couple ac wiring questions from a DIYer
Hey all, I’m running the wires for two compressors for my new central-air system. My Hvac contractor will make the connections. It is about 80 ft from the panel to where the compressors will live. Part of the run will be below grade, I’m planning to run that bit in some galvanized pipe. One compressor will service a 3.5 ton air-handler, the plate says that it requires a 40A circuit-breaker(2 pole); I was going to use 8g wire for that run. The other compressor will service a 1.5 ton handler; that one requires a 20A breaker. I was planning on 10g wire for that run. Should I use a 60A disconnect for the 3.5 ton and a 30A for the 1.5 ton? That is all that seems to be available. I’m running 18/4 from each air-handler to each compressor for control; can I run this in the same conduit as the other wires?
I assume that there is no problem running 8g and 10g inthe same pipe. right? What do guys think?
Replies
First you need to read the nameplates again.
I forget the exact wording, but they are something lime Maximum overload protection (or maximum breaker size).
And minimum circuit size or minimum current requirements.
The latter one determines the size of the wiring.
You are WAY, WAY oversizing the wire.
AC circuit ratings are VERY MUCH DIFFERENT for ordinary branch circuits. The normal rules about wire size vs breaker size don't hold.
Even after adjusting for the distance and the derated for the having 4 current carrying conductors in one conduit.
Yes, you can have both branch circuits in one conduit, but they get derated.
But the 18-4 is low voltage wiring and can't be in the same conduit.
I would use PVC conduit rather than steel.
Disconnects are just that; disconnects not overload protection. A way of disconnecting power at the AC unit for service.
60 amp disconnects are cheap and standard. I would use that size for both of them.
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
I guess oversize is better than undersize... the "min circuit amps" for one of the units is 13.8, the other one is 25.4. I'm now going to try to find out what is correct. Thanks for your help.Pat
"I'm now going to try to find out what is correct. Thanks for your help."Both of them are right.In gneral purose circuits the breaker has to protect agains an overload and a fault (short) in the wiring. You never know how many and what kind of loads will be plugged in.A dedicated circuit is much different.The load is known. 13.8 and 25.4. The are the continous operating currents.The AC units have there own overload protection. In most cases it is a thermal overload and trips of the motor get too hot.So the breaker just need to be larger than those numbers. And they determine the size of the wire needed.But motors, specially AC unit draw a large amount of starting current and a breaker sized to just the running current might have problems tripping durring startup although they have a time inverse curve that does require overloads for a certain period before tripping.So for code allows the breaker to be upsized. Those the max breaker rating.With that size breaker if there is a short or a compressor fault or slow starting either the internal overload or the breaker will trip before the wire gets hot enough to damage the wiring.Also if you run the 2 circuit in the same conduit you will have 4 current carrying conductors and that require derating to 80% to eliminate heat build up.Now #14 wire has an ampacity of 20 amps. And derating to 80% gives you 16 amps.So the first unti can be run on a #14 wire protected by a 20 amp breaker. But I would go ahead and run #12 anyway.Now #12 has a ampacity of 25 amps is just shy of the requirement even before derating.#10 has a 30 amp rating if used at 60*C. NM Romex is limited to 60*C although the wire is rated for 90. So it will handle the load but not if it has to be derated.But if you run it in conducit most likely use THNN/THWN which be run upto 90*C so you are limited to the rating what it connects to. In many cases they will be limited to 75*C.#10 running up to 75*C is rated for 35 amps or 28 after derating..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Ok, so leave it to Bill to clearly explain what most inspectors find obscure..... :)So Bill, in your opinion, is upsizing the conductors a good idea if for no other reason than to help the motor with starting load current? Even with my #2 mains and #8 branch circuit, all lights in the house dim when the compressor starts.Thanks,Scott.Always remember those first immortal words that Adam said to Eve, “You’d better stand back, I don’t know how big this thing’s going to get.â€
It is never "bad" (other than to the pocket book) to upsize.But that might not be the cause of your dimming.First of all a quick "blink" of the lights is normal.But that should be all that it is. If it really dims for over a second then you have a problem.Undersized feeders will cause the voltage to drop and the unit will draw more current and for a longer time until it gets upto speed.It might be a hard starting compressor. There are hard start kits available for the compressor.The other cause is supply problems. Might be a bad connection in the panel/meter/POCO drops. Overloaded transformer, low supply voltage..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Ok, thanks. It's a quick blink.Scott.Always remember those first immortal words that Adam said to Eve, “You’d better stand back, I don’t know how big this thing’s going to get.â€
Just want to tell you, Bill, I enjoy your posts.
And yours too..
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A-holes. Hey every group has to have one. And I have been elected to be the one. I should make that my tagline.
Bill- Not trying to be smart here, just trying to learn. When you say the conductors are WAY oversized, I don't get it. When my unit was installed, the electrician ran #10 copper the 60 feet or so from the panel to the outside unit. (a 2.5 ton 12 SEER Am Standard). Along those same lines, why do you say the OP is oversizing wire? Again, not trying to start a flame, trying to learn.
If he has to derate then he is probably not that much oversized is he? If oversized then at least if he decides to replace with bigger units later he is ok. Only downers to oversizing probably are cost and pain in arse pulling. Both may be significant to some.
Cheers, Bob
Not taking into account any derating that may be required for voltage drop or multiple conductors in a single conduit, 8 awg wire is appropriate for a 40 amp circuit, and 12 awg would be appropriate for a 20 amp circuit (10 awg would be fine as well but it's bigger than you need.)
Assuming they are both 240vac circuits, voltage drop should not be a problem over 80 feet.
If both circuits are routed in the same conduit, the wire ampacity has to be derated by 80%. However, 12 awg is good for 25 amps and 8 awg is good for 50 amps, so if you multiply each by 0.8 you get 20 amps and 40 amps so the wire sizes should still be okay for this application. This is for THHN/THWN copper.
To size the conduit, you'll have one circuit with two #8s and a #10 ground, and one circuit with two #12s and a #12 ground (this again assumes both circuits are 240vac only and you don't need the neutral wire.) That works out to a 1" minimum conduit.
The local disconnects are generally only available in 30 and 60 amp sizes, so you're correct there.
The control wiring does need to be in a separate conduit from the power wiring. If you run both control cables in a single conduit they'll probably fit in a 3/4" conduit, but I'd have to look up the exact size of 4 x #18 cable (I don't have it on my handy cheat sheet.)
Bill's right about AC motor load rules, but the rules seem somewhat obscure; even my inspector couldn't explain the ratings that appeared on my nameplates. I had to dig meticulously through the tables at the end of the code book to find a table that explained what the manufacturer had indicated.
That being said, there's no harm in using oversized conductors as long that they fit easily into the terminal clamps, and you're willing to pay for it.
Scott.
Always remember those first immortal words that Adam said to Eve, “You’d better stand back, I don’t know how big this thing’s going to get.â€
Looks like you forgot a circuit.
You need a receptacle .... for use when servicing the units .... within 25 ft. Might as well run a circuit, and place a receptacle right there.
Good catch!