CLICK HERE AND JOIN OUR COMMUNITY TODAY, IT'S FREE!
Go Back   Electrician Talk - Professional Electrical Contractors Forum > Electrical Forum > General Electrical Discussion

Reply
 
Thread Tools Search this Thread Display Modes
Old 05-09-2009, 09:05 PM   #1
NJ-IEC Member
 
Magnettica's Avatar
 
Join Date: Jan 2007
Location: Rahway, NJ
Posts: 14,161
Default BTU Tonnage and to Amperage ratio

Is there a formula for determining amperage for A/C condensing units if the only info you know is the tonnage?

like...

1-ton equals 12,000 BTU's = 10 amps
2-ton equals 24,000 BTU's = 20 amps
3-ton equals 36,000 BTU's = 30 amps

This is the way I figure it when the nameplate rating is unknown.

__________________
Replies based on NEC 2014
Magnettica is offline   Reply With Quote
The Following User Says Thank You to Magnettica For This Useful Post:
Xknob&Tube (06-20-2014)
Join Contractor Talk

Join the #1 Electrician Forum Today - It's Totally Free!

ElectricianTalk.com - Are you a Professional Electrical Contractor? If so we invite you to join our community and see what it has to offer. Our site is specifically designed for you and it's the leading place for electricians to meet online. No homeowners asking DIY questions. Just fellow tradesmen who enjoy talking about their business, their trade, and anything else that comes up. No matter what your specialty is you'll find that ElectricianTalk.com is a great community to join. Best of all it's totally free!

Join ElectricianTalk.com - Click Here JOIN FOR FREE


Warning: The topics covered on this site include activities in which there exists the potential for serious injury or death. ElectricianTalk.com DOES NOT guarantee the accuracy or completeness of any information contained on this site. Always use proper safety precaution and reference reliable outside sources before attempting any construction or remodeling task!

Old 05-09-2009, 09:15 PM   #2
I own stock in FotoMat!
 
480sparky's Avatar
 
Join Date: Sep 2007
Location: Cornpatch USA
Posts: 35,593
Rewards Points: 26
Default

Quote:
Originally Posted by Magnettica View Post
Is there a formula for determining amperage for A/C condensing units if the only info you know is the tonnage?

like...

1-ton equals 12,000 BTU's = 10 amps
2-ton equals 24,000 BTU's = 20 amps
3-ton equals 36,000 BTU's = 30 amps

This is the way I figure it when the nameplate rating is unknown.
There's no linear relationship that is all that accurate.

Tonnage is a measurement of output. Amperage is a measurement of input.

FWIW, my conversion calculator has 1 BTU/hour = 0.2928104185 watts. Of course, that means the unit has to run 60 minutes for it to be accurate.

The transfer of one measurement to the other, while going upwards hand-in-hand, isn't that accurate. Efficiency of the unit, age, maintenance, etc. all play a part.

Think of converting automobile horsepower to MPG.

__________________

If you didn't exercise your right to vote, you lose your right to complain.

Last edited by 480sparky; 05-09-2009 at 09:18 PM.
480sparky is offline   Reply With Quote
Old 05-09-2009, 10:00 PM   #3
Wire Ninja
 
MDShunk's Avatar
 
Join Date: Jan 2007
Location: Beautiful Cumberland Valley, in PA
Posts: 17,339
Rewards Points: 2,068
Default

1 horsepower per ton has been a pretty decent rule of thumb for decades for air-cooled condensing units (condenser fans included). Smaller units a little more. Larger units, a little less.

As with everything else, try to get actual dataplate information (or at least a model number, cut sheet, or submittal) so you can get the real information.
__________________
One reason not to give DIY advice:
Catch a man a fish and you can sell it to him.
Teach a man to fish and you’ve ruined a good business opportunity.
MDShunk is offline   Reply With Quote
Old 05-09-2009, 11:13 PM   #4
Registered Member
 
Join Date: Feb 2008
Location: Atlanta, Ga/Hamilton, Al
Posts: 4,868
Rewards Points: 2,000
Default

A ton is not 12,000 BTU. It is 12,000 BTU/h, so it is a unit of power equivalent to about 3517 W. So, at 100% efficiency, a 1-ton, 120 V unit would draw about 29 A.

