# Rule of thumb for RTU..



## B4T (Feb 10, 2009)

Is there a formula for figuring out power requirements for single phase and 3/phase A/C RTU per ton.. sometimes the nameplate data is not readily available. 

This is only for rough estimating until I can get "hard" numbers


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## wildleg (Apr 12, 2009)

rule of thumb won't get you right size wire and breaker for the unit. pull a number out of your ass or get the right info so you don't screw yourself (JMO)


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## MDShunk (Jan 7, 2007)

You bet there is. One horsepower per ton. 

Note to the naysayers.... if you don't believe me, get out any manufacturer's literature on RTU's and look for yourself. One horsepower per ton gets you damn close for estimating purposes every time. About 90% of the time, you could pull your circuit conductors based on that rule of thumb too (with the exception being units with resistance backup heat).


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## knowshorts (Jan 9, 2009)

If the nameplate data is not available, why not just look at the M drawings? Or shoot an RFI? Or state in your proposal that an allowance has been made up to 60A or something like that.


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## Innovative (Jan 26, 2010)

MDShunk said:


> You bet there is. One horsepower per ton.
> 
> Note to the naysayers.... if you don't believe me, get out any manufacturer's literature on RTU's and look for yourself. One horsepower per ton gets you damn close for estimating purposes every time. About 90% of the time, you could pull your circuit conductors based on that rule of thumb too (with the exception being units with resistance backup heat).


I disagree.....Case in point
Trane XL16B100ES (480 v 3 phase) 100 Ton Heat pump Min circuit ampacity: 84 amps
Max breaker: 100
Your method: 155 amps

Trane XL196R4512 (240v single phase) 4.5 Ton Heat pump Min circuit ampacity: 38
Max breaker: 50
Your method: 28 amps (based on 5 HP)

These are the last two RTU we hooked up. Both are High efficency units, one a 16 SEER and one a 19.2 SEER with a dual stage compressor. These units were hooked up last week.


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## wildleg (Apr 12, 2009)

Only reason I said no is cause of problems in past - if they can't give me the info I won't nail my neck to the wall, cause no matter what that first price that got thrown out there just keeps coming back like a bad seafood dinner, even if they changed all the equipment. One size pipe and one wire size up can run into a bunch of money, so I just stay away from trying to guess when there's no real benefit to me to do so. I'll exclude it, or put in an allowance for it, but I won't guess at it.


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## jwjrw (Jan 14, 2010)

I never rough estimate anything they want a solid number on with out a cut sheet.


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## 480sparky (Sep 20, 2007)

jwjrw said:


> I never rough estimate anything that want a solid number on with out a cut sheet.



Once again. In English, please.


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## Magnettica (Jan 23, 2007)

An electrician once told me 10 amps for each ton of cooling is close.


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## B4T (Feb 10, 2009)

Magnettica said:


> An electrician once told me 10 amps for each ton of cooling is close.


The RTU I posted pics of a few weeks ago was a (5) ton and got a 60A breaker, so that "formula" might be pretty accurate...


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## jwjrw (Jan 14, 2010)

480sparky said:


> Once again. In English, please.


 
Damn spelling nazi's!! That was english. I just used that when I meant they.:yes:


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## 480sparky (Sep 20, 2007)

jwjrw said:


> Damn spelling nazi's!! That was english. I just used that when I meant they.:yes:



Everything was spelled correctly. I just doesn't make sense.


Oh, and there's no apostrophe in nazis.


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## ToolBag (Aug 29, 2010)

480sparky said:


> Oh, and there's no apostrophe in nazis.


But there is a capital "N".


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## wildleg (Apr 12, 2009)

screw em. they don't deserve a capital n.


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## ampman (Apr 2, 2009)

480sparky said:


> Once again. In English, please.


somehow i knew what he meant


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## ampman (Apr 2, 2009)

12,000 btu = 1 ton most units will have a multiple of 12 in them for the tonage as seen in the nameplate


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## Bob Badger (Apr 19, 2009)

Magnettica said:


> An electrician once told me 10 amps for each ton of cooling is close.


That falls apart for different voltages and single or three phase.

Assuming that electrician was talking about single phase 240 he is in the same ball park as Marc, about 1 HP per ton.

But 10 amps at 480 3 phase is about 7 HP.


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## 220/221 (Sep 25, 2007)

> An electrician once told me 10 amps for each ton of cooling is close.


Was it me? 

That's what I was going to throw out.

If it's single phase, most 3 ton units I do are 30A circuits. 

With the newer, more efficient units they are now making, even the 5 ton units, a 50 A circuit is almost never required.

With three phase, go down a size.


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## MDShunk (Jan 7, 2007)

Innovative said:


> I disagree.....Case in point
> Trane XL16B100ES (480 v 3 phase) 100 Ton Heat pump Min circuit ampacity: 84 amps
> Max breaker: 100
> Your method: 155 amps
> ...


