Got an electrical wiring question if we have a resident electrician

[MattBaggins]

I am going to rewire my upstairs completely and since I have baloon framing I am going to use the attic and drop my wires down to where I want each out let or light switch and run circuits back to gang boxes. I was thinking of pulling a larger wire upwards and setting up a subpanel, working to the new panel. I seem to not fully understand the subpanel and what I have researched on it. My main panel is fed from the the street to a single 100amp breaker that is the main. If I set in a 50amp breaker to feed the subpanel next to the 100amp main, how does this not overload the 100amp main circuit breaker? If I have hypothetically 80 amps downstairs and 40 amps upstairs this is too much for the 100amp main? I should first have the 100amp main replaced with a 150amp or 200amp main?

[EST]

1. You will need not only to replace the breaker, but, since it is

proctecting the input circuit, the input needs to be upgraded to allow for the total amperage you wish to draw.

[MattBaggins]

3. Input wires are 8 or 6 gauge

Should be OK. I really may just have a professional do the subpanel for me. I will have to have them upgrade the main since the power company probable wont shut off at the pole for me ;). I am not brave enough to fool around with the main.

[MattBaggins]

5. Make that 3 gauge

which would require and upgrade I guess

[oneighty]

2. You do not need fifty amps

to feed a branch circuit. That is a lot of power. I have thirty amps feeding a second breaker box and suppling an up stairs apt complete with a fridge and five rooms. Installed by an electrician, although I could have done it myself if physically able.

There are good wiring books available at your local supply houses.

180

[MattBaggins]

4. I have the black and decker book

which is about as good as their power tools.

[oneighty]

7. Here is what i have.

I have two 100 amp service boxes. I have 230 volts/100 amps feeding the main service box. From the main service box I have number ten wire on a thirty amp breaker feeding the second box (Temporary. later I will bring in a second metered power service for the second box feeding the upstairs apt.) But for now the ten wire feeds thirty amps to the second box, there it breaks down into five twenty amp circuits feeding all of the upstairs apt. The thirty amps is plenty of power.

I do all my wiring with twelve wire so twenty amps is ok. Twelve wire with ground is a bit harder to work with but is worth the effort. I used to over wire under fuse.

180

[two gun sid]

6. Do you have access to a clamp-on ammeter?

If you do try taking a reading on your service conductors to see how many amps you are drawing now. If it is a residence I will bet it is well under 100a. If you are still concerned, get a copy of the NEC and go to Art. 220 Part C for service calculations. It is pretty explanatory.

Getting a service change to a 200a service can be expensive. I would say off the top of my head that unless you are adding a bunch of electrical equipment to your upstairs you should be fine. If you do put in a sub panel make sure the neutral is isolated. Don't put the green bonding screw that comes with the panel through the neutral bar and into the panel. Make sure you get a panel that has a isolated neutral. Pull a cable that has 3 wires and a ground from the main to the sub.

Good luck.

[EST]

8. If I were you, and not because I'm an electrician,

I'd pay for a certified electrician to come in and do it. Unless you live in an area that is un zoned, most communities require that all residential electrical work be accompanied by a building permit, be signed off by someone with standing, and conform to NEC standards. This helps protect you, from a safety standpoint as well as insurability. If there should be a fire, for instance, and, due to injuries or other liability, the amount required to settle any resulting claims is substantial, the insurer will be looking for any possible responsible party, other than themselves, for financial accountability. If there is any way they can pin it on unskillful mechanical work, they will happily do so. Your peace of mind can also be pretty important.
If it's still important that you have a bit of working knowledge (and why not?) then read on.
One hundred amps actually isn't very much, although, for a small apartment, it should be sufficient. Be aware, for instance that a mid-size window air conditioning unit will generally draw, while operating normally, approximately 12 to 20 amps. However, it would take about 35 amps when starting. If you consider that you may have about ten amps going to lighting and fifteen amps going to run your sound system and computer and now you can easily be pulling over sixty amps - blowing breakers or, perhaps, endangering your insurance policy. Also, for the larger conductors feeding your panel or sub panel, you will find that 6 or 8 or 10, single conductor wire will not serve. The sizes you will see for, say, a sixty amp service will require cable sizes that get larger as the numbers get bigger instead of smaller. 2-0 or 4-0 (commonly referred to as two ought or four and ought) or larger are what I would expect to see as as input to a 100 amp breaker box. Most of the old, old houses that had REA wiring with, maybe, one light bulb per room with a little single plug outlet attached were set up for 60 or 100 amp input. Of course, not many had even heard of air conditioning. These days, in a small (1500 square foot) house with electric heating, I would expect to see a 200 amp breaker box.
Should you wish to complete the project yourself, (and I do recommend it- can be fun) you can find numerous how-to books that go as deeply as you want. It's not difficult, but can be complex and detailed. Enjoy!