clamp meter reading on 240v AC and watt conversion?

Jayne

Jayne

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I'm sizing a generator and the largest load is the well pump (240v). Using a clamp meter on each of the two hot leads I get a reading of 7A per while it's running. If I'm thinking about that right, it means my choices are:

A) 7A x 120v x 2 = 1680 watts
B) 7A x 240v = 1680 watts
C) 7A x 240v x 2 = 3360 watts
D) A or B
E) none of the above, don't let the computer guy play with the clamp meter

Trying to google that has been maddening.
 
Edit, based on my googling the answer looks like it should be B. 7a 240v total for the pump

Let’s send up the bat signal. @Scsmith42
 
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Power is equal to current times voltage in a purely DC circuit.

P=IE

For AC circuits, power is the same equation, only multiplied by the power factor of the load. You can assume a power factor of about 0.85 for a submersible well pump (close enough, if the label plate doesn't say anything about it).

So this would work out to be:

P (Watts) = 7 Amps X 240 Volts X 0.85 = 1,428 Watts.

(Keep in mind that the current you're measuring is the RUNNING current. Starting current will briefly be higher (as high as 7 to 10 times the running current). But this is for a very brief period and decreases rapidly as the pump comes up to speed.)

NOW...for your purposes, if you never use the power factor in your calculations, you'll just end up with a built-in higher conservative value for required generator wattage. And that's plenty OK.
 
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Good input from all above. Chief’s answer is the most accurate, but your quick and dirty answer is B.

However, that is not the question that you should be asking. Your real concern for generator calculations is current inrush. Of all of the electrical devices that you’ll need to power, the well pump is probably the only one with a significant amount of current inrush, along with perhaps your air conditioner.

Since they will be cycling on and off while the generator is running, you need to take this into account when running your sizing calculations. Everything else (fridge, microwave, lights, TV, etc) can be operating wattage, but for the well and HVAC I’d base your calculations upon start up draw.

Otherwise your generator may be undersized in order to adequately start your pump.
 
Scsmith42 said:
Good input from all above. Chief’s answer is the most accurate, but your quick and dirty answer is B.

However, that is not the question that you should be asking. Your real concern for generator calculations is current inrush. Of all of the electrical devices that you’ll need to power, the well pump is probably the only one with a significant amount of current inrush, along with perhaps your air conditioner.

Since they will be cycling on and off while the generator is running, you need to take this into account when running your sizing calculations. Everything else (fridge, microwave, lights, TV, etc) can be operating wattage, but for the well and HVAC I’d base your calculations upon start up draw.

Otherwise your generator may be undersized in order to adequately start your pump.
Click to expand...

Currently we've got an 8k (10k peak) and it's fine for everything, but is just loafing along burning a ton of gas most of the time. I want to get our runtime longer without stockpiling a lot of unleaded so a "6k" propane (which only has 4800 running watts) covers a lot and uses 1/3 the fuel.

My current thinking is to get another 8k/10k sized dual fuel as the primary and a 2nd 6k dual as the backup. For the first part of our 'disaster scenario' where we expect power back within 5 days we just carefully run the 8k like we do now. Then when it's clear the grid isn't coming back so fast we switch to the 6k and essentially double the runtime of our remaining fuel, we just can't be running the well pump and other stuff at the same time. Will require discipline but after 5 days in with no end in sight.... I think we can do that.

I want to do everything off the 20lb BBQ tanks, no permanent install, no tank rental BS, no permits, no waiting for someone to bring the propane to me.
 
RetiredUSNChief said:
(Keep in mind that the current you're measuring is the RUNNING current. Starting current will briefly be higher (as high as 7 to 10 times the running current). But this is for a very brief period and decreases rapidly as the pump comes up to speed.)
Click to expand...

How is that (surge) possible to cover? If it's 1428 running, but 9996-14280 starting... the existing generator (which has worked for 2-3 outages a year for the last 8 years) is only rated 10k starting load and we've never had zero draw and just turned on the well pump so why hasn't the generator... what would it do? not fail, but not provide enough to start the pump? And why wouldn't the breaker just pop with that initial surge?
 
Breakers are designed with current inrush in mind. They will handle a second or two of current overload before they trip.

Re your current generator, you probably experienced a voltage drop during the well pump startup. The extent of current inrush required varies depending upon the inertial load on the motor.
 

