Residental mains wiring questions (USA)

You will need to add up the ampacities of the single pole breakers on each leg of the service that will feed circuits that are in the old panel. You then size the combined neutral for the higher of the two amperage totals. This assures that the combined neutral will be able to carry the current that would result from the failure of one leg of the service which would raise the current being carried by the neutral to the total of all of the circuits on the intact leg. You then select the conduit that will have a cross sectional area at least 2.5 times the total of the cross sectional area of all of the conductors to be run in the conduit. If you are not using rigid metallic conduit that will be made up wrench tight between the old panel enclosure and the new one then it would be prudent to run a wire Equipment Grounding Conductor (EGC) sized for the largest circuit in the conduit with the other conductors. It is unwise, even though it is permissible, to depend on the continuity of other types of metallic raceway to serve as the EGC. If you use a non metallic raceway between the two cabinets a wire EGC is required.
As to the mast for the service entry conductors it need not be threaded at the top if a non threaded service head is used. In fact many inspectors would fail the installation if a non threaded service head were applied over cut threads. -- Tom Horne

Thanks, Tom.
OK: size for the greater of the 2 phases used for those circuits. Since this depends how one runs those to the new panel, it's important how those are terminated. I was going to balance the loads, overall, but now I see how this impacts the neutral conductor size.
I can find all kinds of charts on-line that show me how many 10 or 12 or 14 ga conductors I can run in a 1-1/4 inch EMT, but not conductor cross-section. Do these exist? Or do I just convert the chart myself (ie, 25 conductors x 12 ga cross-section)?
EMT, so a grounding conductor is a must. By "the largest circuit in the conduit" you mean that if the biggest is 10 ga / 30 A, I should run a ground conductor sized for that one circuit?
Right! Thanks!

No.
In accordance with your local codes. Not all jurisdictions use the same version of the NEC, and some have additional requirements.
Your local Authority Having Jurisdiction (building inspector) has the final say, but any local licensed electrical inspector should be able to answer your questions. I suggest you discuss the installation in advance with the inspector you plan to hire for the final inspection, and also that you consider leaving the old panel intact as a sub-panel with its main fed from a branch breaker in the new panel. It would help a lot if you get whichever code applies in your area and read it carefully before talking to the inspector, so you can ask reasonable questions or better yet have reasonable drawings for review. This is generally cheaper than doing the job twice.
If you don't have the required tools such as torque screwdrivers and wrenches and some training in electrical work, or a willingness to obtain both, then you will be a lot better off just hiring an electrician for this sort of job.

Since you say you want to combine the neutrals, that implies you are running conductors in a raceway. From your description, you will have at least 12 current carrying wires, including your super neutral and that means you have to derate the conductors by 50%. (in the 90c column) That will have you running #10 for your 20a circuits and #12 for your 15a circuits. Your neutral could be as large as a #1 or even 1/0, a calculation is needed, then you double that. You can't really split these up either since the neutral needs to run in the same pipe as the ungrounded conductors it serves. Actually there is a loophole that would allow you to split them up if you have non-metalic boxes on both ends and use a PVC raceway but most panel enclosures are metal. You would need a plastic J box right next to the panel and nipple into it through one hole.

Table 9 in the back of the NEC has the circular mils of all the common wire types, including the insulation, which is what you use for conduit fill. Table 8 has more information about the conductor itself.

Save yourself a world of headaches and don't use conduit. Instead just match the size and type of each cable there and run a matching romex cable over to the new location. Better yet, for any cable runs that are exposed, unstaple them and run them towards the new location to help keep the length of the run to a minimum. You will still need to put the splices in a j-box, but you're going to avoid the neutral issue, conduit bending and fitting, plus you won't have to figure and make allowances for derating of the conductors.

