I have a couple of questions concerning adding a sub panel box.  My main panel is full (except one slot) of tandem breakers along with a few 240 double pole breakers for bigger appliances.  The empty slot is 2nd from bottom on the left with a tandem 120 breaker below it.

I'm going to add a probably 100amp sub panel that can accept tandem breakers and move that bottom tandem breaker to the new box.  I understand the technicalities of adding and wiring up the new sub panel, including isolating the neutral and ground in the sub panel. I'll be moving that bottom tandem breaker to the new sub panel to make room for the 100amp feeder breaker.  The sub panel will only been a couple of feet away in the same attached garage (maybe 10 feet by wire).

2 questions

1. Is #4 AWG wire appropriate for the sub panel?  Or do I need #3 AWG?

2. Can I just extend the two hot wires from the tandem circuit that I'm moving to the new box using a single #10 AWG /2 cable?  no need to extend the neutral and ground wires of that particular circuit, right?

Thanks

Sorry, but I say if you have to ask you should hire someone (I hire people, it's not as if I know the answer but wont tell you).  Who's going to take care of your figs?  I've seen too many failed resuscitations on licensed electricians.

I am with Bob, I would hire one (insured). Also, if they mess up your electrical, they will fix it, not you.

I'm confident in my ability to do such things.   I've added breakers to existing panels before and have done other electrical work (not professionally) before.   I do very good research before attempting, hence the questions that I'm not yet sure about. 

This is the last step of installing/relocating a new water heater. 

I will however name an executor for the fig forest just in case ;)

Seriously, this isn't that big of a job,  just need a few specific details to check myself. 

James,

Agree with above recommendations.. bear in mind that first you must have a pretty good idea of the additional wattage (then current-Amps) requiring more cable and additional breaker (s).. Only after that you enter into cable and breakers calibers/sizing...but I would leave it to the electrician..

Here you have good reading and information on cable sizes/types/capacities. -https://www.icmag.com/modules/Tutorials/ElectricalSafety/1655.htm

Francisco

James,

I have an advanced auto electrical engineering background. While this doesn't apply to homes I can tell you I agree to hire a pro. In my experiences there are many ways to wire something up but only one correct way. The pro will also excuse you from any liability. You don't want to risk electrocution, fire, or damaging any preexisting wiring or things that you plan on plugging in. That is the last thing you would want! I am all for DIY but please don't risk anything even if you are 99% sure you have it figured out. The shock off of a breaker box is enough to take you out for good. Just think of all the orphaned figs! =)

Well. I think I might have a bigger problem than I expected.  As I said, I research well and easily grasp technical concepts.  I've repaired my oven, dishwasher, refrigerator.  I've added a breaker to a box to run a 240v line to a Jacuzzi tub that I installed, where I did all the copper plumbing (sweating the joints).  I'm a very capable handyman. My daughter thinks i can fix anything.. lol...  

But....

The model of my main panel breaker box is GE(something)2020CCU.  I know that means 20 spaces and 20 max circuits.  Guess what.. I think the electrician cheated and did a big no no.  There's 38 slim breakers in this box. None are tandem breaks as I had thought.

All the 240v are also slim single pole breakers where one is in one slot shared with another slim and the other half of the 240 is in the adjacent slot, pinned together.  (if that didn't make sense....  on the right, there's 4 slim individual breakers in the first 2 slots.  The first 2 breakers are on the right bus, the next 2 are on the left bus.  Breaker #2 and #3 are pinned together to provide 240v.  There's 3 sets of 240v configured like this.  The rest of the breakers are all 120v circuits slim breakers.)  

The box is way over the limit of circuits.  We had the house built 11 years ago and obviously it passed inspection at that time.  And obviously it's working. Now I'm very worried about this.  This is the first time I've ever taken the cover off the panel to see what's there.

I don't know if this electrician is still in business, but his number is written on the panel door, so I'll be calling him and probably have another licensed electrician to come out and inspect it too.

My very good friend is an electrician and once gave me some very good advice.  He said, basically, there's only two things you need to know about electricity:

1)  It's invisible
2)  It will kill you

Hire the pro!

That is true, Figaro..  I know well the feeling of 120v..

When I was a kid.. 14 or 15, we had a camper trailer in the front yard.  My dad was in it one day doing homework and I went in to ask if I could go roller skating.  I was wearing rubber sandels.  As I came in and pulled the screen door closed behind me, I thought I felt a shock, but very mild.  I touched it again and didn't feel anything, so we didn't think anything about it.  until...

The next day, my Mom, Dad and I were in the front yard and I needed to get something from the camper.  This time I was barefoot and the ground was wet and a small puddle right before the door of the camper.  I don't remember reaching for the handle, but don't remember grabbing it.  The next memory was of someone screaming and seeing the front yard slide by my vision.

Luckily, the door wan't locked and it swung open, hitting me and knocking me down, which knocked me off the handle.  My dad knew instantly what happened and was already running to knock me down, but I was off before he got there.  The next memory was trying to crawl away, but didn't know from what, and I couldn't move my legs, so i was just dragging myself by my hands.  My Mom caught up to me and held me until I calmed down.

Afterwards, my Dad got his meter and tested the shell of the trailer to the ground and it was a full 120v.  Turned out there was a short in one of the electrical sockets where the a/c was plugged in.

I'll never forget all the details of that event.  Trust me, I respect electricity and know how to be safe around it.

When I did mine, I had an electrician inspect my existing panel, and spec the wires, etc. for the new panel. Cost me $50. That is the technical part. After that it is just grunt work. There isn't anything magic to electrical work - just keep your fingers out of the wrong places and turn the power off when working on stuff. Some things have to be done "hot", but that is where common sense and caution come in. If you have already added breakers, etc. then you should have most of the grunt work skills you need.

I found the specs for my panel and does support 40 breakers, so it's not overloaded. Furthermore, I can move one existing breaker to another position and add a 1" double pole breaker for the water heater without having to add a sub panel. Much easier.

You'll have to make sure you have a 200 amp breaker box, you'll be able to tell by looking at the main breaker it should say 200 or 100 amp. 
Now every state has slightly different code in additional to the NEC. 
Adding 2 slim breakers might work fine and within code. (I'm a certified electrician)

All finished.  No "Shocking" news to report. ;)

I didn't have to add the sub panel.  was able to move one breaker to a new spot and added a new 240v breaker.  Now the panel is full.  I found that pulling the cable through 1/2" conduit impossible w/o lube.. lol

New water heater is working great in it's new location.