Wire Amp size in Garage

[Deere Dude]

I have added a garage/tractor storage to my house. I am putting an overhead heater in; no problem. I want to put a wall AC unit roughly 18000-24000 BTU/H. I don't have a unit picked out yet, but expect it to be 220V. What size amp draw would that size AC take for 220. I understand it is all a guess especially without having an AC unit in hand.

I am dry walling right now so I need an educated guess. I have all kinds of 12-3 wire laying around and a small amount of 10-3 and a lot of 8-3. My lightly educated guess would be a 12-3 wire.

I looked at AC units on line but can't find any that say their amp rating.

Thanks,
Chuck

 

[s219]

The only data point I can share is that my PTAC unit is 15,000 BTU and runs on a 20A 220V circuit.

 

[davesl708]

It will depend on your wire run length. If 20 amp circuit and less than 75' wire run length 12ga. will work. If 30 amp load same run length then 10ga. would be required. Best bet is to check your local code requirements. Local codes vary. Current NEC is 2014.
Consider if you will be using the outlet for other equipment and size the load based on that and possible future demands.
Never hurts to go one wire size over base requirement.

 

[radioman]

the online tools if using 50 ft with 14 amps gives you 14 gauge minimum with 240v. I would not. It will not fly with code . I would use 10/3.

 

[PILOON]

12-2 can handle 20 amps on 110 circuits so at 220 you should be good with 12-3 .

The other option would be to install a sub panel in the garage using your 8-3 as the feed from the main panel. You would protect the feed line at main panel with appropriate breaker and the sub panel would feed garage circuits where I would wire with 12 g. for power tools.
Having garage sub panel 8-3 fed would allow welding, air, heat, compressor and just about whatever eventuality.

In a situation such as you propose I like lighting circuits to be independent from any outlets that could be used for tools as popping a breaker then does not leave you in the dark.

 

[uglyboywith11fingers]

my outside 2 ton (24000 btu) unit is fed off a 20a breaker and about 25 feet of 12-2.
I guess the blower must be a 220v motor as well.

Pete

 

[2458n]

Your going to break something in the future and will need a stick welder. Same thing you are going to want to paint something in the future-air compressor. Run a sub panel using the 8 gauge and be done with it. Someone will want it this way. Do it now and you will not regret it.

 

[LD1]

Does the A/C unit also have an electric heater built in.

I have a 220v unit I used in the shop. I believe it was 20,000BTU or so like you are looking. Nameplate said AC FLA was something like 6 amps. But the electric heater was more like 19A

 

[Deere Dude]

my outside 2 ton (24000 btu) unit is fed off a 20a breaker and about 25 feet of 12-2.
I guess the blower must be a 220v motor as well.

Pete

I am in the process of running a 100 Amp panel into the building but was mainly wondering about a stand alone AC. I remembered I have another AC in an outbuilding that is 220v 22,000 BTU/H and That says 18 amp or close to it so I should be good with 12 gauge wire, because a new unit should be more efficient that my old one. The run is about 30 feet and the building is well insulated so I'll go with that and make sure the AC is small enough for 20 amp 220v.

Thanks for the help guys.
Chuck

 

[Sawyer Rob]

Even though #12 wire is rated for 20A, the wire should NOT be loaded to more than 80% capacity, for continuous duty, and that's why I'd go with #10 in this case...

SR

 

[Tractor Seabee]

Every DIY electrician needs one or more of those books they sell at the big box stores. They are written for the non-electrician in non-electrician language. They have the tables from the code in them and discuss things like not loading a 20A circuit with a connected load of 20A. They are worth a lot more than the cost. The same advice goes for plumbing work and carpenter work where you are trying to at least stab at code issues. Wrong choices in electrical work can burn your house down.

Ron

 

[PILOON]

Every DIY electrician needs one or more of those books they sell at the big box stores. They are written for the non-electrician in non-electrician language. They have the tables from the code in them and discuss things like not loading a 20A circuit with a connected load of 20A. They are worth a lot more than the cost. The same advice goes for plumbing work and carpenter work where you are trying to at least stab at code issues. Wrong choices in electrical work can burn your house down.