You should also know that the manufacturer's are starting to play games with their tonnage ratings. For instance, one particular York 3-ton unit (which should be 36,000 BTU) is only 34,200 and the 3.5 ton unit (which should be 42,000 BTU) is only 35,400!! A ton used to be a ton. More and more, a ton is a marketing unit, and you have to read the dateaplate to see that you're really buying. Much like shop vacs are sold by "peak horsepower", which is like buying a motor based on what the horsepower would be if it was drawing LRA.


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## RIVETER (Sep 26, 2009)

MDShunk said:


> You should also know that the manufacturer's are starting to play games with their tonnage ratings. For instance, one particular York 3-ton unit (which should be 36,000 BTU) is only 34,200 and the 3.5 ton unit (which should be 42,000 BTU) is only 35,400!! A ton used to be a ton. More and more, a ton is a marketing unit, and you have to read the dateaplate to see that you're really buying. Much like shop vacs are sold by "peak horsepower", which is like buying a motor based on what the horsepower would be if it was drawing LRA.


I knew you would have an answer. Are they possibly increasing the size/volume of the coils?


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## MDShunk (Jan 7, 2007)

RIVETER said:


> I knew you would have an answer. Are they possibly increasing the size/volume of the coils?


Yeah, but that's how they're getting the efficiencies up. Has nothing to do with the fact that a certain tonnage (1 ton=12,000 btu of cooling) is being marketed as more than it really is.


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## RIVETER (Sep 26, 2009)

MDShunk said:


> Yeah, but that's how they're getting the efficiencies up. Has nothing to do with the fact that a certain tonnage (1 ton=12,000 btu of cooling) is being marketed as more than it really is.


Okay, is it because it takes less than a ton of cooling to get the effect of a ton of cooling?


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## Jlarson (Jun 28, 2009)

MDShunk said:


> You should also know that the manufacturer's are starting to play games with their tonnage ratings. For instance, one particular York 3-ton unit (which should be 36,000 BTU) is only 34,200 and the 3.5 ton unit (which should be 42,000 BTU) is only 35,400!! A ton used to be a ton. More and more, a ton is a marketing unit, and you have to read the dateaplate to see that you're really buying. Much like shop vacs are sold by "peak horsepower", which is like buying a motor based on what the horsepower would be if it was drawing LRA.


^ totally agree it's all about the $$$. The last RTUs I worked on were 3 ton and pulled like 15 or 20 amps. By the way the AC guys over fused and breakered at like 60 and tapped a evap. off that and started a small fire in one of the freaking discos, ok that's a separate rant.

Plus my shop vac (I bought it cause it was cheap NOT cause of the HP) is not even close to what its rated, if it was running at what the label says it should be able to suck paint off walls across the room.


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## Cletis (Aug 20, 2010)

*Rtu*

I use this homemade formula

Single phase 240 Volt Tons x 10 amps x 1.0 (15SEER) 
Every SEER up or down multiply by 0.1 other direction

3 phase 208V Tons x 10 amps / 1.73 x 1.0 (15 SEER) 
Repeat as needed

If 460 then x 0.5 

Ex. 20 Ton 460 Volt 16 SEER

20 x 10 amps / 1.73 x 0.9 x 0.5 

1 x 10 / 1.73 x 0.9 x 0.5 = 52 amps @ 460V

I was drunk when I made that though? 

Gets me close enough if units are not there yet


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## Innovative (Jan 26, 2010)

MDShunk said:


> You should also know that the manufacturer's are starting to play games with their tonnage ratings. For instance, one particular York 3-ton unit (which should be 36,000 BTU) is only 34,200 and the 3.5 ton unit (which should be 42,000 BTU) is only 35,400!! A ton used to be a ton. More and more, a ton is a marketing unit, and you have to read the dateaplate to see that you're really buying. Much like shop vacs are sold by "peak horsepower", which is like buying a motor based on what the horsepower would be if it was drawing LRA.


Same with boats..... my "35' Donzi" measures 33'10", how is that 35'


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## MDShunk (Jan 7, 2007)

RIVETER said:


> Okay, is it because it takes less than a ton of cooling to get the effect of a ton of cooling?


Nope. It's more like rounding 3/4" to 1".


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## Eddie702 (Aug 7, 2015)

208 0r 240 3 phase is about 4a/ton. Cut it in half for 480v.


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## danickstr (Mar 21, 2010)

2 ton 20 amps
4 ton 40 amps
5 ton 50 amps
Is good for general load calcs


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## MTW (Aug 28, 2013)

danickstr said:


> 2 ton 20 amps
> 4 ton 40 amps
> 5 ton 50 amps
> Is good for general load calcs


Except efficiency is way up now so a 5 ton unit won't draw anything near 50 amps anymore.


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## Bootss (Dec 30, 2011)

MTW said:


> Except efficiency is way up now so a 5 ton unit won't draw anything near 50 amps anymore.


What are these guys doing these days to increase efficiency?


