I have the ability to run a subpanel upstairs and was considering at least running the wire while I have the studs exposed in my kitchen. I will have an electrician wire it up, but I would like to run the wire before my drywallers close my walls up. I will likely use a 30A switch at the main panel and run romex to the sub panel.

What size romex wire should I use? 12/3 or 10/2?

http://www.cerrowire.com/ampacity-charts

Again I would like to run the wire now, not hook it up yet, but have the option to do so at a later date.

What is the capacity of the Sub Panel?

I hope it will drive my 120v circuits upstairs (lights, wall sockets etc.).

No electric heater (HVAC has nat gas), lights, outlet / receptacles.

What I was driving at was that I would size the wire for the full load of the subpanel (i.e. 60amp) in lieu of sizing it for the breaker you put in serving it. some day someone may max it out and the wire would be undersized.

It may be code that you cannot undersize the wire serving it, but I am not that familiar with the NEC.

You could put anything larger than what you will need in there. I'm thinking, be sure to put a whole lot extra length in there for flexibility in locating the panel and terminating the wires.

Good point. I haven't purchased the subpanel yet. I guess I am sort of in the middle of my project when someone mentioned the subpanel upstairs idea.

It seems like 10/3 is likely the way to go.

Note from the table you posted you can't put anything more than 20 amps on a 12 gauge wire. 10 gauge can go to 30 amps. In the NEC tables it is more refined but the obelisk next to 10 and 12 gauge takes you to a footnote that says overcurrent protection for 12 gauge no more than 20 amps and 10 gauge no more than 30 amps.

So with 12 gauge you could run one 20 amp circuit or 10 gauge you could run 15 amp general purpose branch circuits. That isn't going to supply too many lights and outlets in a typical "upstairs" of a house. How many sq feet is "upstairs"?

Lone Stranger said:

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Note from the table you posted you can't put anything more than 20 amps on a 12 gauge wire. 10 gauge can go to 30 amps. In the NEC tables it is more refined but the obelisk next to 10 and 12 gauge takes you to a footnote that says overcurrent protection for 12 gauge no more than 20 amps and 10 gauge no more than 30 amps.

So with 12 gauge you could run one 20 amp circuit or 10 gauge you could run 2, 15 amp general purpose branch circuits. That isn't going to supply too many lights and outlets in a typical "upstairs" of a house. How many sq feet is "upstairs"?

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60 amp minimum required to subpanel by code.

Thank you all. I don't really understand this stuff completely, I was under the impression that I could two 30A circuits on different phases?

If I ran it on a 10/3, I would have 2 wires to run each with 30A circuits on different phases. That way I have 2 120V circuits for a total of 240V, if ever needed. I guess running 6/2, would be easier to understand anyway.

Your comments help me realize that I need to dig in further.

Future-proof ...go big on the wire/panel the first time around.

Why are you putting in a subpanel in your house? You aren't out of room in the main are you?

Edit: disregard reply to Absolute. I was going to clarify his statement which is misleading...But then I got caught up on the subpanel question and abandoned it. Lol.

Sorry, I disregarded your whole post.


I was just going to run a ground wire to the conduits upstairs, but my dad suggested that I run a sub panel upstairs (actually in the attic) and rewire outlets as needed. Scope creep basically.

Also, am likely to get a contractor to build a bathroom upstairs. Hopefully the subpanel will make their lives easier.

TexAg1987 said:

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#6 for a 60 amp panel IMHO

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I mentioned running #6 wire and my dad thought I was crazy. He asked me if I planned to start welding upstairs or running a plasma cutter. He is a retired Square D guy (41 years, his only post college job), so I guess I will stick to the #10 wire.

SpeedyMilt said:

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TexAg1987 said:

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#6 for a 60 amp panel IMHO

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I mentioned running #6 wire and my dad thought I was crazy. He asked me if I planned to start welding upstairs or running a plasma cutter. He is a retired Square D guy (41 years, his only post college job), so I guess I will stick to the #10 wire.

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#10 for 30 amps - OK

But if you are putting in a 60 amp sub panel.......

