Emergency Generator Sizing vs. AC Namplate

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arneson

Member
We are getting ready to buy around ten 15KW to 30KW single phase 240 vac generators in our neighborhood.

The Generac company and several other generator suppliers state that the LRA (locked rotor amps) spec on the AC compressor is what you size the generator for.

My review of the internet and NEC states that the LRA is to allow proper sizing of the cable and disconnect, so that no damage occurs before the breaker blows, and has nothing to do with compressor surge when starting.

Many sources state that the starting surge for a AC compressor is based on RLA (running load amps) and the ampacity spec on the compressor nameplate, which is 1.25 x RLA. You have to add for the compressor fan and house blower.

These sources also state that proper sizing of the single phase 230 generator includes 1.25 x ampacity, and you will have plenty of power to start the compressor. Of course other resistive and reactive loads need to be considered for overall sizing, but lets keep this discussion just focused on the AC compressor.

Question, should we use LRA, which will require a larger generator (LRA is 129 amps for my AC which would require 30KW, or assuming 1.5 surge generator capability 20KW + 10KW fo my other loads for 30KW total)

OR

ampacity x 1.25 (31.3 x 1.25 = 40 amp) 40 x 230 = 9.2KW + 10KW for my other loads for 20KW total?

Regards,

Greg Arneson
 

rick hart

Senior Member
Location
Dallas Texas
Re: Emergency Generator Sizing vs. AC Namplate

LRA unless there is a way to soft start the motor-VFD, etc.

The 125% is used to make sure there is enough copper in the wire to accommodate the short term starting currents. Your problem with a generator is the short term currents are there until the rotor starts to spin. If there is not enough reserve current available the motor will just sit there taking all the generator can put out until the fields collapse.

Besides, the manufacture said to use LRA.
 

bob

Senior Member
Location
Alabama
Re: Emergency Generator Sizing vs. AC Namplate

Question, should we use LRA, which will require a larger generator (LRA is 129 amps for my AC which would require 30KW, or assuming 1.5 surge generator capability 20KW + 10KW fo my other loads for 30KW total)
You must consider the LRA when sizing the generator. Generally it will take about 4 to 5 times the full load amps to start the A/C unit.
The generator must be able to deliver this amount of load until the compressor and fans are running.
OR ampacity x 1.25 (31.3 x 1.25 = 40 amp) 40 x 230 = 9.2KW + 10KW for my other loads for 20KW total?
This has little or nothing to do with sizing the generator to handle the A/C unit.
I ran a quick check and the size you are showing, 30 kw, is in the ball park.
I suggest you get with Generac . They have a software program that will caculate the proper size for your needs.
What area are you in?

[ December 16, 2005, 03:07 PM: Message edited by: bob ]
 

ty

Senior Member
Re: Emergency Generator Sizing vs. AC Namplate

Who is installing the units??

Did you speak to Cutler Hammer?
Honestly, I would go with CH on this size residential unit.

Also CH now has an air conditioner module that has 2 CT's inside the transfer switch and all that you need is a low voltage 2-conductor bell wire ran to the control circut of the A/C (up to a 4-ton unit).

It automatically senses when the A/C tries to start, and if there is enough amperage, the control board will allow it.

[QB] Question, should we use LRA, which will require a larger generator (LRA is 129 amps for my AC which would require 30KW, or assuming 1.5 surge generator capability 20KW + 10KW fo my other loads for 30KW total)

OR

ampacity x 1.25 (31.3 x 1.25 = 40 amp) 40 x 230 = 9.2KW + 10KW for my other loads for 20KW total? {/QB]

Not sure where you got the figures for your unit. Are you certain about 9.2kw?
In any case, it sounds to me like the 20kw would be fine, and a 17.5kw might work also.

In any event, go with the manufacturer recommendations. They have got it worked out to make it simple.
You can go online and use a sizing chart, that will get you in the ball park.

Cutler Hammer's can be linked to from Here.
 

charlie tuna

Senior Member
Location
Florida
Re: Emergency Generator Sizing vs. AC Namplate

remember, the new scroll compressors are being used to get the efficency up on a/c units. they have a very high start current but less moving parts. the average home today has electronics in all their appliances. if you are intending on operating a/c equipment--consider increasing the generator's rating to prevent large voltage drops when the a/c's compressor starts. if it were my home i would go for an additional 10 kw. but, i just had no power for 16 days and never needed my a/c. i was cutting trees for seven days -- and after a day of that, sleep comes easy!!
 

bob

Senior Member
Location
Alabama
Re: Emergency Generator Sizing vs. AC Namplate

Ty
Not sure where you got the figures for your unit. Are you certain about 9.2kw?
He used the FLA x 1.25 to get the 9.2 kW. It should be 31.5 x 240 = 7512 watts. I used the CH
caculator and still came up with a 25 kw gen.
 
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