Question - Generator Sizing and AC Pumps

[7th Kahuna]

I am trying to reach a better understanding of generator sizing when it comes to pumps for refrigerators and AC units.

I have found a wide range of 'directions' online. Maybe one of you guys knows.

As you will be aware, if you are answering this question, starting watts (or amps) are higher than running watts (or amps) for electric motors.

The rule(s) of thumb for running versus starting watts seems to vary from 2x to 6x with 3x seemingly being the most common. My fridge, for instance, is 7.9 amps A.C.. So walking it through, the initial math looks like this:

7.9 x 120 x .8 = 758 watts (running) and 3x running watts equals 2,274 watts (starting).

The charts I have seen suggest a generic rate of 2,800 to 3,000 starting for a refridgerator, which potentially makes sense if during the start-up, the power factor were 1 rather than .8 and thus:

7.9 x 120 x 1 = 948 watts (running) and 3x running being 2,844 watts (starting).

So if all of that makes sense, then I am at a complete loss for the Air Conditioner.

Per the charts, a 10k Btu AC unit requires 1,500 watts (running) and 2,200 watts (starting). 2,200 watts is a lot less than 4,500 watts (3x 1,500). Both electric motors are compressing gas. What is the difference? Is the 2,200 watt suggestion safe?


A secondary question, but one that I think I know the answer to, in calculating the max or surge watts for the generator, the book says add the starting amps for the single highest use appliance to the running amps calculated for the load. This seems over-simplified to me. Surely if you were running a table saw and a microwave from the generator, you could avoid running both at the same time, but an AC unit and a refrigerator could both come on together. Am I over-thinking this? I am adding the starting watts together for those two and subtracting both their running watts in order to calculate the surge requirement.

(Total Running Watts + Surge Watts AC Unit + Surge Watts Fridge - Running Watts AC Unit - Running Watts Fridge = Total Surge Watts)

 

[7th Kahuna]

Re: Question - Generator Sizing and AC Units

Any electricians out there able to comment on this?

I don't know which is worse, posting a question that everyone seems to have know the answer to except you, or posting the question that stumps the group.

 

[theoldwizard1]

Starting current is very difficult to measure. Calculating it, is at best a SWAG ! !

You can greatly reduce your starting current by using a REAL "soft starter" like the MicroAir EasyStart

This is NOT just a big starting capacitor ! It actually limits the amount of current being used (to charge up the capacitors) during the starting process.

 

[7th Kahuna]

Thank you theoldwizard1

" Starting current is very difficult to measure. Calculating it, is at best a SWAG ! ! "

Ok, that helps. Thought I was missing something. Also thanks for the link, I'll take a look.

 

[wyliesdiesels]

Starting current is very difficult to measure. Calculating it, is at best a SWAG ! !

You can greatly reduce your starting current by using a REAL "soft starter" like the MicroAir EasyStart

This is NOT just a big starting capacitor ! It actually limits the amount of current being used (to charge up the capacitors) during the starting process.

Incorrect on both accounts.

Certain multimeters have specific circuitry that is designed to measure and capture(very well), the startup current of inductive equipment.

I own one such meter that does this- Fluke 381.

In order to get an accurate in-rush current measurement, one must use a clamp meter specifically designed to measure in-rush current. not all meters are designed to do this so check the specs...

As far as calculating startup current, it's not difficult to calculate.

https://electricalbaba.com/locked-rotor-starting-current/

 

[AP514]

This is an interesting Thread...almost the exact Question was going to ask.

He has a Pumps/AC and I have an Home AC unit.

The LRA listed on my Home unit says 65. I used an Amp Clamp to try to get the starting AMPs but my Meter only measures 3 times a second.( not in rush ) It read 31.3 Amps at Start.(at C/B )
(maybe for 1 sec) Then quickly dropped to 12 amps and fell from there to about 7.5. depending on what stage was running.
Anyway an LRA of 65..But the AC System is on a 40 AMP - C/B.
40-C/B = 9,600watts X 120% = 11,520 wats (just under 50 AMPs)
LRA-65 Amps =15,600 Watts. I guess the Surge is so short that the C/B does not trip ?? Most Portable Gens are not going to put out that many watts.
So my Question is if a Generator can not put out that many Watts what happens ?
My Honda Gen is a 7,000/12,000.
So, with that in mind How would you figure out if the Generator can handle the draw ?
I was thinking of using a start assist like a 5-2-1 (hard Start assist) , Soft Start or something like them. To help the Gen on the start up surge

 

[HenryAZ]

Use a Kill-o-Watt meter plugged into the wall, set to amps. Plug your device into the Kill-o-Watt meter and start it up to see the actual amperage draw on startup. This only works for 120V, though. For 240V, you'd have to use a clamp meter inside the breaker panel. Have your SO start the device while you stand at the box and read the amperage draw on startup.