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This one would be at the top of my list if I were you. Not dual fuel but the fuel economy of their eco-throttle system is state of the art.
 
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My last name is PIER
Dad threatened bodily harm if I ever forgot Ohm’s Law…..
As far as the grill tanks, you can build your own manifold system to feed the LP genny. I saw one at an extremely remote tower site WAAAYYYY up in WV that had 12 grill tanks on 2 shelves in a covered but airish shelter. When the tech would go up to the site, he’d get 6 grill tanks and haul them up, swap out any empties, and get empties refilled for the next trip up.

You can pressure test your manifold with compressed air and windex.
 
Scsmith42 said:
This one would be at the top of my list if I were you. Not dual fuel but the fuel economy of their eco-throttle system is state of the art.
Click to expand...

I really want dual fuel, storing propane is easier than constantly rotating unleaded. I also have to buy 2 of them.

None of the dual fuel stuff seems to be name brand, and doing a aftermarket propane conversion seems iffy. Some people get it working, some don't and there are start/run issues I would rather not deal with during an emergency.
 
Or you could buy my 26,250 watt Generac with 17,500 watt run watts and power the whole house. Runs off petro, could be converted with a kit but I wouldn’t do it. Yes it takes gas but it’s basically a whole house like the $12,000 installs but on wheels and a cord, or multiples and a little electrical work to a panel and you save $9,000.
72 amps of lovely usage while you continue to live normally.
 
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Jayne said:
How is that (surge) possible to cover? If it's 1428 running, but 9996-14280 starting... the existing generator (which has worked for 2-3 outages a year for the last 8 years) is only rated 10k starting load and we've never had zero draw and just turned on the well pump so why hasn't the generator... what would it do? not fail, but not provide enough to start the pump? And why wouldn't the breaker just pop with that initial surge?
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I don't know what the actual starting current is for your pump motor. That's just a typical maximum range for a single phase motor.

Starting surge is maximum at the instant the motor sees power on startup. As soon as the motor starts turning and building up speed, it starts generating counter electromotive force (because it's also acting as a generator as it is rotating) which opposes the motor current flow. It's part of how the motor develops the torque required to rotate.

It only takes a very brief time for the motor to reach its operating RPM, so this starting surge is very brief.


THAT SAID:

Your well pump does not need to be left powered continuously like normal, cycling automatically, during an emergency power outage scenario. You can plan and coordinate your water usage requirements and only shut the breaker for the well pump when water is actually needed. This way you can temporarily secure other loads for the brief time needed to start/run the well pump.

Remember...during an emergency only essentials should be powered, and only when needed. More than that becomes "convenience" and convenience comes at a much higher price.
 
Typical starting percent for that motor is probally 300-400% but the breaker will not trip on a panel or generator breaker for that instantaneous inrush because the breaker is made to delay tripping.
 
A bit of generator and load theory here. But don't worry...it won't get too technical!

In general, the generator output capacity is limited by the prime mover capacity. Slap a generator on a motor and it will provide as much power as the motor which turns it is capable of providing. Now, the generator may be damaged if the wiring is not capable of handling the current load, but it WILL provide the power regardless however briefly that may be.

Obviously, the motor and generator capacities are typically designed to be "complimentary". It would be inefficient, and more expensive, to pair a 5 KW generator with a 50 HP motor.

As a rule of thumb, there is a relationship between Kilowatts, Horsepower, AC current, and DC current, and it's represented by the ratio 3-4-5-12 (in the same order as KW, HP, AC Amps, DC Amps).

4 horsepower will provide 3 Kilowatts.


Looking at a Generac GP6500 generator, it's rated at 6500 Watts continuous (6.5 KW) with a starting capacity of 8125 Watts. So, how big should the motor be for this?

Using the ratio above, it takes 4 HP to drive 3 KW. So:

6.5 KW times 4/3 gives you 8.67 HP.

Looking at the specs for the GP6500, guess what the motor is rated at? 8.72 HP!

If you try pulling more than 6500 Watts from the generator, the motor will start "bogging down" because it doesn't have enough power to drive the generator at that load. Too much load for too long and the motor may stall out.


