Sub Panel Size and Wire Size

[MGent]

I am finishing my basement and need to install a seperate sub panel for the circuits. I currently have a 200 amp service with one 100 amp panel in my garage and another 100 amp in my basement. The basement panel is almost completely full. I ended up with 11 15amp circuits and 4 20amp circuits for the basement. The 15 amp are all lights, outlets and 2 for disposal and dishwasher. The 20 amp are for microwave, refrig, bathroom, and bar outlets. My questions are:

1. What size sub panel should I use
2. Breaker size
3. How to determine amperage for common outlets...etc.
4. Wire size from panel to sub

Thanks in advance!

[John Nelson]

You want to add a fourth panel? Do you want to take power from the main panel or one of the other subpanels? Have you done a demand locad calculation?

Is simply replacing the existing subpanel in the basement with a larger one an option?

You mention that you have an existing subpanel in the basement, and you listed all the loads on it. What you didn't say is what loads will be on the new subpanel you are asking about. And tell us how many square feet of finished space will be supported by this new panel.

I'm a bit confused so far.

The way to size a panel is:
1. Figure out what loads will run from it, including planning something spare for the future.
2. Figure out how many watts each of those loads uses.
3. Figure out what will realistically be on simultaneously.
4. Add all those up to get some number of watts.
5. Divide those watts by 240 to get amps.
6. Add a fudge factor to give you a margin of error.
7. Put in a subpanel rated for at least that many amps.
8. Put a breaker in the feeding panel for no more than that many amps.
9. Size the feeder wire based on the feeding panel breaker size.

[MGent]

John: The circuits listed are the ones I need breakers for. The existing panel is for the main floor of the home. The basement is roughly 4000sq. ft. The conversion I get but my question is how do you determine general outlets not knowing what will be plugging into them? Thanks again!

[John Nelson]

So what did "almost full" mean in your first post? Have you already bought and installed this subpanel? If so, it's too late to ask what size it should be. What size is it?

[MGent]

There are 4 slots open in the existing panel and I didn't want to use the double circuit breakers in the existing panel. If I am not mistaken I believe I would be over the existing panels limit of circuits any way. No, I have not bought the sub panel yet since I am not sure what size. I was thinking about the 125 amp main lug but only using about 60 amps for the new circuits. I didn't know if that would be sufficient or not.

[John Nelson]

A 125 main-lug panel would be fine. Feeding it with a 60-amp double-pole breaker is also fine. Use 6/3 for the feeder.

I'd still like to know how many square feet the new panel will service. That piece of information normally goes into the calculation.

If you know anything at all about how a receptacle is going to be used, you can use that in your calculations. If you know nothing, then it is common to assume 180 watts per receptacle.

[MGent]

The basement is about 4000 sq. ft. That is why there are so many home runs back. 11 - 15 amp and 4 - 20 amp.

[John Nelson]

That's a huge basement.

What are you going to put down there? You mentioned a disposal, dishwasher, microwave, refrig, bathroom, and bar. If the rest is just lighting, computers and televisions, then you're probably okay. If you plan a motorized treadmill, tanning bed, space heaters, workshop or other high consumers, you might need more power. What rooms do you plan?

By the way, I'd use 20-amp circuits for the dishwasher and disposal. In fact, I'd use all 20-amp circuits where you plan 15-amp circuits.

[MGent]

John: Thanks again for the input! The rooms are the following:
1. Family Room
2. Bar
3. 3/4 Bath
4. Pool Table Room
5. Wine/Humidor Room
6. Safe Room (All 4 sides are concrete)(Under back porch)
7. Exercise Room
8. Theatre Room
9. Home audio/video equipment room (Already wired since I needed the room done for the audio/video equipment that is needed upstairs.

The only big thing I plan on is the treadmill. There would not be any other things running at the same time with a big drawl in the basement. That one was an oversight. I should have put in a dedicated circuit. Too late now though as the drywall is about ready for paint. I have always used the 15 amp for dishwashers and disposals in every house I built with no issues. I thought that was standard. I have the upper end dishwasher/disposals upstairs and they have never triped a breaker. This is the last house I will wire myself. Next one, I am sub'ing everything out!!

[John Nelson]

You will probably live to regret not putting in the dedicated circuit for the treadmill. You might want to consider doing it now, before the painting when the pain will be much less.

Some but not all dishwashers require a dedicated 20-amp circuit. Not having one may limit your choices of dishwashers in the future. But you should be fine in most cases with the 15-amp circuit. I wouldn't worry as much about this as the treadmill.

[french277V]

Wow now that is huge basement

Anyway really myself and John N. really suggest that you run the seperated circuit for threadmill before you get to the painting section it far easier to do that before you start the paint.

If after painted it can be pretty tricky to get in there without goof up something.

The reason that I know some of the threadmill can kick out 15 amp breaker when ya crank up to full speed with 20 amp breaker it will hold up pretty good there.

Merci,Marc