Ron

Re 20 amp circuits: thing about circuits is that starting loads might be 20 amps but generally running load is 80 or 90%. Either way the CB is the weak link and even at max amps the wire won't heat up as the CB is the protection.
Most devices that draw 20amps continuous would have a probable 25amp start surge.
In a shop U want 20 as many if not most power tools now want 15amps run and often draw 20 start plus often we want to run 2-3 tools at the same time.
Ever notice the adds? 'powerful one hp motor', well that's maxing a typical 15 amp circuit.

 

[docone]

IMHO bigger is ALWAYS better when running wire. If you have #8 available, use it. It's much more difficult to upgrade later on.

 

[445A]

Every DIY electrician needs one or more of those books they sell at the big box stores. They are written for the non-electrician in non-electrician language. They have the tables from the code in them and discuss things like not loading a 20A circuit with a connected load of 20A. They are worth a lot more than the cost.

The little reference book is called " Ugly's " and is available at Home Delay/Home Depot in the electrical dept. I have 2 of them and I don't think I paid more than $10 each. A bunch of valuable information for a small price, IMO. 445A

 

[uglyboywith11fingers]

I recently replaced my electric water heater... it's rated 240volts, with 4500 watt elements.
It's fed from a 20a breaker with 12-2 wiring.
I was surprised to see, when making the connections, that the wiring inside the heater, from connection box to thermostats/elements, is 14 gauge.
I called the manufacturer... they said 14 gauge wire can handle 20 amps when the conductors are not jacketed... they can dissipate heat better.
Seems to me that inside a water heater is not ideal for dissipating heat...:confused2:

Pete

 

[s219]

I recently replaced my electric water heater... it's rated 240volts, with 4500 watt elements.
It's fed from a 20a breaker with 12-2 wiring.
I was surprised to see, when making the connections, that the wiring inside the heater, from connection box to thermostats/elements, is 14 gauge.
I called the manufacturer... they said 14 gauge wire can handle 20 amps when the conductors are not jacketed... they can dissipate heat better.
Seems to me that inside a water heater is not ideal for dissipating heat...:confused2:

Pete

It's always more complicated than just the gauge of the wiring, and frequently comes down to the insulation type and the installation type. For example, some outdoor wire is rated for direct bury but not conduit, precisely due to heating considerations.

One recent learning experience was with 2-2-2-4 Aluminum wire. If used for a branch circuit, it's rated for 90A. If used to feed an entire home, it's rated for 100A. Seems counterintuitive, but it's based on duty cycle, as a service entry feed is rated for 85% load on average (if I remember the number right). But a branch circuit has to be capable of the full amperage 100% of the time. Lots of little tidbits in the code book if you have time to read it.

 

[Deere Dude]

Even though #12 wire is rated for 20A, the wire should NOT be loaded to more than 80% capacity, for continuous duty, and that's why I'd go with #10 in this case...

SR

After reading these posts and a PM or so, I pulled my #12 out and did run a #10-3 wire with ground. I think the AC I get will be no more that 17-18K Btu so it should be good. I probably could have used a 10-2 wire, but I read someplace that the 10-3 is needed for 120v electronics on the unit. It doesn't really matter now because a 10-3 should handle any wall AC I get unless it's a monster.

 

[Sawyer Rob]

After reading these posts and a PM or so, I pulled my #12 out and did run a #10-3 wire with ground. I think the AC I get will be no more that 17-18K Btu so it should be good. I probably could have used a 10-2 wire, but I read someplace that the 10-3 is needed for 120v electronics on the unit. It doesn't really matter now because a 10-3 should handle any wall AC I get unless it's a monster.

10/3 was the right choice, good for you for doing it RIGHT in the first place!!

SR

 

[Tractor Seabee]

When dealing with smaller AC units a rule of thumb is 12,000 BTUs cooling capacity = 1 HP = 13-16 Amps at 120V. depending on efficiency factor, for compressor motors. For 240 V amps are X .5. In my old refrigeration days I experienced household/light commercial voltages as low as 90 V and those old heavy motors just ran hotter, not so today. Heat pumps; you have to look at the auxiliary electric heaters as they often exceed the motor load. Theoretically the circuitry does not allow both to run at the same time; but, sh** happens. The new high efficiency units probably run less amps than 1 HP used to. We don't need AC here in Puget Sound.

Caveat: the above info may not be totally accurate but it is a close approximation. Todays imports may be using terminology and metrics based on calories, dynes, or joules, instead of BTU.

Ron