I'm not an expert at all, having recently dug into much of this when adding an extra circuit to my garage, so take everything I say with a grain of salt. That said, I would say this isn't much of a guessing game or anything - if you want to run 30 amps, you use 10, 20 amps use 12, etc.

If you follow your dads advice, you're basically setting up a circuit in a way that it can draw more current (breaker will allow it) than the wire can handle. Even if you DON'T overload it, somebody in the future that isn't you may not realize "hey, this panel is rated for 60 amps, but it's only supported by a wire that is rated for 30 amps so I shouldn't depend on these breakers". You're setting yourself up mechanically to pull more power than your infrasture will be able to handle. Ultimately no, you won't be welding, and the "normal" thing to do at that point might be to drop down to a 30 amp switch and 10 gauge wire - but if code requires 60 amps to a subpanel, you can't do that. So dropping the wire but not the panel seems like a dangerous recipe. Especially for use in a way that could lead to people that don't understand the situation simply assuming if it plugs in and doesn't flip the breaker, all is well.

He mentioned that the subpanel should be purchased for how many slots are needed. He was adamant that I do not let the panel dictate how much amperage I run.
JDCAG, you are correct. I dont' think that I am going to run 60 amps anymore as it doesn't seem like I have enough load upstairs to justify it. Even if the new (not yet built) bathroom has an electrical heater, my dad is pretty sure I will not past 20amps, so 30 should be plenty.
On a side note, he drew me the following diagram:

He is pretty adamant that I go with Square D panels / breakers and that I stay away from the Homeline and go with QO.

Speedy- if you don't mind let me ask you some questions.


Why do you want to add a sub panel? I'm not trying to be dense, I get that you want to add a bathroom. But is there available capacity in your main panel? How many circuits are already on the upper floor? I guess it just seems to add a degree of complexity that also adds a failure point, so kinda curious. If you have available slots in your main panel it will be far easier to rough in 3 runs of 12/2 wg romex and be done.

I'm an electrical P.E. and I do this stuff for a living though mostly industrial construction. However I just remodeled 2 bathrooms so am up to speed on applicable code.

Regardless, I second your Dad's advice on the Square D QO line. SqD QO is more expensive but you're not adding enough to make an appreciable cost difference. I would also strongly recommend you reconsider only running 10 ga to a sub panel. It can be properly protected with a 30A breaker but that does not provide much capacity in general and won't add adequate capacity for a bathroom.


Next, with all deference to your Dad, I strongly recommend you get copies of the applicable codes for your city relative to bathroom electrical. Most cities adopt IRC or NEC, or both. Read NEC about circuits for bathroom lighting, electrical outlets, vent fans, and heaters. It might surprise you to learn that you could have at least 3 20A circuits just for a bathroom with a heater. Further, don't make hasty assumptions about maximum power load you'll have. Add up hair dryers, curling irons, and a heater, all of which could be on at the same time, and you can easily have 30-35A total on multiple circuits. All out of a proposed sub panel fed with a 2 pole 30A branch circuit?

Something to think about anyway.

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Why do you want to add a sub panel?

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I purchased a 1920s house about 7 months ago and I have conduit electrical sockets upstairs and I don't have ground (conduits must have lost it). Initially I was going to run a ground up to my attic and ground out some conduits. That seemed less than elegant and that is where the subpanel idea came in. If I ever want to rewire some or all of the outlets upstairs, I would have a ground in the subpanel, as well as switches.
The subpanel idea would allow me to run the wire now while my studs are exposed downstairs and upgrade later.

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How many circuits are already on the upper floor?

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I am not sure. Most of it is run by 1 breaker, but there maybe 2. The wiring is pretty old.

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How many circuits are available in your main panel?

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I don't recall at the moment, but several, although my electrician may take up a couple more slots for the kitchen. The panel seems to be only 5ish years old.

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strongly recommend you reconsider only running 10 ga to a sub panel

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- Thank you.

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"with all deference to your Dad"

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- No need. He nor I would be offended. He knows more about industrial code in Louisiana than anything in OKC. He also mentioned that breakers have advanced a bit in houses since he retired.

