Central AC Wiring

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  #1  
Old 07-17-06, 02:50 PM
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Central AC Wiring

Hello there,

I am going to be having Central Air installed in our home shortly. I've got a couple of questions regarding the wiring.

We are installing 2 condensors (our home is 3 floors, hence the need for the 2 units). We have one 150 amp panel and another 100 amp panel. Both are wired independently (used to have 2 meters - was a duplex).

My thoughts are to wire one of the units to the 150 amp panel and another to the 100 amp panel. The 150 amp panel handles the stove (oven only), hot water heater and 1st floor lighting. The 100 amp panel handles 2nd/3rd floor lighting and some baseboard heat (that is hardly used any longer).

Just wondering if anyone has any thoughts/concerns on this. Installer is indicating he needs 2 30 amp AC disconnects. All I can find is 60 amp units at the big box stores. What size wiring/circuit breakers would I need to install for these? I'm not worried about doing the work (I've done quite a bit of smaller wiring before), just want to make sure I do everything correctly. The run from both units to the panels will be 40-50 feet approx.

Thanks in advance.
 
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Old 07-17-06, 03:28 PM
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Originally Posted by swallman
We have one 150 amp panel and another 100 amp panel. Both are wired independently (used to have 2 meters - was a duplex).
How are they wired now? Are they both connected to the meter providing you with a 250A service? Is one panel wired from the other giving you either a 150A or 100A service? Or, are they both wired to a main panel (possibly built-in to the meter box) providing you with some other service level like 200A?

My thoughts are to wire one of the units to the 150 amp panel and another to the 100 amp panel.
Depends on your answer to the above.

Installer is indicating he needs 2 30 amp AC disconnects. All I can find is 60 amp units at the big box stores.
The non-fused 60A disconnect is the correct part; 30A is the minimum size, but is never used anymore since 60A disconnects are so cheap.

What size wiring/circuit breakers would I need to install for these?
Depends on the nameplate rating of the A/C unit. You can get this info from the metal plate on the A/C condensor unit, from the manufacturer with model number, or from the installation instructions usually.
 
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Old 07-17-06, 03:29 PM
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Most small A/C type disconnects are 60 amp and are fine for anything from 15 to 60 amps.
I assume if he requested 30 amp disconnects a 30 amp circuit is also adequate. If you run #10 (10/2NM most likely) you should be fine. Without the exact specs off the unit there is no way to tell for sure though.
You WILL need the specs to size your breakers though. DO NOT go by what he tells you, get the corrrect info off the units.
 
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Old 07-19-06, 10:27 AM
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That's a good question (that I unfortunately do not know the answer to). We have a single drop off the pole to the side of our home. On the side of our home, we have a single meter, and alongside of that there is a space where a 2nd meter could go (currently empty). From the meters, there is conduit going to each panel separately on the outside of our home. I do know that the 100 amp panel is not wired from the main panel. If you turn off the main breaker in the 150 amp panel, the 100 amp panel continues to work. Could it somehow be bridged inside the meter ? If so, wouldn't we at least have 250 amp service (150 + 100) ?

Originally Posted by ibpooks
How are they wired now? Are they both connected to the meter providing you with a 250A service? Is one panel wired from the other giving you either a 150A or 100A service? Or, are they both wired to a main panel (possibly built-in to the meter box) providing you with some other service level like 200A?






Depends on your answer to the above.



The non-fused 60A disconnect is the correct part; 30A is the minimum size, but is never used anymore since 60A disconnects are so cheap.



Depends on the nameplate rating of the A/C unit. You can get this info from the metal plate on the A/C condensor unit, from the manufacturer with model number, or from the installation instructions usually.
 
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Old 07-21-06, 03:47 PM
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Originally Posted by swallman
That's a good question (that I unfortunately do not know the answer to). We have a single drop off the pole to the side of our home. On the side of our home, we have a single meter, and alongside of that there is a space where a 2nd meter could go (currently empty). From the meters, there is conduit going to each panel separately on the outside of our home. I do know that the 100 amp panel is not wired from the main panel. If you turn off the main breaker in the 150 amp panel, the 100 amp panel continues to work. Could it somehow be bridged inside the meter ? If so, wouldn't we at least have 250 amp service (150 + 100) ?
Bumping to the top for more feedback!
 
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Old 07-22-06, 10:56 AM
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The ampacity of the Service Conductors determines the kilowatt rating of the Service, not the sum of the ratings of the "Main" cicuit-breakers, i.e., 150 + 100 = 250 amp.