 

[Showkey]

You guys realize in the real world when the outside temperatures are 100* plus.....cycling AC under these conditions

AC unit might draw more ........Aka start hard
Generator might produce less........winding and unit hotter than the sun !!!!

Add some generators are a whole lot better than others......under stress, heat and load.

So yes........SWAG .........might be the answer.

 

[wyliesdiesels]

Use a Kill-o-Watt meter plugged into the wall, set to amps. Plug your device into the Kill-o-Watt meter and start it up to see the actual amperage draw on startup. This only works for 120V, though. For 240V, you'd have to use a clamp meter inside the breaker panel. Have your SO start the device while you stand at the box and read the amperage draw on startup.

and the OP is asking about 240v equipment so that wont work...

 

[theoldwizard1]

Incorrect on both accounts.

Certain multimeters have specific circuitry that is designed to measure and capture(very well), the startup current of inductive equipment.

I own one such meter that does this- Fluke 381.

Okay I should have used the word EXPENSIVE instead of DIFFICULT !

Your a professional. A $500+ meter is the "cost of doing business". Some guy noodling on a scratch pad is NOT going to buy one.

As far as calculating startup current, it's not difficult to calculate.

https://electricalbaba.com/locked-rotor-starting-current/

As the rotor is at rest during starting and voltage is rated voltage, the starting current will be equal to locked rotor current.

What those calculations do NOT include is the current required to charge the starting capacitor. Remember, uncharged capacitors look like a short circuit.
Yes, this is a very, very short spike (msecs) and occurs even before the voltages achieves its running voltage. This probably does not matter for power from the power line or even a generator, but it does to an inverter.

 

[theoldwizard1]

Anyway an LRA of 65..But the AC System is on a 40 AMP - C/B.
40-C/B = 9,600watts X 120% = 11,520 wats (just under 50 AMPs)
LRA-65 Amps =15,600 Watts. I guess the Surge is so short that the C/B does not trip ??

Correct

So my Question is if a Generator can not put out that many Watts what happens ?
When there is insufficient power from a generator the voltage sags. Lower voltage means higher current which means even more voltage sag.

My Honda Gen is a 7,000/12,000.
So, with that in mind How would you figure out if the Generator can handle the draw ?

Its a **** shoot !

I was thinking of using a start assist like a 5-2-1 (hard Start assist) , Soft Start or something like them. To help the Gen on the start up surge

A 5-2-1® COMPRESSOR SAVER® significantly reduces the amount of time your compressor is on high amp start-up. Some top-of-the-line A/C units come from the factory with 5-2-1® technology built-in. Now you can have a 5-2-1® COMPRESSOR SAVER® installed in your A/C unit by a professional.

I am not familiar with this device so I am going by the above description.

IMHO, it is NOT a true "soft start". It is just a start capacitor that is switched off by an external relay instead of a centrifugal motor start switch.

The problem is, un-charged starting capacitors look like a short circuit and require a lot of power to get them charged so that they can apply the extra power to the motor.

While it seems counterintuitive, you don't want to charge the start capacitor quickly ! Slower means less peak current. The way you "slow down" charging the starting capacitor is by switching in a current limiting resistor and then switching it out once the motor is running.

When I say "slow down" I mean changing 100 msec (?) starting time to maybe 1-2 seconds (?). This is a REAL "soft starter" MicroAir EasyStart

 

[AP514]

Well, took the cover off the 18 seer-2.5ton AC and my unit already has the HARD START KITS installed from the Mfr. (American Standard) For Both the Low and High stage Compressors.
Now a Soft Start really only will give good results if my Compressor is a Scroll Type.. That is Direct from the Easy Start.
Also my 2nd stage hits 64Amps and my fans are already running so I do not see how EASY START would cut the Amps down. Thoughts ? ?
Maybe I am missing something on how the SOFT START works ?