Electrical wire "requirements" for central air unit...

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Was poking around my basement looking for paths to run new electrical wire/circuits when I traced the wire for my central air.
2 things struck me as odd:
First is there appears to be about 40 feet extra wire in the run to my outside a/c unit. My guess is that the compressor unit was moved from one side of the house to another but whomever installed the unit on the move didn't bother to shorten the run. So the wire runs along a joist then suddenly enters a junction box and reverses course 20 feet back.
Secondly the wire that comes out of the breaker panel to the "halfway" junction box is 12/2 (originally from 1967 builder) but the wire coming out of the box that finishes the run to the a/c is 10/2 and is much newer.
A/c unit is 15 years old, 1.5 ton. Claims to be 19 amps start, 13 amps nominal.
So my questions are this: is 12/2 actually acceptable, and is 12/2 acceptable for todays central air units as well (2 ton lets presume)? Also is there some voltage drop from the extra 40 feet of wire that could be costing me some money? Remember, in that 40 feet, 20 feet is 12 gauge and 20 feet is 10 gauge.
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Without going into all of the intricacies of article 440, yes #12 is fine if the minimum circuit ampacity or the FLA is 13a.
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On 8/13/2012 8:42 PM, Duesenberg wrote: ...

12 is rated for 20A load so is ok _if_ it is on a 20A breaker. That's conservative for the #10 which is 30A-rated for protection.
There's a tiny bit more voltage drop/ft in the 12 as compared to the 20 but you're not going to see it. I didn't bother to look it up but the total 40-ft extra isn't going to be enough to matter.
As you noted, undoubtedly the run to the box was original and a run was made from there to make the connection to the replacement unit. Neater could have been to have made new home run, but not surprising it was done as was. Again, other than checking the breaker rating, nothing particularly unusual in rework for the situation described.
--
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You're ok. But, if it were me, I would shorten the 12 GA run as much as possible and use as much of the 10GA as possible. SInce there is already a junction box, you can move it to the most convenient spot for maximizing the 10GA and minimizing the 12 GA. It would't make more than a 1-2 % difference in your overall bill, but it's a little less heat generated inside the house and a little more energy delivered to the AC compressor, so why not?
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On 8/13/2012 9:48 PM, hr(bob) snipped-for-privacy@att.net wrote: ...

The full 40-ft length difference between #12 and #10 <0.75% at rated load so if he can get rid of half of it it's roughly has of that -- while theoretically it is a help, yes, it's so small as to be essentially negligible. There's bound to be lots of other places to effect far more significant savings.
Now, if the overall length of #12 is 100-ft and he goes back to the home run; then it might be enough to _eventually_ pay for the wire over a few years...
Just cutting out the kink in the run is a nicety but it won't be noticeable other than the satisfaction of having done it and knowing that have done... :)
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Might be able to run the 12 ga all the way to the outdoor unit?
And then, the 10 gage heavier wire could go to the metal recycling place, copper is worth money. As theives and goblins well know.
Real money savings is cleaning the outdoor unit so it runs more efficiently.
Christopher A. Young Learn more about Jesus www.lds.org .
The full 40-ft length difference between #12 and #10 <0.75% at rated load so if he can get rid of half of it it's roughly has of that -- while theoretically it is a help, yes, it's so small as to be essentially negligible. There's bound to be lots of other places to effect far more significant savings.
Now, if the overall length of #12 is 100-ft and he goes back to the home run; then it might be enough to _eventually_ pay for the wire over a few years...
Just cutting out the kink in the run is a nicety but it won't be noticeable other than the satisfaction of having done it and knowing that have done... :)
--



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On Mon, 13 Aug 2012 19:48:16 -0700 (PDT), "hr(bob) snipped-for-privacy@att.net"

That would be my suggestion too. And if he ends up with only a small amount of 12 GA, why not change it to 10 GA after moving the junction box.
Of course I'm still left with the question. What size breaker is on it? If it's a 20A, then the 12 GA is actually fine. If it's bigger, such as a 25A or 30A, then that 12 GA wire is too small. The OP did not say the breaker size.
19A starting, is below 20A, but barely. If the breaker is 20A, he could just leave it too, or clean it up just to look better and maybe get a very slight better voltage to the unit when starting. If it was mine, I'd clean it up, and I'd save as much 10 GA as possible.
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On Tue, 14 Aug 2012 00:44:05 -0500, snipped-for-privacy@thecave.com wrote:

Not true at all on an air conditioner. That is why I did not want to get into article 440. It makes people's head hurt.
The real answer is not in article 240.4(D) where that 12ga = 20a comes from. You really need to read the label on the condenser and use 310.16 to size the wire.
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On Tue, 14 Aug 2012 02:14:22 -0400, snipped-for-privacy@aol.com wrote:

Not to be a smartass, but I can only guess you're referring to articles in the NEC code book. This is almost as bad as people who quote the bible in religious topics, since there are hundreds of different bibles. Which one are they referring to? I can only assume the code book you have is the latest edition????
I understand that the NEC code book is available online (somewhere). It sure would help if you'd simply quote the actual text of the article. Not all of us have these books, and those of us who do, may not have the latest edition. Unless we're in the biz, it's not something the average homeowner has on hand, or updates every edition.
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On Aug 14, 6:26am, snipped-for-privacy@thecave.com wrote:

You can access the NEC online for free. And because it's complicated, that's why he didn't explain it. Basicly, as he said, the eqpt specifies the min/max breaker and the min circuit amps. I have a 5 ton that has a 50 amp breaker, could use up to at least 60 with a circuit that has 8 gauge wire. That's because the starting current is a lot higher than the run current and the eqpt has it's own overload protection.
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On 8/14/2012 1:44 AM, snipped-for-privacy@thecave.com wrote:

Can't change all the 12 to 10 easily. I can remove 20 of the 45' of 12 gauge, put in the junction then run the 10 gauge after the junction to the a/c. Too much drywall and stucco ceiling to rip out the 12 guage left from the panel to the new junction box location.
It'll be easier to start from 10 guage at the panel and make the run from that, which I'll consider when it comes time to replace the condenser unit.
Breaker is 30 amp, indeed, but there is an outdoor disconnect that has fuses. When it stops raining i'll check to see the ratings on those fuses.
Will also see if I can find the built in disconnect ratings from my unit.
As it stands, I'll most likely leave things alone until next air conditioner purchase, whihc hopefully won't be for a long time... Was asking these questions out of curiosity more than anything...
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It sounds like what you have is OK. The new A/C unit may actually have a lower current requirement than the old one anyway.
What you are looking at is the special rules that apply to motors. As a general rule you can have a breaker of 250% of the full load amps of a motor and the wire is rated at 125% of FLA. For the purposes of this computation 12 gauge wire is rated at 25 amps (310.16) (That does not apply to general purpose branch circuits)
In an air conditioner you have a mixed load and that computation can get far more complicated so we depend on the "minimum circuit ampacity" and "Maximum over current device" to size the wire and the breaker. They usually don't look like they go together but you have trust the engineer who spec'ed that label.
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Where do you live that has rain, we have had 0.3 inches in the last 4 weeks, just W of Chicago. Corn and lawns are dead.
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On 8/14/2012 9:36 PM, hr(bob) snipped-for-privacy@att.net wrote:

Hour west of Toronto. Went 45 maybe even 50, but for sure 45 days without rain, and now we've had 4 in a row with. Grass is green again, but alot of crops will get this rain too late. Even had warning out about the "rivers might be too flowing too fast so keep an eye on the kids" from the local water and conservation authorities...
Calling for rain on Friday again...
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On 8/15/2012 8:30 AM, Duesenberg wrote:

Make that 5 days in a row. Was only supposed to be a 30% chance today...
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In any case, the current breaker should be 20 amps. If you do install all 10 gauge, it's likely you can still use a 20 amp breaker. Wow, my 3.5 ton unit is drawing 6 Amps only, and a 20 amp breaker is stated.
Greg
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If the breaker is a 30A, 12 GA wire is too small. Personally, I would not leave it. Either change the wire to all 10 GA, or change the breaker to a 20A. If there's nothing else connected to that line, you're probably safe as far as fires, but legally, you are under rated on that wire. It's not just about fire safety, but about protecting the motor of the AC. Any motor that has to work too hard to start, will burn out sooner. It's similar to someone trying to use a heavy duty power tool on a 100 foot 18 GA extension cord. The tool will run, but slower than normal and it's life will be shortened.
Sometimes you can be lucky and tape new wire to the old stuff and pull it thru sheetrocked walls and ceilings. Depends on how it was stapled.
If nothing else, remove as much of the 12 GA as possible. The shorter distance helps.
If a 20A breaker trips, you will know that it's drawing more power.
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On Aug 15, 3:13am, snipped-for-privacy@thecave.com wrote:

As gfretw has explained several times now that would be true if this were a general purpose branch circuit. It's not. It's an AC unit, in which case the circuit amps and breaker sizing must conform to the manufacturer's requirements. That's because these types of loads can have high starting currents and they also have their own additional over current protection in the eqpt.
>If there's nothing else connected to that line,

Not true. It is perfectly premissible as long as it conforms to the manufacturer's reqts. And it sounds like it very likely does.
It's not just about fire safety, but about protecting the

It probably will trip, but it's meaningless as many AC units that are properly installed would trip the breaker during start-up if the breaker was sized according to the rules for regular branch circuits.
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On 8/15/2012 6:57 AM, snipped-for-privacy@optonline.net wrote:

I agree except it still has to conform to the electrical code. But as gfretwell has posted, "the intricacies of article 440" have different rules than what we are used to.
Adding some detail - for hard wired motors, and in particular compressors, the circuit breaker provides "short circuit protection". Overload protection is at the compressor and is likely a klixon type thermal switch. In other applications a "motor starter" with "overloads" might be used. The breaker may have a significantly higher rating than the wire rating. The overload protection is at the load end of the branch circuit.
Some compressor name plates say "maximum fuse size". When it says "fuse" there must be a fuse somewhere in the supply. It is easy to install a disconnect with a pull-out fuse block.
--
bud--

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wrote:

When they say "fuse", they mean that there must be a fuse like the type connected to a stick of dynamite. So, just hook a stick of dynamite to the wire and be done with it...... :)
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