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I will be doing an addition to our home over the next year. One of the first steps of this is upgrading our existing service to 400 amp and then pulling a 200 amp subpanel that will be mounted on the back of the new attached garage we are building approx 100 feet from the main panel (i can mount interior or exterior). The 200amp sub panel will feed a 5 ton AC unit and standard outlets and lighting in a 24x30 garage. I intend to later pull a 100 amp feed off the 200 amp panel for a later planned shop. Heres my question - I intend to run the feed through the entire length of the attic of my home and into the new attached garage to the sub panel. What wire size do I need for 200 amps at 100ft and is it required to run in conduit through the attic? I am in Arizona and it gets hot up there too. Thanks for any and all help!!!
 

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First things we need to know, location of 400 service as in on pole or on home. 320/400 meter? Meter/main or meter with disconnects adjacent? Trying to establish configuration
 

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Discussion Starter #3
400 amp service is attached to the home into a siemens 320/400 meter/main box with a 200 main and 30 breaker slots and a provision for an additional 200 main that will feed the subpanel.
 

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200A @100' should be fine to use 3/0 wire. I'd be inclined to use conduit and pull THHN but you might want to go another route.


This will be pricey. $3/ft for THHN 3/0 so with four conductors, $12/ft or $1,200 for wire alone. I'd be sure you really need a 200A panel. A 5 TON AC and a few standard receptacles don't need 200A.



Maybe you run the future panel from the main service panel and downgrade today's panel to 100A and save some coin on supplies. Essentially two 100A panel off the main service instead of a (an expensive) daisy chain.
 

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Okay thanks for the info! If I went with just a 100a sub panel, what gauge would I need to be at for 100ft? I’m being told I can use 2-2-2-4 for it.....Thanks again for the help and info!!!!
 

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What wire size do I need for 200 amps at 100ft and is it required to run in conduit through the attic?
Generally you're going to need 4/0 aluminum or 2/0 copper. Some circumstances will require larger. Conduit will be required if you run individual conductors but not if you run cable. Aluminum SER cable is usually going to be the least expensive method. 4/0-4/0-4/0-2/0 aluminum SER runs $3-$4 per foot.

I am in Arizona and it gets hot up there too.
This is one of the factors which may require larger conductors. What's an honest assessment of how hot the attic temperature sustains on a regular basis?

Can you keep the wiring out of the insulation where it is run in the attic? If not, what version of the NEC is your locality using?

If I went with just a 100a sub panel, what gauge would I need to be at for 100ft? I’m being told I can use 2-2-2-4 for it.....
Assuming you mean aluminum, #2 isn't sufficient for 100A with your size of service. You would need to run #1 at least.
 

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I was referring to 2-2-2-4 copper cable. I can keep the wire out of insulation. Dduring the summer months, it can reach up to 130 degrees in the attic mid day on a 110+ degree day. NEC version 2012. I would much prefer to run cable than individual conductors in conduit.
 

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Dduring the summer months, it can reach up to 130 degrees in the attic mid day on a 110+ degree day. NEC version 2012.
The temperature doesn't surprise me all that much and will probably require you to go to 250 MCM aluminum. It adds a little cost and it's certainly less fun to work with.

2012 isn't a valid NEC version. They run in 3 year cycles and didn't publish one in that year. Can you find out which you are using? It's going to make a difference.
 

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Discussion Starter #9
Its nec 2011, I saw 2012 for IRC my bad. Yeah mcm is never fun, ive done a lot of DC power work with 500 and 750 mcm copper for cell site battery banks. I am fine just doing a 100 amp subpanel rather than the 200.
 

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Its nec 2011, I saw 2012 for IRC my bad.
With that version of the NEC you may use 4/0 aluminum SER for a 200A feeder. It still bases the service entrance conductors and maximum feeder sizes on physical size alone instead of ampacity. Temperature correction doesn't apply. But do me a favor and get the okay from your inspector before spending the money for the cable.

None of that helps you on a 100A feeder, though. Your feeder will be required to carry the full 100 Amps after any derating is factored in. With temperature correction you will need 1/0 aluminum or #1 copper.
 

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With that version of the NEC you may use 4/0 aluminum SER for a 200A feeder. It still bases the service entrance conductors and maximum feeder sizes on physical size alone instead of ampacity. Temperature correction doesn't apply. But do me a favor and get the okay from your inspector before spending the money for the cable.

None of that helps you on a 100A feeder, though. Your feeder will be required to carry the full 100 Amps after any derating is factored in. With temperature correction you will need 1/0 aluminum or #1 copper.



The code allowing 4/0 aluminum for 200A is where it carries the entire load of a dwelling, a outbuilding does not fit that description, the only way around that is if the calculated load is under 180 amperes, then the rule allowing the next higher breaker/fuse size comes into the picture and the next higher size is 200A.
 

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Discussion Starter #12
Alright guys thanks for the help and heres the plan - 4/0 alum feeding a 100a subpanel in the new attached garage. The new 400a panel has a 200a main breaker and provisions to add two more 100a breakers of which one will feed the new attached garage and one will feed the future 30x40 shop. Thanks again for the help everyone!
 

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Be advised, it is highly unlikely that 4/0 aluminum will fit into the lugs of a 100 amp circuit breaker.
 
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