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I strongly recommend you get copies of the applicable codes for your city relative to bathroom electrical

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Will do. I don't think I understood much of this on the front end, so the codes wouldn't have made as much sense. I will likely push back the drywallers a couple days so I can read up on bathroom code prior to that.

In light of your explanation I get the need for a sub panel- I would probably add one too. Go ahead and put a 60A panel in but run conductors for 60A (depending on insulation type it will likely be #6). This will be enough capacity for your future bathroom and allow you to move the rest of your upstairs lighting & receptacles to the new panel when you work on them. This will provide ability to upgrade to current code levels with grounded outlets.

Can you define what "upstairs" means? Is this two small bedrooms and a crapper, or another 2000 square feet?

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He mentioned that the subpanel should be purchased for how many slots are needed.

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Sure, but knowing the load kinda helps.

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60 amp minimum required to subpanel by code.

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The subpanel is fed by a feeder, not a service. The rules of 230 don't apply. It's also not a separate structure or outside feeder - unless the old house has severe issues - so 225 doesn't apply either. The sub-panel, upstream OCPD, and conductors are determined by the load.

There's a couple of ways to approach it because it's an existing dwelling unit. The basic idea is the upstairs load is based on 2 w/sqft lighting, 1 w/sqft general receptacles, nameplate load of any fixed appliance or HVAC, and then diversity taken based on a couple of specific points. Depending on the square footage, you *might* get away with a 30A sub-panel for the second floor. Keep in mind the NEC isn't a design handbook, but rather the bare minimum. Only you know what you plan on plugging in to it, how many space heaters will eventually show up, and what hair care constitutes in your place.

As noted above, there are specific requirements to branch circuits for a bathroom. You need one circuit to feed nothing but the bathroom's receptacles. Since you would be adding a new panel in a dwelling unit (aka house), you'd need to provide arc fault circuit interrupter protection to the new circuits in bedrooms and such.

I think Underoos and I are on the same page. There's no way I would undertake a remodel like that with only a 30A panel. In addition to the dedicated circuit for bathroom outlets he mentioned I think you will have to have another dedicated 20A circuit for a heater should you include one. If you install a jetted tub it will have to have it's dedicated circuit. At the end of the day you will greatly restrict your future remodel options if you only rough in for a 30A panel. I would also like to see the answers to Underoos' questions on square footage & usage.

AgEngineer72 said:

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I think Underoos and I are on the same page. There's no way I would undertake a remodel like that with only a 30A panel. In addition to the dedicated circuit for bathroom outlets he mentioned I think you will have to have another dedicated 20A circuit for a heater should you include one. If you install a jetted tub it will have to have it's dedicated circuit. At the end of the day you will greatly restrict your future remodel options if you only rough in for a 30A panel. I would also like to see the answers to Underoos' questions on square footage & usage.

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Thank you all.

I think the sq ft is around 1,200 for the upstairs. It is a really old house that had its electricity put in pretty late in the game. I am not great with load, but at the moment, a hair dryer would be highest load. All of the light bulbs are LED. The new bathroom won't have a jetted tub (likely won't have a tub) as my wife and I wouldn't use it and there is a cast iron tub in the other bathroom (that is right, the ~2600 sq ft house only has 1 bathroom).

At this point I think I am going to take your advice and run 6 ga wire. I still need to read the code for OKC and may have a follow up question, but for now I am think 6/3 will be safest.

Thank you again.

AgEngineer72 said:

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I think Underoos and I are on the same page. There's no way I would undertake a remodel like that with only a 30A panel. In addition to the dedicated circuit for bathroom outlets he mentioned I think you will have to have another dedicated 20A circuit for a heater should you include one. If you install a jetted tub it will have to have it's dedicated circuit. At the end of the day you will greatly restrict your future remodel options if you only rough in for a 30A panel. I would also like to see the answers to Underoos' questions on square footage & usage.

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We renovated a 1960's bathroom and have a heater and a jetted tub. Both required 20a service.

I would do the full 60.