Installing 125A Subpanel

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  #1  
Old 01-25-06, 08:24 PM
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Installing 125A Subpanel

I'm currently remodeling my second floor, including bathroom and office, and decided a running from a subpanel then routing to each room would be easier than running each from the main panel.

I have a 125A Subpanel coming with arc-fault breakers coming, but have a few questions:

1) What gauge and type of wire do I need to run to the panel? I looked at a chart at Lowe's and it looks like 2AWG, but I'd assume it'd be in the 0's.

2) I assume I need a double breaker whose sum is 125A on my main breaker panel to feed this subpanel?

3) Do I run a copper ground wire from this panel to a grounding rod, or does the fact that the main panel is grounded in this way suffice?

Once I get past this issue everything will be fine. Thanks.
 
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  #2  
Old 01-25-06, 09:04 PM
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The size wire will be determined by the size of the breaker feeding the subpanel. The 125 A rating is the max that the subpanel is rated for.

Have you determined the loads that will be served by the subpanel?
 
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Old 01-25-06, 09:08 PM
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The questions that you are asking suggest that you have a bunch more reading to do.

The issue is that an electrical installation includes many details, all of which must be correct. There are so many details that you won't even know to ask about them, and we won't remember to tell you about all of them. This is why you will get lots of advice suggesting that you read several books on home wiring, and then come back to check your plans and ask questions about details.

The second to worst outcome if you miss a detail is that you will need to pull out and redo most of your work.

The worst outcome if you miss an important detail will require talking to your insurance company and lawyers.

I will do my best to answer your specific questions, but please remember that these answers are by their nature incomplete. You must fill in a ton of details.

1. If you use copper conductors suitable for operation at 75C or greater, and all of your terminations are 75C or greater, then you can use #1 conductors at 130A. If you have 60C conductors or terminations, or use one of the wiring methods that must be calculated at 60C, then you would need #1/0 conductors for 125A

2. You would use a 125A double breaker. This is 125A on _each_ lever, _not_ 125A total. The fact that there are 2 poles means that you have 240V available at 125A.

3. You run an equipment ground wire from the subpanel to the main panel. You do not need an additional ground rod, and most importantly you _do not_ run a separate grounding electrode system. You _must_ connect an equipment grounding conductor to the main panel.

4. You did not ask, but I will remind you: you must run _separate_ ground and neutral wires from your main panel to the subpanel, and must have _separate_ ground and neutral bus bars in the subpanel.

_I_ am certain that there are other issues that should be confirmed before you continue. Please do the reading, and good luck with your project.

-Jon
 
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Old 01-25-06, 09:45 PM
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Don't forget your bath GFI...

Good to hear you have AFI coming in your panel, these look similar to GFI breakers but have a totally diffierent purpose. Give us an idea of the layout of your bathroom circuit, and we can fill you in on what needs protected. ie... is there a fan and/or a light over the shower/tub? Also, is there going to be a kitchen(ette) that may require GFI protection? While GFI breakers give total circuit protection, they cost about three times the price of a GFI receptacle. This price will be repeated when the GFI portion of the breaker/receptacle fails. If you've never installed a GFI or AFI breaker, pay close attention to your neturals for those circuits. The netural coming to the panel hooks to a lug on the breaker and a netural from those breakers hooks to the netural bus.



Originally Posted by ignernt
I'm currently remodeling my second floor, including bathroom and office, and decided a running from a subpanel then routing to each room would be easier than running each from the main panel.

I have a 125A Subpanel coming with arc-fault breakers coming, but have a few questions:

1) What gauge and type of wire do I need to run to the panel? I looked at a chart at Lowe's and it looks like 2AWG, but I'd assume it'd be in the 0's.

2) I assume I need a double breaker whose sum is 125A on my main breaker panel to feed this subpanel?

3) Do I run a copper ground wire from this panel to a grounding rod, or does the fact that the main panel is grounded in this way suffice?

Once I get past this issue everything will be fine. Thanks.
 
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Old 01-25-06, 11:20 PM
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You need to ask your electrical inspector if they allow feeders sized by NEC table 310.15(B)(6) for sub panels. Quite a few do, and if yours does then the #2 copper or 1/0 AL would be OK. If not you need to use #1 CU or 2/0 AL.

Typically you would use type SE style SER (or SER) size 2-2-2-4 copper or size 1/0-1/0-1/0-2 AL with the smaller conductor being the ground.

If your inspector won't allow sizing from table 310.15(B)(6) then you will need 1-1-1-3 CU or 2/0-2/0-2/0-1 AL

This is assuming, of course, you actually feed the panel with a 125 AMP breaker. The loads you put on the panel may allow a smaller feeder and breaker. Even allowing for future needs you may be able to use a 70-100 AMP feed to the sub.

UNK
 
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Old 01-26-06, 12:22 PM
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The upstairs has four rooms.