If you know the efficiency of the unit, you can get a rough estimate of current draw.
InPhase277 is offline   Reply With Quote
Old 05-09-2009, 11:41 PM   #5
Wire Ninja
 
MDShunk's Avatar
 
Join Date: Jan 2007
Location: Beautiful Cumberland Valley, in PA
Posts: 17,339
Rewards Points: 2,068
Default

Quote:
Originally Posted by InPhase277 View Post
A ton is not 12,000 BTU. It is 12,000 BTU/h, so it is a unit of power equivalent to about 3517 W. So, at 100% efficiency, a 1-ton, 120 V unit would draw about 29 A.

If you know the efficiency of the unit, you can get a rough estimate of current draw.
Uh.... not really. The compressor does not directly change electrical energy to cooling tons. It moves heat from one place to another. The breakdown is more realistically 1200 watts per ton when you consider the effect of the refrigerant, which is where the 1 horsepower per ton rule of thumb comes from.

Think about it.... if you were right, (and you aren't), consider the size of the circuit that would be required for a 10 or 12 thousand BTU window type air conditioner.
__________________
One reason not to give DIY advice:
Catch a man a fish and you can sell it to him.
Teach a man to fish and you’ve ruined a good business opportunity.
MDShunk is offline   Reply With Quote
Old 05-09-2009, 11:44 PM   #6
Senior Member
 
Join Date: Apr 2009
Location: NASA
Posts: 7,521
Rewards Points: 2,000
Default

I guess we are assuming 240 volt single phase equipment?

Seems we would need a KW per ton factor for it to be useful for any voltage or phase.
Bob Badger is offline   Reply With Quote
Old 05-09-2009, 11:46 PM   #7
Wire Ninja
 
MDShunk's Avatar
 
Join Date: Jan 2007
Location: Beautiful Cumberland Valley, in PA
Posts: 17,339
Rewards Points: 2,068
Default

Quote:
Originally Posted by Bob Badger View Post
I guess we are assuming 240 volt single phase equipment?

Seems we would need a KW per ton factor for it to be useful for any voltage or phase.
1.2 kw/ton for air cooled. .75 kw/ton for water cooled (chillers).
__________________
One reason not to give DIY advice:
Catch a man a fish and you can sell it to him.
Teach a man to fish and you’ve ruined a good business opportunity.
MDShunk is offline   Reply With Quote
Old 05-09-2009, 11:47 PM   #8
NJ-IEC Member
 
Magnettica's Avatar
 
Join Date: Jan 2007
Location: Rahway, NJ
Posts: 14,161
Default

I'm just having this discussion because often times I've been told "we've got a 4-ton unit needs to be wired up" and that's all information I get until I get there.
__________________
Replies based on NEC 2014
Magnettica is offline   Reply With Quote
Old 05-09-2009, 11:54 PM   #9
Wire Ninja
 
MDShunk's Avatar
 
Join Date: Jan 2007
Location: Beautiful Cumberland Valley, in PA
Posts: 17,339
Rewards Points: 2,068
Default

Quote:
Originally Posted by Magnettica View Post
I'm just having this discussion because often times I've been told "we've got a 4-ton unit needs to be wired up" and that's all information I get until I get there.
You're pretty safe to rough-in wire sized to Table 430.248 for a 4 horsepower motor. You'll have to wait for a dataplate to select a breaker, but 99 times out of 100, the motor amp draw chart will work if you go with one horsepower per ton. I just did a 3-ton last week, and the NEC says 18.7 for a 3 horsepower motor. The actual dataplate was 18.8. Pretty darned close.
__________________
One reason not to give DIY advice:
Catch a man a fish and you can sell it to him.
Teach a man to fish and you’ve ruined a good business opportunity.