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## Black Dog (Oct 16, 2011)

danickstr said:


> 2 ton 20 amps
> 4 ton 40 amps
> 5 ton 50 amps
> Is good for general load calcs



*Rest in peace Black4Truck.....B4T....Bob.*


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## MTW (Aug 28, 2013)

Lep said:


> What are these guys doing these days to increase efficiency?


13 SEER is the minimum efficiency allowed now which is possible because of much larger condensing coils and scroll compressors. Also, fudging the BTU numbers as previously mentioned.


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## 220/221 (Sep 25, 2007)

Cletis *and* B4T :thumbup:

I miss the olden days.


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## telsa (May 22, 2015)

Lep said:


> What are these guys doing these days to increase efficiency?


The big 'thing' has been the move to scroll compressors.

Another big 'thing' is the move to variable speed fan blowers -- basically putting them on a dedicated VFD with a tie in to BAS. (Building Automation)

The more $$$ then the trickier the bells and whistles become.

Pilot production of fully VFD compressors has been under way for some time. These have been put in tricked up show houses... the old "House of Tomorrow" trade exhibits. Hermetically sealed three-phase refrigeration motors last virtually forever.

The continued drop in the cost of solid state power circuits should make this a standard product feature some time soon.

The energy savings possible by slowing down fluid flows -- either the refrigerant, itself, or the cooling air blast -- are nothing short of remarkable.

%%%

FYI:

A 'one - ton' unit is short hand for the cooling power of a single ton of ice melting within 24 hours -- ie 'one ton of ice' is the ton being referred to.

It got started as a marketing ploy (as was Watt's 'horsepower') back when cooling by ice was the only known method, and major users had it dropped off every morning, tons at a time. Ice put Green Bay on the map.

The latent heat of a ton of ice is 288,000 BTUs or 12,000 BTUs per hour. 

%%%

As to the original query... in our era of Google and pdfs -- I can't imagine why one need guess.


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## Dash Dingo (Mar 3, 2012)

Magnettica said:


> An electrician once told me 10 amps for each ton of cooling is close.



What is measured in tons?


Sent from my iPhone using Tapatalk


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## Chris1971 (Dec 27, 2010)

220/221 said:


> Cletis *and* B4T :thumbup:
> 
> I miss the olden days.


:thumbup:


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## Big John (May 23, 2010)

Dash Dingo said:


> What is measured in tons?


 It's how you determine the cooling capacity of an AC: My condensor is at least a ton because I damn near got a hernia trying to lift it.:jester:

I think it actually refers to the energy needed to melt 1 ton of ice in an hour. Holdover from the days when that was how they cooled the forced-air.


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## mgraw (Jan 14, 2011)

telsa said:


> The big 'thing' has been the move to scroll compressors.
> 
> Another big 'thing' is the move to variable speed fan blowers -- basically putting them on a dedicated VFD with a tie in to BAS. (Building Automation)
> 
> ...


All of that is already here. Foreign manufacturers are a few years ahead of US manufacturers with this stuff. Last year I did the electrical on some replacement units that were all DC. Power was connected to an inverter on both the air handler and condenser. On this job the building had eight 4 ton units. Two 100 amp 208V 3PH circuits feed the eight condensers. Eight 1PH 208V 80 amp circuits feed the air handlers. They were replaced with four 8 ton condensers and sixteent 2 ton air handlers. Two 125 amp 208V 3PH circuits for the condensers and two 1PH 208V 20 amp circuits for the air handlers.


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## Southeast Power (Jan 18, 2009)

220/221 said:


> Cletis *and* B4T :thumbup:
> 
> I miss the olden days.


Cletis got kicked in the head by his mule. 
He turned into the savont we all know here as Tesla.


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## MTW (Aug 28, 2013)

When in doubt, run an empty raceway.


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## telsa (May 22, 2015)

Big John said:


> It's how you determine the cooling capacity of an AC: My condensor is at least a ton because I damn near got a hernia trying to lift it.:jester:
> 
> I think it actually refers to the energy needed to melt 1 ton of ice in an hour. Holdover from the days when that was how they cooled the forced-air.


*That's ONE TON per 24 Hours.* :thumbsup:

A ton of ice is 288,000 BTUs in latent heat of absorption.

This works out to 12,000 BTU per hour.


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## Magnettica (Jan 23, 2007)

Dash Dingo said:


> What is measured in tons?
> 
> 
> Sent from my iPhone using Tapatalk


1 ton of cooling is measurement in BTU's. 

12,000 BTU's is (1) ton of cooling.


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## IEC (Sep 20, 2015)

Typical and average thumb rules are fine. RFI's are better.

In today's digital age of almost instant communication, I can't understand why anyone would forgo an RFI for a rule of thumb. When I was running work back in 2000, an RFI could take a week or even two to be answered. Today, it's an hour or two if the GC is on top of things. 

I like Cletis' formula. Looks pretty accurate to me.


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