Will an ammeter register a motor's startup surge?

I’m trying to determine how much amps my well pump is drawing so that I’ll know how big a generator to buy in case the electricity goes off. If I get a $50 ammeter from Home Depot, will it be able to register the surge that starts up my pump or just the on-going current when the pump is running?


I doubt a consumer-grade DMM is going to do what you want.
You not only need to measure the peak current draw, but the duration of the surge. That requires a fast data-logging meter, or an oscilloscope.

I would get in touch with the manufacturer and ask them.

Of the pump or the ammeter? The pump was installed 30 years ago. I bought the house 25 years ago and have no information about it.

I have been though that. I just got a generator that had the capacity greater then that of the well pump circuit breaker. It works, though lights dim when the well kicks on. It also will run everything in the house w/o having to try to ration.

Also realize that you could not use the meter in-line with this much power, but would have to use a clamp on one where the ammeter had a terminal that loops / clips over one of the power leads.

A standard calculation I’ve used in industry is a stalled / starting motor inrush is 6X the rated FLA. You won’t get a consumer grade ammeter to respond quick enough to “see” the motor inrush. The worst case scenario is ohms law calculation of line voltage divided by motor winding resistance to get the immediate inrush as the current is at its theoretcal maximum as the magnetic field builds to turn the motor. <I think

ERRR… Maybe, maybe not. It depends what part of China your meter is manufactured in.

Irrespective of meter capability, go by the FLA rating on the motor when sizing your generator. Most gensets have the ability to tolerate large momentary surges from motor start cycles, well over their rated “continuous” rating.

What is the line voltage and breaker size to the well? I would just do a quick power calculation, and size the generator accordingly.

Good general rule of thumb… Sort of… Kind of. Start current is always much less than locked rotor rating however.

BTW: I have a Harbor Freight meter inherited from a deceased friend that actually does a pretty good job of giving a brief (if you pay close attention) indication of start up current. Go figure.:rolleyes:

I work on well pumps for a living.

You are over thinking the surge rating. Simply size the generator large enough for the pumps rating and the surge rating of the generator will be sufficient.

Even a cheap amp probe will register the initial surge of the motor starting, but it’s actual ability to measure it accurately will vary. I used to prefer using an analog meter for motor testing because it was the simplest method to see the peak. Unfortunately analog meters that also meet the small space requirements of pressure switches are hard to come by. With a good digital meter it’s actually better information but I have to toggle back through the mapping to see it. In my toolbag I just carry a cheap digital meter. It’s rare that I actually need precision data on a household system. I just check that the amperage spikes on start up and immediately settles into it’s normal operating range. The expensive meters stay in the truck where they won’t get banged up, if there is something unusual I’ll test with my good meters.
This is the meter in my bag:

If the pump’s 25years old odds are it has a Franklin motor, though pretty much every other submersible will have similar electrical characteristics.

Here is Franklin’s generator chart

Looking quickly I didn’t find a good chart for amps to hp. If you tell me voltage, amp draw and 2wire or 3wire I can tell you what the HP rating of the pump is.

That’s there the min/max feature comes in handy. Tell your meter to record the highest amp load and then go start the pump and it’ll catch those brief spikes.
Sure a $50 meter might not get you as good of a reading as a $300 fluke or a scope, but it’s better then watching an autoranging DMM that won’t even move the decimal or choose a unit before everything settles out.

As I said, YMMV.
It all depends on your equipment, and the ability to decipher it’s output.

I would also consider adding a start capacitor in parallel with the existing start capacitor (many inexpensive options available on Amazon). It would reduce your start current and make it easier for the generator.

As a note, the new capacitors do not have as much life as the old ones because the fluid in them was changed due to environmental concerns. So keep the old capacitor.

regarding capacitors, make sure to get an AmRad any time you can (they’re some of, if not the best capacitors available today,
they’re also made in the u.s.a.).


regardless the mfd rating, get the 440 volt ones as opposed to the 370 volt ones, they’re a little beefier.

I have a Klein CL 1000 that has the capture mode. It works well but I don’t know the surge rating it is capable of reading. Various models are available at Home Depot although I got mine online. It was well under $100.

I tested all my major loads with it when I was setting up my generator. Most inexpensive generators don’t have much of a surge rating. If the full output is 5000 watts, they surge to 5500 or possibly 6000.

My Onan 6500 has a full 13,000 watts for 6 seconds surge rating. It has a freakin 950cc motor, nearly as much as some cars. The carryover capability is enormous.


Your running amps will be no more than eight. HVAC guy checking in and loving the run around you get from electricians.

Proper measurement of current draw of electric motors is notoriously difficult.

This was how Joseph Newman was able to get EEs to sign off on his ~perpetual motion machine. The people testing it didn’t have the right knowledge and equipment to really measure things. His device produced a lot of odd transients that messed up ordinary meters.

Startup surges are sort of like that. You rely on calculations and empirical knowledge more than anything.

I am guessing they were trying to measure power in an effort to calculate efficiency. But measuring real power is difficult. It can’t be done with ordinary handheld DMMs. The EEs who were performing the measurements should have known better.

If increasing the capacitance of the start capacitor is always a good thing to do, why didn’t the manufacturer use a higher capacitance to begin with? This is bad advice, as it implies the manufacturer didn’t know what they were doing when they chose the capacitance value of the start capacitor.

It’s a 3 wire 220 with a double 20 amp breaker. The “first water” in this area is around 25 feet. I’m guessing that the pump is around 60 feet. I can’t imagine that it’s more than 1/2 hp.

this is how we had to do it when I worked at a test lab. we had some beefy calibrated current shunts in-line with the circuit, and measured the voltage drop across the shunt at a high sampling rate.