The Service Condutors are the three conductors between the utility's cable-drop connection and the Service equiptments.The conductor size, such as "0000", and the conductor metal, copper or aluminum, determines the ampacity of the Service Conductors.

If you have S-C's with an ampacity of 200 amps , the power-rating of the Service = 200 amps X 240 volts = 48000 watts = 48 kilowatts.

Good Luck, and Learn & Enjoy from the Experience!!!
 
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Old 07-22-06, 11:04 AM
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Is there a disconnect at the meter (or anywhere actually) that will shut down both panels? If not, you will need to correct that situation before doing anything else.

You are not allowed to have the situation you have in a single residence without a common disconnect.

After you fix that, you can tap any panel you desire as long as it has the capacity to handle what you are putting in it. The determination is usually a matter of convenience (and the resulting costs) than anything, as long as the previous requirements are met.
 
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Old 07-22-06, 11:14 AM
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Originally Posted by nap
You are not allowed to have the situation you have in a single residence without a common disconnect.
Can you tell us where you find this?
I know of no such rule. Residential or commercial.
 
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Old 07-22-06, 02:31 PM
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Well I may be reading more into this than there actually is but from the description of the building (duplex) that had two services. I would presume they are in different rooms.

Code allows up to six disconnects grouped in a common area (some inspectors will not allow you to move your feet to reach them, some will allow some movement) to disconnect the entire service. Now there is only one service so they need to be able to be disconnected from a common location.

If the panels are not fed one from the other, and they are in different rooms, that would be a violation of the code.

Additionally per 230.40 each service drop or lateral will only supply one set of service entrance conductors. If there is no common disconnect (I'm including the up to six possibility as a "common disconnect"), then it is against code. What this is describing to me is that there was two services at this building but having one of the meters pulled and somehow they are tied together. By the OP's statement it appears they are both from the common meter.

Like I said before, if the panels are side by side and each has a main, that would be different and allowed but since it was a duplex, I assumed they are in different rooms.
 
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Old 07-24-06, 03:00 PM
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Originally Posted by PATTBAA
The ampacity of the Service Conductors determines the kilowatt rating of the Service, not the sum of the ratings of the "Main" cicuit-breakers, i.e., 150 + 100 = 250 amp.

The Service Condutors are the three conductors between the utility's cable-drop connection and the Service equiptments.The conductor size, such as "0000", and the conductor metal, copper or aluminum, determines the ampacity of the Service Conductors.

If you have S-C's with an ampacity of 200 amps , the power-rating of the Service = 200 amps X 240 volts = 48000 watts = 48 kilowatts.

Good Luck, and Learn & Enjoy from the Experience!!!
I understand that the service conductors would determine the maximum capacity. The wiring has to be large enough to carry the capacity from the pole to the home.

However, if there are in fact 2 separate services, wouldn't the electric utility have made sure that the service conductors were large enough to supply both 100 and 150 amp panels (at the same time) ? Or at least close (200 amps) ??
 
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Old 07-24-06, 03:22 PM
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Originally Posted by nap
Well I may be reading more into this than there actually is but from the description of the building (duplex) that had two services. I would presume they are in different rooms.

Code allows up to six disconnects grouped in a common area (some inspectors will not allow you to move your feet to reach them, some will allow some movement) to disconnect the entire service. Now there is only one service so they need to be able to be disconnected from a common location.

If the panels are not fed one from the other, and they are in different rooms, that would be a violation of the code.

Additionally per 230.40 each service drop or lateral will only supply one set of service entrance conductors. If there is no common disconnect (I'm including the up to six possibility as a "common disconnect"), then it is against code. What this is describing to me is that there was two services at this building but having one of the meters pulled and somehow they are tied together. By the OP's statement it appears they are both from the common meter.

Like I said before, if the panels are side by side and each has a main, that would be different and allowed but since it was a duplex, I assumed they are in different rooms.
If what you are describing is accurate, then it sounds like this was not wired to code from the beginning. There is only a single service drop to the home. From that drop, it feeds into 2 meter boxes, only one of which has a meter now. I'm not aware of any disconnect at the meter (at least none that I can see).

Inside the home, the panels are at the bottom and top of a stairwell respectively.

This is a 111 year old home (not sure if that makes a difference or not). It was originally wired with knob/tube, but was upgraded to romex at the same time that the circuit panels were installed.

I posted a picture of what the outside service looks like here:

www.couleetech.com/P7100116.JPG

You might have to zoom in a bit to see it (on the right-hand side of the photo).
 
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