Last edited by MDShunk; 05-09-2009 at 11:57 PM.
MDShunk is offline   Reply With Quote
The Following User Says Thank You to MDShunk For This Useful Post:
The_Modifier (06-20-2014)
Old 05-10-2009, 12:07 AM   #10
NJ-IEC Member
 
Magnettica's Avatar
 
Join Date: Jan 2007
Location: Rahway, NJ
Posts: 14,161
Default

Quote:
Originally Posted by MDShunk View Post
You're pretty safe to rough-in wire sized to Table 430.248 for a 4 horsepower motor. You'll have to wait for a dataplate to select a breaker, but 99 times out of 100, the motor amp draw chart will work if you go with one horsepower per ton. I just did a 3-ton last week, and the NEC says 18.7 for a 3 horsepower motor. The actual dataplate was 18.8. Pretty darned close.
That's an even better way to know it. Thanks.
__________________
Replies based on NEC 2014
Magnettica is offline   Reply With Quote
Old 05-10-2009, 10:06 AM   #11
Registered Member
 
Join Date: Feb 2008
Location: Atlanta, Ga/Hamilton, Al
Posts: 4,868
Rewards Points: 2,000
Default

Quote:
Originally Posted by MDShunk View Post
Uh.... not really. The compressor does not directly change electrical energy to cooling tons. It moves heat from one place to another. The breakdown is more realistically 1200 watts per ton when you consider the effect of the refrigerant, which is where the 1 horsepower per ton rule of thumb comes from.

Think about it.... if you were right, (and you aren't), consider the size of the circuit that would be required for a 10 or 12 thousand BTU window type air conditioner.
Yeah, absolutely right. I wasn't thinking thermodynamically, or soberly. But can a brutha get a pass? I had four Bacardi and Cokes at the steak house.
InPhase277 is offline   Reply With Quote
Old 05-10-2009, 10:11 AM   #12
Wire Ninja
 
MDShunk's Avatar
 
Join Date: Jan 2007
Location: Beautiful Cumberland Valley, in PA
Posts: 17,339
Rewards Points: 2,068
Default

Quote:
Originally Posted by InPhase277 View Post
Yeah, absolutely right. I wasn't thinking thermodynamically, or soberly. But can a brutha get a pass? I had four Bacardi and Cokes at the steak house.
You bet. Word on the street is that Coke is sometimes unfit to drink without requisite amounts of Bicardi or Captain.
__________________
One reason not to give DIY advice:
Catch a man a fish and you can sell it to him.
Teach a man to fish and you’ve ruined a good business opportunity.
MDShunk is offline   Reply With Quote
Old 06-20-2014, 12:14 PM   #13
Member
 
Join Date: Jul 2008
Location: Southern Colorado
Posts: 48
Rewards Points: 31
Default

Thanks for the chart! An inspector made me replace my 30 amp breaker with a 25 amp once.

__________________
Cut your own wood and it will warm you twice
Xknob&Tube is offline   Reply With Quote
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
wire amperage rating dsmith411 General Electrical Discussion 14 06-19-2010 11:56 AM
What is the effect of voltage and amperage on a person? Frasbee Workplace Safety 28 04-21-2009 12:27 PM
Increasing Amperage in retail commercial space. jttrieu General Electrical Discussion 2 09-28-2008 07:55 PM
What amperage isolator switch required Mr P.Dillon General Electrical Discussion 1 07-18-2007 05:30 PM


All times are GMT -4. The time now is 06:10 AM.


Copyright © 2006-2014 Escalate Media LP. All Rights Reserved
Our Pro Sites Network
ContractorTalk.com | DrywallTalk.com | HVACSite.com | PaintTalk.com | PlumbingZone.com | RoofingTalk.com