THAT SAID:

The electrical generator will be further protected electrically by a couple means:

1. Breaker output. This is obvious...if the breaker trips, you've drawn too much current for too long.

2. AC Regulation. This is a "maybe" and is typically associated with big honkin' generators. Simple generators are just a straight generator which is designed to give the required output voltage and frequency at normal operating RPM and therefore will not need any further regulation. Since they're not designed to operate in parallel with other power sources, they have no need for voltage and frequency regulators. Large capacity generators (the kind that are transported on trucks and the like) will likely have voltage and frequency regulators which will allow them to operate in parallel with other generators for the purpose of transferring load from one generator to another.
 
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RetiredUSNChief said:
Looking at the specs for the GP6500, guess what the motor is rated at? 8.72 HP!
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Witchcraft! Burn him! BURN him!

(appreciate the info though, the more I learn the more confident in the decisions I'll become).

RetiredUSNChief said:
If you try pulling more than 6500 Watts from the generator, the motor will start "bogging down" because it doesn't have enough power to drive the generator at that load. Too much load for too long and the motor may stall out.
Click to expand...

I've observed that, when the coffee maker or the well pump kick on, you can hear the generator 'grunt' a little and then stabilize right away.

I think I've overloaded the wife with details and options on this. I'm starting to rethink the plan a bit, having N+1 generators is cool, but we're still relying on just the one well pump too. Lots of single points of failure. It might make more sense to have one generator and one well, and then a better secondary source of water. Larger failure domain, but less single points to render us out of water.
 
Honda should be able to provide specs in regards to what type of loads their eqpt can handle and they should be able to provide a trip curve for their output circuit breaker. Brand X, not so much.
 
If you are not buying a generator you can connect to the service panel and give you extended sustainability why even bother. I was living in the city between a fire station and EMS building which assured I got power restored before anyone so I kept a smaller generator that never got unboxed.

But given the events of the last few years I went with something for a complete living as normal. It was an awakening being in Moore County when the grid went down. Even my daughters 12,000w genny not hooked into the panel yet was close to useless for living.
 
BlackGun said:
If you are not buying a generator you can connect to the service panel and give you extended sustainability why even bother.
Click to expand...

I already have one and it's got a transfer switch and panel and everything, and it runs the household well enough. It's just big, noisy and runs on unleaded which is harder to stockpile than propane. The exercise is (or was) to see if we could get away with smaller and more fuel efficient while buying new vs oversizing and having to stock more fuel.

BlackGun said:
I was living in the city between a fire station and EMS building which assured I got power restored before anyone so I kept a smaller generator that never got unboxed.
Click to expand...

We lose power several times a year for days at a time. It's why this 'prep' is so easy to sell to the wife and get her onboard with the idea, we use it frequently enough that it's obviously useful.
 
If the goal is to use less fuel. Why not just pick up one of the small suit case 2000-2500 watt honda or clone gens to run lights or few things and then only run the big one when you want showers, hot water and run freezer fridges to keep them cool? The little ones will run a long time on a gallon of gas and run many things that you use for convience. Then your still only stocking one fuel and have some redundancy and increases your time you can run on your fuel supply.
 
Jp8819 said:
If the goal is to use less fuel. Why not just pick up one of the small suit case 2000-2500 watt honda or clone gens to run lights or few things and then only run the big one when you want showers, hot water and run freezer fridges to keep them cool? The little ones will run a long time on a gallon of gas and run many things that you use for convience. Then your still only stocking one fuel and have some redundancy and increases your time you can run on your fuel supply.
Click to expand...

A large and a small are an option, there are so many choices on this.
 
I think I've made my choice: Westinghouse 12500 Watt Dual Fuel

Pros:

- has enough power on propane to match my existing unleaded generator so I know it will work even with the surge load
- propane supply is in the 'right' spot to work with my application where it's going to physically sit
- fuel switch is easy to get to, battery is easy to get to, oil fill is easy, exhaust is going the right way to work with the existing vent
- fits budget and let me get two

Meh:

- remote start is cute, and helpful if it works but probably just something else to fail, hopefully the starting circuit is simple enough to rig in an emergency
- not inverter, but that apparently won't work with my loads anyway
- possibly made in China (or Vietnam which wouldn't be as bad)
 
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Jayne said:
Hmm... I put the clamp meter into MAX mode and turned on the pump, got a max reading of 19A.

