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I am trying to size a generator by measuring power from the breaker.

I would to get some advice from you guys on my plan.

I put an A/C clamp around both the cables from each terminal wire ("red arrows") on the breaker box.

I am using the formula:

Power [watts] = Current [amps] X Voltage [volts]

I have microwave, oven, vacuum cleaner, TVs, lights and I calculate 9.24 kW. I know I probably won't have these on during an outage but just trying to get a feel for power usage for my home.

For the left, I show 30a, for the right, 47a.

(30x120+47x120)/1000 = 9.24 kW

A/C, minisplit, dryer are not on.

  1. Does this calculation sound correct to you guys?
  2. How much power/kilowatt draw should I add for a 4 ton A/C unit? It's rather cold here, otherwise, I could fire it up and measure the breaker power.

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    Probably an unproductive approach to it. You can start with a budget. How many digits? $20,000? $2000? $400? Why do you want a generator? What do you want to achieve during power failures? Just keep the house from freezing and the food from spoiling? Or keep the whole house cool for days in hot summer weather? List the things that are important.
    – jay613
    Commented Feb 7, 2022 at 2:07
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    In addition: automatic transfer switch where the power goes out and the generator fires up, no human involved, or manual transfer switch. The former case would apply to Gil's "worst case method" while the latter case allows considerably smaller investment by load management by human. Turn this load off while running that load since power is limited. In my case I want my mini-splits to run, as they are my heat, but I also know what it takes to run them, worst-case. It would be a very odd event that I'd feel the need to run them for A/C in a power outage.
    – Ecnerwal
    Commented Feb 7, 2022 at 3:00
  • What do you want to run on your generator? That's the first and largest question you need to ask here Commented Feb 7, 2022 at 5:41
  • For power for dryer and A/C units check the labels. The dryer should be inside the door, the A/C units will list "minimum circuit ampacity". Oven is anybody's guess what you will use. Commented Feb 8, 2022 at 3:24

2 Answers 2

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Question 1 no. Question 2 "look at the nameplate".

The rest of this answer is a frame challenge - that's why it doesn't 'answer the question'. The frame challenge is: No, you can't do that. Code says you don't get to invent your own self-styled technique for doing load calculations. You have to do it by the book.

Noting that a generator is involved, there's another book too: your power company's Service Agreement / Rules & Regs.

Automatic transfer switch

With an automatic transfer switch, Code absolutely requires the generator be large enough to pick up "the entire load to be served". That's because you may not be there to run around turning off water heater, A/C etc. to stop the engine from bogging.

What is "The entire load to be served"? That is definitively answered by a Load Calculation on the house. The Load Calculation is a standard procedure in NEC article 220 that requires an inventory of large loads, a fixed allocation for kitchen, bathroom, laundry 120V circuits, and then "3 watts per square foot" as a catch-all for lighting loads and miscellaneous receptacle circuits. So you don't need to find the nameplate on your laser printer or cable box; that's in the catch-all.

"Throwing all your loads on and clamping your service wires" is not a substitute for a Load Calculation. There is an allowance in NEC for observed loads, but it needs to be better done than a one-time "clamp job".

There are a couple of exceptions. First, the Load Calculation itself allows loads to be excluded that will never run simultaneously; such as electric heating and (obviously electric) air conditioning.

Second, for "generator sizing" purposes, you can exclude a load wired through a "Load Shed" or "Demand-side management" device. In the simplest form, it's a box you put in front of the dryer or water heater that automatically disconnects them if AC power frequency and voltage suggest a lugging generator.

In the more complex form, you have emerging technology like Eaton, Square D, Leviton and Span's "smart service panels". The boutique product is the choke gulp $6000 Span service panel, where every breaker has energy monitoring and programmable disconnect. You can tell the Span "When on generator, don't let the house exceed 6KW". As obscene as the Span's price is, it solves problems that might be even more costly to solve otherwise.

Manual transfer switch (much easier)

However, if you are using a manual interlock or transfer switch (Square D or an aftermarket maker will make an interlock for that panel in the $60-80 range; that plus a $12 breaker takes care of that requirement), then you are allowed any size generator you want, since you'll be presumed to be there to cut off the water heater if it cycles at an inopportune moment.


Note that from a sizing perspective, if you get a generator that is capable of 9.24 KW (38.5A @ 240V), it will not be able to carry a split load of 30A and 47A. At the risk of stating the obvious, 47 is larger than 38.5.

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  • I am not sure I follow. If I turn on the devices I plan to use then measure the current draw on those devices, how does that not give me the most accurate representation of the size of the generator I would need to support those devices?
    – amrog
    Commented Feb 7, 2022 at 16:49
  • @amrog Since you are using an automatic transfer switch, you won't be able to pick and choose which appliances are running when the power fails. Heck, you may not even be on site, might be the only people there are guests or family members, and they don't know what to do. On the other hand if it's a manual transfer switch you're allowed to do anything you want, so you're right on track. Commented Feb 7, 2022 at 19:47
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    @amrog I added answers to your literal questions. As to why you can't freestyle your own load calculation, it's "Because NFPA said so". My dowsing of their reasons is that people "Free-styling" load calculations tend to be wrong - pollyanna at best and self-serving at worst. NFPA has invested huge science in a gold-standard method that's proven the test of time, and it's easy to implement without measuring. Commented Feb 8, 2022 at 21:38
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    Which you'd have to repeat on a hot day in the summer with the A/C running flat out. Science takes time, so relax and reschedule your install for next year...while looking up the proper approved procedure for this type of science ("observed loads") already referred to in the answer, and equipping yourself to do that, or hiring it out. Likewise, unless you have instantaneous measurement of amps and volts, the number you get by assuming constant voltage for measured amps is not the actual power. Go science power factor for AC power and learn a thing or three.
    – Ecnerwal
    Commented Feb 8, 2022 at 23:49
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    @amrog it isn't good science just because you do it. Much the opposite, it's vanity and reveals a common blunder: to quote the master: "Each field has its secret insider gotchas. These are certain to cause major grief to the casual inquirer. Accurately measuring rms power or doing low ∆t calorimetry are two obvious examples." (and I didn't even realize it mentioned RMS power when I went to quote it, speaking of that, are you dealing with VA properly?) See also bullet points 7-8 next column. Commented Feb 9, 2022 at 1:25
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When I did my whole house generator I went with worse case load, not what I assumed might be on. Failure does not check if the Dryer etc are on or off. I measured the current in each leg when everything was turned on including the lights. That came to about 12KW. The next larger generator they had was a 19 KW one for only a few hundred more so I went that way. That was about 20 years ago, it is still operating properly today without any problems. I do get it serviced every year and tech said I was smart upsizing, it puts less strain on the gas engine that drives it and it will last longer. The oil today is still clear, not black. It does run on natural gas which helps keep it clean.

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