Can I put my AC on a 20A circuit?

[Matt Meiser]

The AC unit I'm moving from the house to the shop is currently on a 30A circuit, apparently with 25A fuses in the disconnect outside. I haven't actually looked out there, but we have the receipt from when it was purchased and there's 2 25A fuses on it.

The documentation for the unit lists the following specs:
Min Circuit Ampacity: 19A
Max Overcurrent Device: 25A

If I'm reading this right the dedicated 20A 220V circuit that I ran for a window AC will be fine for this unit? If so, I can use the existing receptacle box as a junction box and extend it a few feet and through the wall to where the condenser will sit. Otherwise I need to run conduit, wire, etc and its a lot more work--so here's hoping I'm reading right!

[Mike Cruz]

Not sure, Matt, but consider wire size, too. Make sure your wire size is big enough...I would say 12 gauge at the smallest. Maybe others will be able to answer this better.

[Matt Meiser]

The existing wire is 12ga since its a 20a circuit.

[Mike Reinholtz]

Sounds like you should be in good shape with a 20A circuit, if you are comfortable with and have an amp clamp I would suggest turning it on and checking the amp draw (preferably on a warm day). If the wires are 12 gauge, you should be in good shape to run 20 amps but this would really be the limit. When the unit being replaced is fed by conduit (rather than Romex) I like to replace 12 awg with 10 just for a little cheap insurance. The only concern I would have is the additional start-up (inrush) current when the unit starts again on a hot day. That's the only time I can see you having a problem. Best bet to combat that would be making sure the compressor capacitor is good.

Any more info like tonnage, make, age?

On a side note I do heating and air for a living and would be happy to help, but you'd have to wait till next June when we move to Ada, MI.

[Rollie Meyers]

Sounds like you should be in good shape with a 20A circuit, if you are comfortable with and have an amp clamp I would suggest turning it on and checking the amp draw (preferably on a warm day). If the wires are 12 gauge, you should be in good shape to run 20 amps but this would really be the limit. When the unit being replaced is fed by conduit (rather than Romex) I like to replace 12 awg with 10 just for a little cheap insurance. The only concern I would have is the additional start-up (inrush) current when the unit starts again on a hot day. That's the only time I can see you having a problem. Best bet to combat that would be making sure the compressor capacitor is good.

Any more info like tonnage, make, age?

On a side note I do heating and air for a living and would be happy to help, but you'd have to wait till next June when we move to Ada, MI.

The rules for A/C equipment are WAAAAAAAAY different then for "normal" branch circuits, and the manufacturers do the work for sizing the branch circuit for you, Art 440 allows table 310.16 ampacities to be used which means 14 AWG is 20A, 12 AWG is 25A & 10 AWG 35A, this means the OP's A/C unit can be run w/ 14 AWG on a 25A MAX circuit, this is allowed because the compressor has overload protection & the fuse or circuit breaker is there for short circuit protection. One note is that a lot of electricians do not understand that rule,even fewer A/C techs do get it, plus Home Inspectors go ballistic because many don't know about it either...


Please note:The above ampacities do not apply to branch circuit sizing in any way.

Here is one of many threads at Mike Holt's forum on this very subject.

http://forums.mikeholt.com/showthread.php?t=129358

[Matt Meiser]

Mike, the AC unit is about 13 years old, 2.5T, 10 SEER by Armstrong. Unfortunately after rechecking how this circuit runs, there is Romex from the panel through the attic, then out to a junction box at the ceiling. From there, it drops down the wall in conduit to the receptacle. I'll be re-running from the junction box out to the disconnect.

In the house, the circuit is currently a 30A breaker with 10ga romex to the disconnect where there are 25A fuses I think. Its raining so I'm not checking today.

(BTW, Ada/Ida, two different, oft confused places. DirecTV has twice scheduled our job right after someone in Ada. Unfortunately that one letter makes a 3 hour difference. Grand Rapids area is a nice place to live though. Ada is better known so I get the short end of the stick)

Rollie, after I posted this I found some threads like the one you refered to which really confused me. But if I'm understanding correctly, the gist is that the wire is sized based on the min circuit ampacity, and then you can select a breaker based on the max overcurrent protection device? So as you are saying, you can actually use a 25A breaker on a 14g circuit, as crazy as it sounds? In that thread, the poster could actually have used a 50a breaker with a 10ga wire.

