic, I don’t think generators care about power factor.

 

[Highbeam]

Ammeter on one leg, times 240 volts. But don’t double it, Highbeam, what’s flowing in on one leg is flowing out on the other.

This is ignoring Power Factor, but let’s not get academic, I don’t think generators care about power factor.

Its not flowing out the other but being pulled out the other leg. The doubling happened when you multiplied by 240 instead of 120 which is what you will find in that one line you placed in the ammeter jaws.

We get the same number though!

 

[MTY]

In single phase, 240 is the potential difference between 2 legs of 120 spaced 180 degrees from one another. I do not understand either pulling or flowing.

 

[blades]

the simple redneck way - well pump on it own circuit - what is the amp rating of the twin breaker? so just as an example breaker 30A/leg at 120V =3600w x2 = 7200W Therefore your 6500w/ 8500w surge gen set should be ok but only for the well by itself as you really do not have a lot of extra capacity left when the well pumps kicks on. as noted depth of well, age of system, pressure set at, size of pressure tank all come into play as well. It 's a lot like a wood stove - big stove you can make small fires in- little stove got to push the envelope in the case of gen set pushing the envelope is not a good idea.

 

[Ashful]

In single phase, 240 is the potential difference between 2 legs of 120 spaced 180 degrees from one another. I do not understand either pulling or flowing.

Some people (eg. Highbeam’s post above) like to think in terms of ground reference. Whatever helps you solidify the theory in your own mind, use it. If you want to get into the real physics of it, I’m game!

 

[Highbeam]

I think of it as electricity and if I need to, water flow. Amps are a measure of current, the flow of electrons. The voltage is the pressure pushing these electrons.

 

[blades]

Same thing I tell a lot of persons when they ask on homes and vehicles. Computer control refinements make life interesting for a lot of people on equipment - device always has potential but nothing happens until the potential is allowed to escape.

 

[fbelec]

the gen you have should work fine. ductless splits airconditioning or heat pump with one of those units do not have surge current. they start up small and ramp up to what is needed. you will probably find that it will run both. if you were to try to figure out gen needed for a house ang used the breaker as a reference point one would need a power plant rather than a generator

 

[billb3]

a momentary start up surge can be 5 or 6 times the normal operating amperage. I think I read someplace that some of the newer energy star fridges and freezers are low, like 1.4X. But I coul dbe mis-remembering.
Some compressors have the start-up surge right there on the label. If the label can still be read.

I have a Kill-A-Watt and I'm not so sure the "meter" is fast enough to measure peaks. Even some cheaper multi-meters aren't all that accurate.