Can you keep the old panel where it is? If so, the easiest way to feed the stuff already in there is to leave everything alone. It's also cheaper.
If you start filling up a pipe with all the circuits you have, you'll be facing a problem with having to derate the ampacity of what you put in. That means bigger wires and pipe than you figured.
Feed the existing main breaker from the new panel. That way you only have to run three conductors and a ground. The breaker in the new panel feeding this supply will have to be sized no larger than what you already have.
*IF* the old panel has 100 amp. bussing but a 60 amp main breaker, you can change the breaker to a 100 and then feed with the appropriate sized wire and breaker. There is usually a label inside the panel with maximum amperage ratings on it.
IF the feed to this panel is underground, the following will not apply.
There is a non-metallic sheathed cable available for sixty or hundred amp. capacity. It looks like Loomex. or Romex or whatever is available where you live. (3 conductor and ground)
It resembles a fat range/dryer feed and has four wires in it, a red, black, white and ground. There is an 'NMD-9' or 'NMD-90' or some such rating on the jacket. It's a temperature rating. The NMD means 'Non Metallic Dry' (location).
Using that cable saves a lot of grief. It's infinitely easier than pipe and not limited to the 360 degrees maximum of allowed bends.
The Neutral in the old panel will most likely have to be disconnected from ground. Most jurisdictions require that the Neutral and Ground be bonded together *only* at the main service disconnect. Your new ground wire will obviously be picking up all the old existing ones.
There should be a brass bonding screw in the neutral bar that screws into the back of the case. Really ancient panels will have a wire jumper between the ground and neutral bar. Remove the brass screw or jumper to ground in the old panel. Leave it in the new one.
Is the old one a 100 amp panel? Around here, 3 AWG copper or 2 AWG Aluminium are the correct sizes and 1 1/4 inch conduit, should you decide not to use non metallic sheathed cable. Aluminium conductors need an antioxidant paste smeared on at the terminations, before assembly, like No-Alox or Penetrox or equivalent.
Most of the residential weatherheads I've used have two set screws to clamp to the side of the pipe, making threads unnecessary. See what's in use around you. Some utilities MAY want a threaded head. The hub side has to be threaded.
You have to watch the amount of pipe sticking through the roof. There is a limit to how high above the roof the attachment point of the overhead wire can be. They're worried about bending.
Here, IF the conduit doesn't go through the roof it can be the 'thin wall' conduit and not the rigid needed otherwise. There still has to be a threaded hub on the meter base. You use a weathertight connector on the thin wall pipe to connect. It's a grounding the pipe thing.
Here, we can even use PVC pipe IF it doesn't go through the roof. Plastic costs more than metal and is only used if the labour savings make sense.
Check around the neighbourhood and see what they are using. You may get lucky and find a good sales person at a local Home Hardware type store. Make sure what you buy meets LOCAL code before spending money.
All disclaimers apply. Your safety is YOUR concern. If you blow yourself up, burn down the neighbourhood or lose your hair, it's NOT my fault.
mike

Should he decide to go this route, I'd suggest keeping the old panel enclosure where it is and strip the breakers and bars out of it.
Leave all the old circuits right where they are and then match them, as per your instructions. This leaves lots of room for the splicing and has the benefit of having only the existing panel cover as an eyesore, instead of multiple junction boxes.
This all assumes the stuff is in the same building and that his new meter and panel aren't out on a pole next to the building. That would be a different thing.
mike

The old box probably won't pass inspection. It's an old Zinsco(sp?) box with the ratty breakers and the cover's missing. I plan to replace it with a proper J-box. The romex will terminate in the box without a problem, I think. These are all ungrounded circuits, but a separate ground wire will be run from each outlet over the roof (it's a flat roof that's being overhauled) to the new main panel. The ground conductor doesn't have to run along side the power conductors, does it?
10 existing circuits: 6x15A, 3x20A, 1x30A (220v). Can't I just oversize the conduit and extend the 14 ga (for 15A circuits), 12 ga (for 20A circuts), and 10 ga (for 30A circuits)? My understanding is that the issue was heating in the conduit and that if you oversize the conduit (EMT) that you will avoid approaching the heating limit. No?
Service feed is arial, from the pole to a mast on the roof.
There are no existing grounds, but nonetheless, I will separate the box ground and neutral in the old panel (now sub panel).
Thanks. I just wanted to know if an unthreaded could be used at the top. Looks straightforward.
The utility's reference manual (that they gladly hand out) states the max, min, and other parameters. Seems pretty clear and they provide a phone number for answers.
It has to go through the roof, so 2" threaded according to the utility (PG&E).
I'm a big boy. Just ask my GF. ;-) No worries, mate. Thanks.

