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Old 06-07-2009, 11:10 AM   #1
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I am installing an 18K BTU split AC which takes 240V. My service panel is full. there is only (1) half size slot available - good for a thin single pole breaker. My plan is to run a sub panel from the service panel and install the sub panel (30 Amp breaker) in the back yard where the AC outdoor unit will be mounted.

One other thought is to piggy back (2) wires using wire nut and a pig tail into a single pole 15 Amp breaker to make room for a double pole breaker, run (4) wire conductor to the back yard to a fused disconnect.

any thoughts would be appreciated.

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Old 06-07-2009, 11:53 AM   #2
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If you install a subpanel you are still going to need to make room for a double pole breaker to feed the sub. Since your going to need to move over a circuit or two over to the subpanel, it would be easier to mount it inside near the main panel.

Combining two circuits isn't a good idea without knowing the load on those circuits.


Last edited by surrogate; 06-07-2009 at 12:00 PM.
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Old 06-07-2009, 12:03 PM   #3
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I am installing an 18K BTU split AC which takes 240V. My service panel is full. there is only (1) half size slot available - good for a thin single pole breaker. My plan is to run a sub panel from the service panel and install the sub panel (30 Amp breaker) in the back yard where the AC outdoor unit will be mounted.

One other thought is to piggy back (2) wires using wire nut and a pig tail into a single pole 15 Amp breaker to make room for a double pole breaker, run (4) wire conductor to the back yard to a fused disconnect.

any thoughts would be appreciated.
What size is your service panel. You may need to upgrade the service. Did you have AC before and you are replacing, or is this a first time install?
Might be a good time to determine if an upgrade is required, if this is a new installation.

You can install a sub panel if you have the capacity to do so. (you still need two full size breaker slots) Install it right next to the old panel (clearances observed). Many homeowners have to upgrade their services when installing new equipment. You may just be one of them.
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Old 06-07-2009, 12:05 PM   #4
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Is it not OK (code allowable?) to double tap to the main-incoming lines to feed the sub?

Else, is it OK (code allowable?) to double tap into a single pole breaker? This will combine (2) 120V circuits into (1) single pole breaker, thereby allowing a space for (1) double pole breaker for the AC disconnect?
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Old 06-07-2009, 12:10 PM   #5
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What size is your service panel. You may need to upgrade the service. Did you have AC before and you are replacing, or is this a first time install?
Might be a good time to determine if an upgrade is required, if this is a new installation.

You can install a sub panel if you have the capacity to do so. (you still need two full size breaker slots) Install it right next to the old panel (clearances observed). Many homeowners have to upgrade their services when installing new equipment. You may just be one of them.
good point JV. it is a new install. i sure will review review this. the AC by the way requires only a 20-30 Amp breaker so it is not too big a load.
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Old 06-07-2009, 01:03 PM   #6
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One of the best things I have done was to upgrade my service and add a box with lots of room and easy access. Makes adding and changing things around much easier. Adds value to the house - buyers appreciate upgraded electric service.

So I agree with the recommendation to consider a service upgrade - add some money to what would be spent to do otherwise and come out with more for the money.
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Old 06-07-2009, 07:51 PM   #7
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Physical space isn't the only issue to take into consideration.


When I need to make space in a panel, I install a new panel

If you need to make space in a panel, you would find some 15 amp circuits and "double them up" on twin breakers, making space for your full sized AC breaker. You would take special precautions and ask more questions if any of these 15 amp circuits in question have red wires (3 wire homeruns).
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Old 06-08-2009, 11:19 AM   #8
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good point JV. it is a new install. i sure will review review this. the AC by the way requires only a 20-30 Amp breaker so it is not too big a load.
You still have not told us what the existing service amps are. Please respond to this question. Look at the main breaker or disconnect for this information.

My gut tells me you need the upgrade. But you can prove me wrong with your answer.

Is this a window unit?
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Old 06-08-2009, 11:30 AM   #9
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Else, is it OK (code allowable?) to double tap into a single pole breaker? This will combine (2) 120V circuits into (1) single pole breaker, thereby allowing a space for (1) double pole breaker for the AC disconnect?
You can only place two conductors on the same single pole breaker if the manufacturer specifically identifies that they allow it. What brand/part numbers are your breakers?

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... you would find some 15 amp circuits and "double them up" on twin breakers, making space for your full sized AC breaker. You would take special precautions and ask more questions if any of these 15 amp circuits in question have red wires (3 wire homeruns).
One of those "special precautions" referred to by 220/221 is that you can only use the twin breakers (a.k.a. tandem breakers) if your panel is listed for them, and ONLY in the spaces where your panel instructions specifically state they can go. What is your panel brand/part number?
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Old 06-08-2009, 02:12 PM   #10
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One of those "special precautions" referred to by 220/221 is that you can only use the twin breakers (a.k.a. tandem breakers) if your panel is listed for them, and ONLY in the spaces where your panel instructions specifically state they can go. What is your panel brand/part number?
Not to put words in his keyboard, but I think 220/221 was referring to MWBCs and/or 240V branch circuits. Though you are 100% correct.

If you stuck both hot legs of an MWBC on the same tandem breaker, you'd burn the neutral sooner or later (and I do mean burn).

Worst if you did it to a 240V circuit would be you'd have 0V at the outlet. Still not wise though.
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Old 06-08-2009, 02:58 PM   #11
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Not to put words in his keyboard, but I think 220/221 was referring to MWBCs and/or 240V branch circuits
yep, I knew that, I just wanted to keep with the "special precautions" theme!
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Old 06-08-2009, 03:03 PM   #12
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yep, I knew that, I just wanted to keep with the "special precautions" theme!
Cool.. definitely sage advice on your part.

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