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?

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.

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.

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.

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.

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.

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.

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.

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 I don't think you sound like that at all. It's good advice.

What the code will allow and what is the most energy efficient way are not one in the same.

Bob

Thanks everyone for making it easier for me to understand what I'm doing, and more importantly, know why I'm doing it.

I checked out article 440.6 and it referred me to table 310.16. Thanks for the tip, Bob.

Oh, and lol @ "copper.org"

--Ron

[This message has been edited by ShockMe77 (edited 10-02-2005).]

I always upsize here in AZ where the air conditioner can just about be a continous load. Besides, my AC guy requests it and he pays for it. There is a 125% requirement for fixed electric heating, well out here our cooling load is much bigger.

BigB that is a good idea.

Just as a point of information there is already 125% figured into the MCA for the start up current on the unit so you are already covered for the continuous load if it is one.

IMO an AC would not be a continuous load unless you leave the window open or it was severely undersized.

Bob



[This message has been edited by iwire (edited 10-02-2005).]

IWIRE

A 12AWG on a 50a ocpd, wouldn`t that be a violation of 240.4(D)?

Maybe I`m misreading your answer or the thread all together.


P.S. Is there a "quote" button anywhere?

No it not, check out the first sentence of 240.4(D)


It happens that for HVAC units it is permitted by 240.4(E) through (G)

No there is not a 'quote' button.

If you follow this link https://www.electrical-contractor.net/ubb/ubbcode.html you will see how to quote or bold etc.

Bob



[This message has been edited by iwire (edited 10-02-2005).]

Bob I know A/C isn't a big deal in Ma and humidity is not an issue in Az but here in Florida a properly designed system is going to be a continuous load. If your A/C cycles too much it won't get the humidity out of the air and you get that clammy cold you feel if you are in a movie theater that is empty.

Morning Greg, you got me wondering what you guys use for cooling equipment.

It seems to me that if the AC is sized to run continuously with an outside temp of say 70 F that it will not cool the place very well when the outside temp reaches 100 F.

And if it sized to run continuously with an outside temp of 100 F it will certainly not run continuously at temperatures below 100 F.

Of course that can be overcome using multiple cooling stages but around here that is only done in commercial work.

All it takes is the compressor to kick off once every three hours and it is not by definition a continuous load.

MCA is 125% of the largest load plus all the other loads and is usually used to size the conductors. It is also the minimum fuse size. Max Fuse/Breaker is a little misleading. It is actually just the Max Fuse size or a non-HACR breaker size. This is 225% of the largest load plus all other loads. If you use a HACR breaker you can go up to 400% of the largest load plus all other loads as the max. Why someone would do that I do not know. Unless it is availability.

Another thing to keep in mind is that some of the loads are locked out when the other ones are running, such as heaters being locked out during the cooling cycle. Whichever load is higher (heat or cool) can throw a kink into the RLA/MCA calculations. But with heaters being resistive the motor load usually trumps on the max fuse size due to inrush on the inductive side.

Another thing my company does is put a note by the disconnect on the drawings saying to use the 60 degree column of 310-16 if the load is less than 100A and the 75 degree if it is over 100A. We use the 90 degree column for all of the wiring internal to our units. Other manufacturers may be different as we do heavy commercial, industrial and large residential and not regular residential units.

They usually use a design temp of 95f <ambient> or so with a 20-25 degree delta.
Larger houses will have 2 or more systems, particularly if they are multi-level. When we have the 95 degree day it will come with 90+ humidity so getting the water out is as important as getting out the heat.