If you can protect the ground wire it can be run separately.
The derating has nothing to do with the size of the pipe. It is just the number of conductors in the pipe.
Generally speaking you can't split circuit conductors into separate raceways or go through separate holes but that is a "metal" problem having to do with inductive heating. . What you can do is run 2 or 3 PVC conduits to split them up, use a big plastic box at each end and use a single short nipple into the existing and new panelboard enclosures. The only function of the box is as a pull box where you combine the conductors. That way, where they are split will be all plastic (no inductive heating) then use short nipples to get in the metal enclosures through one hole. You do not need to derate if the nipple is less than 2 feet long.

I understand what inductive heating is, but not "you can't split circuit conductors". I had planned to extend the circuits (hot & neutrals) from the old panel to the new panel (about 50 feet apart). I won't be separating hots & neutrals.
Or by "separating" are you referring to my question regarding combining all the neutrals into a common neutral? Is this why I would have to be concerned with inductive heating, if I go with a common neutral?
Thanks.

And if I want to use EMT and avoid inductive heating, I could just use separate neutral conductors for each circuit in the conduit?
Thanks.

See if you can use Ground Fault Interrupters in the feeds to these circuits. Put an 1110 box or something similar on the new panel board and use a 'dead front' GFCI in it.They're cheaper than GFCI breakers. If allowed, it will save you having to run ground wires.
Then, feed the old circuit from there. I used to do that on rewires to ungrounded circuits. It saved trying to fish in ground wires. Your local code may be different. That wiring on the roof scares me. Any way to run the stuff under the floor? Crawl space, basement?
There are three things here. One, the temperature around the pipe. Two, the number of wires in the pipe and three, the allowed pipe fill percentage. Find a web page that has the Electrical Code Tables and look up table 310-15xxxx or somewhere around there.
Your derating *may* be as high as 50 percent, I'm guessing. So, a 15 amp wire, derated by 50 percent is now 15 / 0.5 = 30 amps. Then divide by the temperature derating percentage, if applicable. Your 14 AWG has become at least a 10 AWG.
Going up in pipe size does NOT allow you cut back on the wire size.
See where this going? Once you get all the wires sized, add up the total of the cross section areas. Divide that by the allowed pipe fill, probably 40 percent (0.4 division).
That gives you the MINIMUM allowed pipe size. If it doesn't equal a regular size, you have to go UP to the next one. If this adds up to a three or four inch pipe, you will have your hands full and wallet empty.
Perhaps consider a new sub panel with only a feed to it. The costs, even with ground fault breakers, would be less than the above stuff and take many hours less to do.
Good luck, regardless.
I think I know her sister...

Huh? Your outlet wires go over the roof? Uh, no you don't take seperate runs for the ground wire, all need to go together. You mention 'passing inspection'. That won't...
Yes, they do.
You can get and use conduit as large as you want.
Consider that the main breaker must be within a given distance of where the service entry wire comes into the building. A long run of SE to a 'remote' box will likely also raise some questions with the inspector.
I'd strongly recommend a session with the building inspector in your area, describe what you want to do, and ask him/her if that will pass. If they say "no", then look for other alternatives. If they say it is "OK", ask if they have any suggestions or things to watch for.
Their advice, without any doubt, will be worth more than all the advice you will ever get on the Internet.

weatherhead=20 No. The NEC rule is one neutral per breaker.
Where to you find non-threaded weatherheads?

a 15 amp wire (there is no such thing, you are referring to a 14 AWG wire), derated by 50% would be: 15 * 0.5, or 7.5 amps max. Not 30 amps!
Again, totally flawed math. The 14 AWG would be (roughly) the equal to 18 AWG. Not 10 AWG!

That is just asking for multiple code violations.

Yes, I was responding to the common neutral idea. Bear in mind if you are running separate neutrals each one adds to the wire count for derating so to avoid it you are in 3 pipes for your installation from what I see. That is the right way to do it although I am not really sure what the violation would be if you did use a common neutral. Another option is to double up your circuits in a multiwire (edison, shared neutral or whatever they call it in your area). That would require double pole breakers but you reduce the neutrals by half and cut the voltage drop in that section of wire. The down side is if one trips you lose 2.