I see I can get a 25A breaker to fit my Siemens panel, which unfortunately is only available full-height so it it eats up the space that a current Q220220 breaker that provides 2 20A 220 volt circuits (one is unused currently) leaving me only a spare 15A 110 circuit out there but I guess that's life. Or I can get a triplex breaker with 2 15 or 20A 110's and the 25A 220. But I think I'll use what I've got for now and deal with it if I have problems with it tripping on hot days. Why the guy who built my house and shop chose such small panels I guess I'll never know.

Or can I get away with a larger breaker since the disconnect is fused and the breaker would only be providing short circuit protection to the wire between the panel and the disconnect?

[Rollie Meyers]

Mike, the AC unit is about 13 years old, 2.5T, 10 SEER by Armstrong. Unfortunately after rechecking how this circuit runs, there is Romex from the panel through the attic, then out to a junction box at the ceiling. From there, it drops down the wall in conduit to the receptacle. I'll be re-running from the junction box out to the disconnect.

In the house, the circuit is currently a 30A breaker with 10ga romex to the disconnect where there are 25A fuses I think. Its raining so I'm not checking today.

(BTW, Ada/Ida, two different, oft confused places. DirecTV has twice scheduled our job right after someone in Ada. Unfortunately that one letter makes a 3 hour difference. Grand Rapids area is a nice place to live though. Ada is better known so I get the short end of the stick)

Rollie, after I posted this I found some threads like the one you refered to which really confused me. But if I'm understanding correctly, the gist is that the wire is sized based on the min circuit ampacity, and then you can select a breaker based on the max overcurrent protection device? So as you are saying, you can actually use a 25A breaker on a 14g circuit, as crazy as it sounds? In that thread, the poster could actually have used a 50a breaker with a 10ga wire.

I see I can get a 25A breaker to fit my Siemens panel, which unfortunately is only available full-height so it it eats up the space that a current Q220220 breaker that provides 2 20A 220 volt circuits (one is unused currently) leaving me only a spare 15A 110 circuit out there but I guess that's life. Or I can get a triplex breaker with 2 15 or 20A 110's and the 25A 220. But I think I'll use what I've got for now and deal with it if I have problems with it tripping on hot days. Why the guy who built my house and shop chose such small panels I guess I'll never know.

Or can I get away with a larger breaker since the disconnect is fused and the breaker would only be providing short circuit protection to the wire between the panel and the disconnect?

I see nothing wrong with 25A fuses in the disco & a 30A breaker supplying it.

[Matt Meiser]

With the 12ga conductors, right?

[Mike Reinholtz]

Matt- I know the feeling on the confusion between the 2 cities, when we closed on the land we recently bought the paperwork had the town written both ways. I decided to google the two and found they are in fact different towns, surprised the title company too!

As for your wire sizing questions, I used the same source I always go to for wiring concerns-- Ugly's electrical reference book. The reason I tend to go a little bigger on wire size is simply based on experience. Often times I go to a site where the maintenance is less than ideal and the condenser is plugged with cotton wood, grass clippings, plastic bags, etc and the head pressure is much higher than it should be, leading to the compressor working harder and higher amp draws. This leads to pressures staying higher between calls for cooling, so when the system comes back on the compressor has to work even harder to turn over, leading to higher amperage. Worst case is when the unit is a little small for the space and runs for extended periods, heating up undersized wiring (namely where I live now in SoCal with 113 being yesterday's high). Long story short, I was taught through my apprenticeship to play it safe.

Sounds like you should be all set.

[Dan Hintz]

So there's no confusion in the future... the higher amperage circuits with the "lower than typical" gauge wires are for dedicated circuits only (in this case, A/C units). I've never felt right about such things, but I understand the reasoning behind it. I can see some hack, though, deciding to add on "just ooooone little circuit box" and overpowering the wire, causing a fire.