Two of them are bedrooms, and the biggest probable load will be a TV and DVD player.

The third room will be the bathroom, and I had planned on two circuits to share the lighting, exhaust fan, and outlets. There will be one overhead light in the center of the room, lights to either side of the mirror, and a light in the shower. I may use a space heater in the bathroom, a panel heater, or a floor heater under the tile.

The last room is the office, and once again I expect two circuits, possibly three to accomodate lighting in three areas, and enough outlets to handle two computers, three 19" LCD monitors, 2 DVD players and all my other video / editing equipment.

The breakers are 15A and the wiring obviously 14/2.
 
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Old 01-26-06, 12:54 PM
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As I suggested, you have a bit of reading to do. I strongly recommend any of the books on home wiring by Rex Cauldwell, combined with the book 'wiring simplified'. The way that you describe things, I am not sure that you understand how to calculate loads on a circuit.

The number of circuits required for lighting does not change if you subdivide the lighting controls. You could have 10 lamps on a single switch, or 10 lamps on 10 switches, all on a single circuit. All that matters is that the wattage of the lamps not exceed the ratings of the switch or circuit.

Code requires that the bathroom receptacle be on a 20A circuit with 12ga wire. This can either be a circuit that feeds _only_ bathroom _receptacles_ (possibly in multiple bathrooms), or it can be a circuit that only feeds loads in a _single_ bathroom (but any load including the receptacles and lights).

Should you use a heater in the bathroom, it will almost certainly need to be on its own 240V 20A circuit, although it probably won't use the full 4800W available on such a circuit.

You can divide up the load into as many circuits as you wish, but you need to do the load calculation to determine the size of the feeder to the subpanel.

For the rest of the rooms you need to do a load calculation based upon the size of the rooms and your expected usage. Rex Cauldwell 'wiring a house' describes load calculations, and you will also find discussion of this in the forums.

That said, based upon your description, my _hunch_ is that you will have more power than you could ever want to use if you run a 60A feeder to this subpanel. This would mean putting a 60A breaker into your main panel, and using wires suitable for 60A to feed the panel. These would be #6 or #4 conductors, depending upon specifics.

Again, do the reading before you buy any more parts, or you will spend way more money than you need to; first buying stuff that you don't need, and then again when you rip it out to fix something that you missed.

Good luck! Doing a complete rebuild is a much more enjoyable project that fishing one circuit in an old house

-Jon
 
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Old 01-26-06, 01:31 PM
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And just because you install a 125A subpanel doesn't mean you have to feed it with 125A. It depends on the loads you will be feeding from that subpanel.

The last two subpanels I did, in fact, were 125A panels fed from 50A breakers with #8 copper wire.
 
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Old 01-26-06, 01:47 PM
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Since you NEED a 20 amp circuit for the bathroom, you will be using at least some 12 gage wire.

I strongly suggest that you make ALL the receptacle circuits 20 amp circuits. You will appreciate this later on, especially for the office.

Go ahead with 15 amp circuits for the lighting if you want, but these too can be 20 amp.
 
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Old 01-26-06, 02:56 PM
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Thanks

Thanks for all the input all. I guess I probably jumped the gun with ordering things without knowing specifics i.e. figuring out specific loads, but at the same time I have a hookup with Cutler Hammer on the breakers/panels, so even if I decide to go with 20A arc-faults instead of 15's I can always EBay the 15's and break even at least. I don't think I misunderstand loads as much as I like to go overkill on everything, hence the three circuits to the office.

As for the space heater, I was thinking more along the lines of a panel heater from www.eheat.com. The load given is only like 3A.

And as for it being more enjoyable to do a rebuild... I love to create, and that's the reason why I'm going all out like this. There's nothing like having somebody say, "You did THIS?" So long as it's not the firemen.
 
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Old 01-26-06, 03:33 PM
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I think this has been covered here before, so I don't mean to de-rail your thread on the heating tangent. I think you may want to consider more traditional heating methods than e-heat.

Originally Posted by ignernt
As for the space heater, I was thinking more along the lines of a panel heater from www.eheat.com. The load given is only like 3A.
Personally, I am a skeptical of the eheat products. I really cringe when one of the main advertising points on their front page is "NO FIRE RISK." They must be really full of themselves or out in lala land to make a claim like that for an electric heater. Every electrical appliance has a fire risk; theirs may be safer than others, but it is certainly not no risk.

They also throw around the word "efficient" an awful lot, yet it really has nothing to do with the performance of the heater. All resistive electric heaters are virtually 100% efficient by definition. A 1000W electric heater produces 1000W of heat; their claims of incredible heating capabilities with only 400W simply aren't supported by physics in my opinion.

I don't have any experience with these products, but I just bring this up as a "buyer beware" point. Look for some reviews or something like that before spending a lot of money.

There's nothing like having somebody say, "You did THIS?" So long as it's not the firemen.
That's a good one!
 
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