I'm taking that to mean my inrush power is 19Ax240v = 4560 watts.
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Like I said before breakers delay inrush. Your concern is the starting watt current of the generator. For example I have one that is 17,500 but the starting watts are 26,250. That is a massive difference. Generators list run watts but the starting watts are discovered in the specifications list.

If I lived east of Raleigh I would prepare for a comfortable situation and damned the let me just get by situation. The other generator I have is 12,000w but I have city water, gas water heater, and, gas furnaces. Mine is dual fuel but I will run it off gas for the most bang for the buck unless stations in multiple counties go out.
 
BlackGun said:
Like I said before breakers delay inrush. Your concern is the starting watt current of the generator. For example I have one that is 17,500 but the starting watts are 26,250. That is a massive difference. Generators list run watts but the starting watts are discovered in the specifications list.
Click to expand...

The smaller version of the one I picked shows (for propane) 4800 running with 5900 starting. That makes me think I could get that as the 2nd generator and if the first fails, then we only get to run 1 thing at a time but it can run the well pump which is the most important thing. Once we pump enough water then we can turn that load off and run the fridge/microwave/coffee maker/whatever easily. It's not as comfortable having to manage the power load but we're talking a worst case here where we've lost the primary.

The upside to the small one is less fuel.

Another thought would be to get TWO of the smaller ones. Rewire the panel so that the well pump runs on one and the rest of the house on the other so we only fire up the 2nd when we need water. The rest of the time we're running on the single smaller one using much less fuel.

Ugh, going in circles again.
 
Jayne said:
Hmm... I put the clamp meter into MAX mode and turned on the pump, got a max reading of 19A.

I'm taking that to mean my inrush power is 19Ax240v = 4560 watts.
Click to expand...

You should be good to go, given the 12.5 KW generator you want to go with.
 
Jayne said:
What about the smaller option? 6600 Watt

In theory it handles the 19A surge, and will run way longer on the same fuel.
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This generator is rated at 6600 Watts peak and 5300 Watts continuous. It should handle it.

Normal running current being what you said it is, it'll draw about 1500 Watts and that means this generator will last a lot longer with gas than the 12.5 KW one will, obviously.

That said...with 4560 Watts starting surge, you can carry about 2000 Watts load on the generator and be able to handle the starting current on the pump when it kicks in. If you're loaded much more than this, it might stall out. Which means you just need to plan your pump use and temporarily reduce your other loads when starting it.
 
RetiredUSNChief said:
This generator is rated at 6600 Watts peak and 5300 Watts continuous. It should handle it.
Click to expand...

It's only 4800/5900 on propane, but still below that max.

RetiredUSNChief said:
That said...with 4560 Watts starting surge, you can carry about 2000 Watts load on the generator and be able to handle the starting current on the pump when it kicks in. If you're loaded much more than this, it might stall out. Which means you just need to plan your pump use and temporarily reduce your other loads when starting it.
Click to expand...

Or a meastly 1430 watts on propane.
 
For what it’s worth, I’ve found analog meters to be more accurate when measuring current inrush than digital, but other’s experiences may vary.

I used to operate a 3/4hp 200’ deep submersible with a 5500 watt generator. Granted it was an older 2 cylinder generator running at 1,800 rpm.
 
Jayne said:
It's only 4800/5900 on propane, but still below that max.



Or a meastly 1430 watts on propane.
Click to expand...

Again...remember that in a real emergency, you'll be (or SHOULD be) doing everything you can to stretch your resources. The goal of an emergency generator is to supply power to ESSENTIALS and then ONLY when needed.

Keep your freezer frozen and your fridge cool. In the winter, run what needs to be run to keep from freezing. If you have medical equipment, power that when needed. When water is needed, shift things around so you can start/run your pump as required, then before you secure your pump top off all your water storage containers so you don't have to run the water again for a while.

Many people (like my wife) have it in their heads that something like an emergency generator is supposed to "maintain their standard of living" the same as it was before losing power. Nope. You're on energy rations and resources are scarce. I'm sure you understand, having read all your posts above, that the time to worry about having enough fuel is BEFORE you suddenly realize you're 6 hours away from an empty generator tank...and that fuel conservation starts the minute you realize you need to run your emergency generator.

Heck, you've even pointed out that fuel conservation starts by choosing the correct size generator for your needs, because a higher capacity than what you need will burn through your fuel faster for the same load.
 
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