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Old 03-12-2013, 12:06 AM   #1
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100 amp service to 200 amp service


I am planning on buying a house built in 1897. It has a 100 amp main panel with a 100 amp sub-panel for the upstairs, but only is powered with 50 amps (has a 50 amp breaker in the main panel for the subpanel is what my inspector said). Currently it has 2 furnaces and A-coils but no central A/C units (I will need 2) for dual zone hvac. There is currently no wiring for the A/C units so it will have to be run and since there is no wiring there is no circuit breakers for them and there are only 2 spots left in the main panel, but I don't think there will be enough amps left in the house to add 2 A/c units. So I am thinking I will have to get 200amp service All the the wiring has been updated to Romex 3-wire. The wiring that feeds the 100 amp subpanel looks like it is twice the size of the wiring which feeds the 100 amp main panel wiring. Granted I am going to have to make sure it is big enough to make it 100 amp. So my long drawn out questions are:
Can I just Remove the 100 amp main panel box and install a 200amp main panel box with new breakers and if need be have the utility company change out the wiring to the house to 200 amp service and meter, and without making it sound that easy, that is it?
And if the wiring is big enough for the upstairs sub-panel can I just change that breaker from 50 amp to 100 amp and have 100 amp service upstairs?
And who makes the best most economical main panel and circuit breakers? (I think GE appliances are junk, so there breaker boxes probably aren't any better).

Thanks for any input

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Old 03-12-2013, 06:09 AM   #2
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100 amp service to 200 amp service


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Originally Posted by rugerduke View Post
Can I just Remove the 100 amp main panel box and install a 200amp main panel box with new breakers and if need be have the utility company change out the wiring to the house to 200 amp service and meter, and without making it sound that easy, that is it?
Most likely this would be YOUR responsibility...

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And if the wiring is big enough for the upstairs sub-panel can I just change that breaker from 50 amp to 100 amp and have 100 amp service upstairs?
If the wire is in fact rated for 100 amps, but you probably won't ever need 100 amps, so its probably not worth worrying about.



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Originally Posted by rugerduke View Post
And who makes the best most economical main panel and circuit breakers? (I think GE appliances are junk, so there breaker boxes probably aren't any better).

Thanks for any input
IMO, GE is fine, my current employer likes square D, but they cost more, they make a home line version which is typically cheaper. Personally, it's residential, so they pretty much all fall in the same category...

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Old 03-12-2013, 07:42 AM   #3
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100 amp service to 200 amp service


When you pull the permit for installing the new air conditioner compressor you will need to do a load analysis (refer to the NEC) for the entire house. This may or may not reveal the need to upgrade the service.

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Originally Posted by rugerduke View Post
Can I just Remove the 100 amp main panel box and install a 200amp main panel box with new breakers and if need be have the utility company change out the wiring to the house to 200 amp service and meter, and without making it sound that easy, that is it?
Yes you can do that.

Upgrading the service and main panel may or may not need a separate permit (depends on the city). Depending on space available you could install a new main panel and meter socket and weather head in advance. Also have a cable from the old panel ready to connect to a breaker pair in the new panel.

You can place an order with the power company for an upgraded service and on the appointed day they change over to your new meter box and panel and you connect the cable from the old main panel to the new main panel. This way the power to the house will be cut for only a hour or two and on just one occasion.

Then you can hook up the new AC compressor at your leisure.

The existing upstairs subpanel can remain connected to the old main panel for the time being.

Using the immediately preceding method, if you wish to, you can move all the circuits including the upstairs subpanel feed to the new main panel and finally remove the old main panel.
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Last edited by AllanJ; 03-12-2013 at 07:50 AM.
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Old 03-13-2013, 10:55 PM   #4
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100 amp service to 200 amp service


thanks, also I was wondering what size wire is necessary from the meter box to the service panel for 200 amp service I looked a few places online and they said 2/0 copper THHN/THWN, but when I looked at the specs from a company it said it was recommended for a max of 165amps and the 3/0 copper THHN/THWN is recommended for a max of 200 amp. Granted it depends on the code in my area but would 200amp service need the 3/0 copper THHN/THWN?

Thanks in advance
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Old 03-14-2013, 04:51 AM   #5
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100 amp service to 200 amp service


Wires that carry a houses' entire load use a special table, 310.15(B)(6), 2008 NEC. For copper the hots would be 2/0, for aluminum the hots would be 4/0. Three wire from the meter. Or three wire with ground after the first disconnect.

If you're replacing the meter can anyway, and there's no outdoor breaker panel, I recommend a meter/main combo with breaker spaces. Or at least an outdoor main breaker panel, with feed through lugs, next to the meter. This is a handy place to feed your A/C compressors, if they're being installed near the meter. It gives you outdoor expandability. And all the wires downstream are protected by a main breaker, a nifty safety feature.

Unfortunately, after an outdoor panel the inspector will seldom let you use table 310.15(B)(6). And depending on a lot of variables, including the residential load calculation, the hots would be, 3/0 Cu or 250 kcmil AL.

This brings us back to your original post and wire ampacity ("...if the wiring is big enough for the upstairs sub-panel..."). You see, ampacity is determined from a chart and then that number is plugged into a formula. I need to know: is it cable or individual wires in conduit; Al or Cu; does it touch (above or under) insulation; what's the hottest possible ambient temperature; what's the letter code on the cable (SE,USE etc.) or wires (THHN,THWN-2, etc.); what size and how many hots; wet (outdoors) or dry (indoors) location; and what's the temperature rating of the lugs or terminals these conductors land on, at both ends.

Last edited by Glennsparky; 03-14-2013 at 05:05 AM.
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Old 03-14-2013, 05:52 AM   #6
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100 amp service to 200 amp service


By the sound of the OP I really hope he is not planning on doing this work himself.
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Old 03-14-2013, 07:48 AM   #7
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100 amp service to 200 amp service


Ok thanks for the input. And I do plan on doing the work myself I just like to get all the information available and then researching it with city code so I know what I am looking at with the technical lingo in involved with it. And then choose the best easiest way to do it possible because I don't want inspection to come and fail or spend more money then I didn't plan on. It is called counting the cost.
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Old 03-14-2013, 11:53 AM   #8
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100 amp service to 200 amp service


Make sure you can do the work before you do too much research. Some places don't allow homeowners to do some, or all, of the work you describe.
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Old 03-14-2013, 12:30 PM   #9
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100 amp service to 200 amp service


When I did my main panel upgrade from 150 to 200 amp the poco just came and pulled the socket. They said I had the correct wire to the house but it was my responsibility to get the correct wire from the meter to the panel. They did say that if I would need a larger service they would come and trench new wire in at no cost to me. After a couple calls to the inspector I had everything I needed. He was very upfront with me and answered every question I had. IMO the inspector is your best friend. Get them involved early and keep them up to date on your progress they are more then willing to give advice. Turned out if I didn't change any of the wire or circuits then I would not need to use arc fault breakers. Total cost for my Square D QO panel and all new breaker was just over $225.

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