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#56852 - 10/01/05 03:08 PM 3.5 ton A/C condensor
ShockMe77 Offline
Member

Registered: 06/11/05
Posts: 823
Loc: Rahway, New Jersey
Where I work, I do alot of wiring for residential air conditioning "over-head" units. I instinctively know to bring with me from the shop 8.2 romex when I'll be wiring a 3.5 ton system. My question is, where does the term "3.5 ton" come from and what exactly is 3.5 tons a measurement of?

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#56853 - 10/01/05 03:29 PM Re: 3.5 ton A/C condensor
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
 Quote:
Question:
What is a BTU? What is a ton of cooling?

Answer:
A BTU (British thermal unit) is the amount of heat that will raise or lower the temperature of one pound of water one degree Fahrenheit. One ton of cooling equals 12,000 BTU's.
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#56854 - 10/01/05 03:32 PM Re: 3.5 ton A/C condensor
ShockMe77 Offline
Member

Registered: 06/11/05
Posts: 823
Loc: Rahway, New Jersey
Thanks Bob. I knew had something to do with BTU's. But I didn't know that 12,000 BTU's equaled 1 ton.

Thanks.

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#56855 - 10/01/05 06:09 PM Re: 3.5 ton A/C condensor
CTwireman Offline
Member

Registered: 02/07/02
Posts: 839
Loc: Connecticut, USA
ShockMe,

Do you just run an 8/2, or do you go by the nameplate on the unit? Remember, some high efficiency units may only require #14 or #12. With the high price of copper, you may want to rethink this approach. Simply going by the tonnage is not a good method in my opinion.

Case in point: Once upon a time, I had no clue how to size the circuit for an AC unit. The AC guy said "Run a 10/2 and put it on a 30 amp breaker." A few years later, when I learned the right way, I read the nameplate on the unit: Min fuse breaker- 20 amps, Max fuse/breaker size - 25 amps. I removed the 30 amp breaker and installed a 20. Lesson learned.
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#56856 - 10/01/05 07:11 PM Re: 3.5 ton A/C condensor
ShockMe77 Offline
Member

Registered: 06/11/05
Posts: 823
Loc: Rahway, New Jersey
Ct, I always check the maximum fuse rating before doing anything. That's the way I was taught and I believe its also code. I bring the 8.2 Romex with me because in case I need it and usually do when wiring a 3.5 ton. Our company buys wire in bulk so the 8.2 romex is on a reel, I think its 1,000 ft per reel, and I only stock 14, 12, and 10 romex on the truck.

By the way, funny you should mention using a 20 amp breaker when the max fuse is 25 amps because that happened to me on a job yesterday. I had to run 10/2 instead of the 8/2 because I forgot to check to see if my helper put it on the truck in the morning. But that's another story. Anyway, could you tell me where it says in the code that its ok to protect a 35 amp max fuse a/c unit with only a 30 amp breaker? Thanks.

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#56857 - 10/01/05 07:19 PM Re: 3.5 ton A/C condensor
Tesla Offline
Member

Registered: 06/16/04
Posts: 1280
Loc: Sacramento, CA
288,000 Btu's are exchanged when 2,000 lbs of ice are melted.

When artificial cold was first sold it came as blocks of ice, typically cut out of nearby lakes and stashed for sale in the spring.

When the new fangled refrigerators moved in on the old 'ice box' market the salesman had to explain just how valuable the machine was. Customers were won over by, "This baby is the same as having one ton of ice delivered each day to your place of business."

One ton of ice each day has been shortened to "One ton of cooling".

1 ton of ice per day = 288,000 Btu per day / 24 hours = 12,000 Btu per hour

The most intense loading of an airconditioner will necessarily occur during 40+ Centigrade weather. Do not undersize its conductors. This is one area where an owner would be wise to upsize -- and forget the code minimums.

Someday the government will get wise and insist that these conductors be upsized as a matter of public policy. They directly affect the peak load driving capital expenditures for all power providers.

The Federal government has already weighed in on the units themselves with various energy efficiency standards.
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Tesla

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#56858 - 10/01/05 09:03 PM Re: 3.5 ton A/C condensor
CTwireman Offline
Member

Registered: 02/07/02
Posts: 839
Loc: Connecticut, USA
 Quote:
Anyway, could you tell me where it says in the code that its ok to protect a 35 amp max fuse a/c unit with only a 30 amp breaker? Thanks.


I don't know where it is in the code, but I know you can size your breaker or fuse based on the mininum or maximum size as stated on the unit's nameplate. The choice is yours.

Lately, I have only dealt with high-efficiency units which have the same apacity for both the min. and max., usually only 20 amps.
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Peter

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#56859 - 10/02/05 06:49 AM Re: 3.5 ton A/C condensor
ShockMe77 Offline
Member

Registered: 06/11/05
Posts: 823
Loc: Rahway, New Jersey
I'll keep that in mind. For some reason I was under the impression that the maximum fuse rate was the one to go by. I'm actually taking a course now at my local Vo-tech so I can learn everything involved in a central a/c system instead of being a robot who does this, does that, but has no clue why.

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#56860 - 10/02/05 07:03 AM Re: 3.5 ton A/C condensor
iwire Offline
Moderator

Registered: 01/05/03
Posts: 4343
Loc: North Attleboro, MA USA
Shockme, The way you are doing it is fine, it certainly meets code.

However you can also wire like Peter suggested.

If you had an HVAC unit that had the following information.

Minimum Circuit Ampacity 24.6

Maximum Fuse or Breaker 50

You could wire it with 12 AWG connected to a 50 amp breaker or fuse.

As far as a code reference you have to jump around but if you look at 440.6 it will get you started.
_________________________
Bob Badger
Construction & Maintenance Electrician
Massachusetts

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#56861 - 10/02/05 08:25 AM Re: 3.5 ton A/C condensor
gfretwell Offline

Member

Registered: 07/20/04
Posts: 9045
Loc: Estero,Fl,usa
I don't want to sound like copper.org but in something like an AC it is worthwhile upsizing the conductors a size from the strictest "hold your nose legal" value you get from 310.16
It may not be as big a deal in the frozen north but here in Florida the AC load is the biggest user of power and adding extra I2R heat to the house whacks you twice.
_________________________
Greg Fretwell

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