Author Topic: How many watts for 2.5 ton central AC unit  (Read 5911 times)

Offline jrace4fun

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How many watts for 2.5 ton central AC unit
« on: June 02, 2013, 05:05:51 PM »
How do I figure out how many watts my 2.5 ton AC needs to start & run? Is there a way for me to measure how many watts it is consuming on start up and when it is running?

I've seen a range between 4000 and 20000 watts. That's a big range.

My current generator has 5700 continuous and 7150 surge watts and it's not enough power. It will turn on but it runs very slow. This generator has no problem running two 1.0 ton portable air conditioner units but I really want something that is going to power my central ac unit.

Here is my air conditioner information http://flic.kr/p/eB48KN

Offline TexasGirl

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Re: How many watts for 2.5 ton central AC unit
« Reply #1 on: June 02, 2013, 07:46:59 PM »
Large motor loads on a generator are figured differently from convenience outlet loads, which are calculated in watts and volt/amperes (yes, there is a difference). 

The major component of an airconditioner compressor load is the "hard start" surge required.  Larger generators usually have a nameplate value for the largest standard motor horsepower they can start.  Conventional compressor motors will usually require more surge capacity.  Each generator is constructed differently, and handles surges differently.  One 8kw might work fine, and the next may not.  Some have a heavy "flywheel" effect and others not.  You get the idea...

First choice is to contact the (or a) generator manufacturer and simply ask.  Second choice is to look at the A/C nameplate to see the minimum circuit fuse size, find a generator which has 125% of that value.  I'd still want a manufacturer to recommend one, though, that's a large purchase.

~TG

Offline Roknrandy

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Re: How many watts for 2.5 ton central AC unit
« Reply #2 on: June 03, 2013, 09:41:40 AM »
Ac's need large starting wattage
                                      running     additional starting
Window AC – 10,000 BTU     1200              1800
Window AC – 12,000 BTU     3300              4000

Central AC – 24,000 BTU    3800                5000
Central AC – 40,000 BTU    6000                6700

You have to add in the Furnace motor also

Furnace Fan Blower 1/2 HP  800                  2400
Furnace Fan Blower 1/3 HP  700                  1400

(Plus anything else you plan on running is additional)

This is why its suggested you run a window unit for one room so you doent need a giant genset and the massive amounts of gas (large gens will use a gallon an hour or more)

Im getting ready to post a video in the next 2 weeks about this very subject.

Ya know, the more I think about it the madder I get.  Our motto, what we believed down in our heart, went something like this "ye though I walk through the Valley of Death we will fear no evil...for we are the baddest mother strawberry pickers in the Valley".

FAILURE TO PLAN ON YOUR PART DOES NOT CONSTITUTE AN EMERGENCY ON MY PART

Offline jrace4fun

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Re: How many watts for 2.5 ton central AC unit
« Reply #3 on: June 03, 2013, 03:49:12 PM »
Thanks for the information. I'm thinking about getting a large generator just to run the central ac. This way I'm not burning a ton a fuel and I also have some redundancy built into the system.

Roknrandy, where can I watch your video?
« Last Edit: June 03, 2013, 03:55:39 PM by jrace4fun »

Offline mxitman

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Re: How many watts for 2.5 ton central AC unit
« Reply #4 on: June 03, 2013, 04:23:25 PM »
Hey, I've got allot of experience in this realm, being an HVAC/Electrician not to mention installing high end residential generators and I sell my own brand of single phase transfer switches.

Just off your nameplate; your looking at 15A running, thats 14.1 RLA for the compressor and then .9 for your cond fan. RLA is running load amps, your minimum ampacity is 19, and the min fuse size is 25.

I would base it off that to start with, but you must also consider the indoor blower unit which is probably 120V and 3-7 Amps...for 2.5 ton my guess is 3.5-4.5 Amps.

So overall your looking at say 30 Amps. Volts x Amps = Watts. Outdoor unit is roughly 5200-6000 watts running, starting is usually rated at 125% of that. So now your talking at least 7500 watts just for the outdoor unit. The indoor unit will probably need at least 800, bot more like 1000-1200 watts just to be sure.

The most accurate way to do this is by measuring the starting amps with an accurate meter, that's how we do it for minimal use...but most of the time I've also sized for additional loads.

Like was said before, not all manufacturer's are the same, you need to look past residential generator units if you want a 8000K Watt generator to work for you. If you got a 10K or 12K you will probably be okay, start the indoor unit first though to ramp up the generator.

I like to use work duty rated generators like WINCO, Multiquip, MTQ... Onan, Generac, anything you would probably see at a construction site will be rock solid for AC units.
http://ricksdiy.com http://heezy.com/ http://woodybeds.com

I'd rather have it and not need it, than not have it when I do need it.

Offline Roknrandy

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Re: How many watts for 2.5 ton central AC unit
« Reply #5 on: June 03, 2013, 05:02:13 PM »
Thanks for the information. I'm thinking about getting a large generator just to run the central ac. This way I'm not burning a ton a fuel and I also have some redundancy built into the system.

Roknrandy, where can I watch your video?

My youtube channel http://www.youtube.com/user/roknrandy

Ya know, the more I think about it the madder I get.  Our motto, what we believed down in our heart, went something like this "ye though I walk through the Valley of Death we will fear no evil...for we are the baddest mother strawberry pickers in the Valley".

FAILURE TO PLAN ON YOUR PART DOES NOT CONSTITUTE AN EMERGENCY ON MY PART

Offline Tony Lekas

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Re: How many watts for 2.5 ton central AC unit
« Reply #6 on: November 09, 2013, 01:25:33 AM »
mxitman - Over what time period is the starting surge measured.  I have a clamp on amp meter with an 100ms inrush setting.  On my 240V well pump I am seeing 49A inrush with 6.9A running.  Almost 12KW to start!  For my 120V freezer the running current is low but the inrush is 44A!

I started looking into this because I could not start my freezer or fridge with a Cobra 800W or Whistler 1600W inverter.  I finally could with a Whistler 3000W one.  In fact it could start both at the same time so long as they had been off for 5 min to let the pressure equalize.

I am wondering if the 100ms peak is the right measurement for a generator.  I would expect there to be limits on what the generator could put out for a short term peak.  But I could see a second not as short issue if the motor slows significantly during the peak.  I would expect that to take more than 100ms although maybe not.

I am looking into backup power options and I don't want to have to run a generator all the time.

Offline backwoods_engineer

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Re: How many watts for 2.5 ton central AC unit
« Reply #7 on: November 09, 2013, 08:26:24 AM »
Thanks for the information. I'm thinking about getting a large generator just to run the central ac. This way I'm not burning a ton a fuel and I also have some redundancy built into the system.

Running a central A/C system off a generator is not a fuel-efficient option for even a short-term loss in electrical grid.  You don't need to cool the whole house; figure out a couple of critical areas (living room, bedroom) and find window or portable air conditioning units for them.  I have a window A/C unit  in my office upstairs (doesn't get cool enough in that corner of the house, with all my electronics equipment running).  I can easily run it off my little 1800W generator.  If the power goes down in the summer, I can bring that window unit down the the master bedroom, and the three of us can sleep in there in comfort.

Also, those generators will DIE if you run them 24/7 for more than a couple days.  They are simply not meant for continuous operation.  If you want that, buy a military surplus generator.