Grounded neutral in an old sub-panel

The short answer is no. Run a new feeder. You can't run a neutral separate from the hot leads and you can't use the bare wire in your existing cable for a neutral.

Hire an electrician.
Chances are if the neutural are gounded, you'd have a hard time dismantling the old one without feeling the buzz.
If you're going to do it yourself anyway, you'll need to run another wire to connect ground to something grounded, check your local regulation for proper ground rod or hookup. Also replace the old neutural wire with those that are insulated.

Holy smoke!
I *hope* you mean on the *interior* side of that flat roof... That's a Code violation. The ground conductor is required to be run in the same cable or raceway as the circuit conductors. By installing a new feeder, with three insulated conductors of the proper size. IN CONDUIT THE WHOLE WAY. If you use metal conduit securely bonded to the boxes at both ends, Code allows the conduit itself to be the equipment grounding conductor. Otherwise, you'll need to run a fourth conductor for equipment ground.
Also, make sure that the feeder is protected by an appropriately-sized breaker in the main panel (not taken directly off the hot lugs of the main breaker, as so many DIYidiots seem to be fond of doing).

NEC requires ground to be in the conduit with the feeds? Rats! The roof is new and while I am experienced with electrics, I don't want to puncture that expensive new roof for conduit & conductors, unless there's no other way...
Breaker already exists in the (1980's vintage) service-entrance panel for the sub-panel feeds.
JJ

You are kidding, right? Run, run away fast... Totally replace the existing setup, get those wires off the roof!
The current system is a hazard, and needs full replacement. If you touch it, do it right. If you don't do it right, and later there's an accident (and there *will* be) you will be held responsible.
No, that is not an neutral conductor. It is a ground conductor.
Install it according to code. There is no other alternative. That includes getting that non-code, hazardous, existing wiring under the roofing removed, and replaced with soemthing that meets code.

OK, since it looks like going up on the roof is the current :-) solution...
This provides 2 possibilities for the conduit: 1) run supply feeds (2 hots + neutral + ground) to the sub-panel or 2) gut the sub-panel and use it as a junction box; run circuit conductors from the junction box back to the service-entrance panel (which contains the main disconnect, sub-panel disconnect, and breaker for the air-conditioning unit) where there are busses and spaces for more breakers.
What do y'all think the best option is?
In either case, through and over the roof with conduit. This means exposed to the desert weather (winter, sometimes to freezing; spring & fall, rain; summer, 140 F degrees - but its a *dry* heat :-).
What size and type of conduit and conductor should I use for: 1) supplying a 100A sub-panel? Individual conductors or SE? or 2) connecting the 10 individual circuits back to the SE panel? Itemized list of conductors for this option? Can I use one large neutral conductor? (7 fifteen amp circuits, 1 twenty amp, 1 two-pole thirty amp circuit (4-wire for dryer)).
JJ

| The existing (old) sub-panel is fed by 2 phases and an uninsulated neutral | conductor. There is no ground bus, return conductor to the service entrance, | nor connection to a ground rod. The conductors are in conduit when leaving | the main panel and arriving at the sub-panel, but not in between (they're | stapled to the flat roof under the roof's insulation & paper). | | I want to replace the sub-panel with a modern one with safe breakers and add | a ground conductor, which will be run via another route (I want to keep the | project manageable, so don't want to run new conductors). | | Since the neutral conductor is uninsulated, it is in contact with the | sub-panel (it is in the conduit bringing the feed conductors into the | sub-panel). In a sub-panel the neutral and ground are not supposed to be | connected.
That neutral conductor is, in today's terms, a grounding conductor (so it seems from your description).
| My question is this: | How do I install this new sub-panel and new ground conductor such that the | neutral and ground are separate?
Add a new insulated neutral conductor that runs with the 2 working phases all the way. If the existing phase wires are separate wires instead of pre-assembled cable, then why can't you run a 3rd wire along with them?
Another alternative: install a 240 to 120/240 volt transformer and derive a new neutral at the subpanel. That new neutral would be bonded to the ground at the transformer itself. Transformers are expensive and waste a bit of power, so this is only an option if the wiring cost would be very high.
Otherwise, you need a new 3+1 wire feeder.

Look, it is clear you are in over you head on this project. Get a qualified electrican to do the job, and they will both do the job right, and you'll be happier.
Asking 'what size conduit' is a very basic question, and if you can't figure that out then the rest of the job is probalby going ot be just as difficult, and you are likely to make mistakes that will later come and haunt (or kill) you.
A sub-panel works. Size of conduit is based on size of wire. Size of wire is based on current loads and breakers.
You must have a hot, neutral and ground for *each* circuit when you have multiple circuits. You cannot combind these as you are suggesting.
Your box is going to be holding about 150 amps of breakers. How far between it and the main box? You are talking 0 AWG wire probably, maybe something larger if the distance is great.
This is not a trivial problem or project. Please get professional help.

Yep. There is an exception for retrofitting a ground wire on a branch circuit to provide a ground for 3-prong receptacles, but it doesn't apply to your situation.
Abandon the existing feeder in place, and run a new one on the inside of the roof.

Actually, it *is* an uninsulated neutral (because it's carrying return current). He has no ground.

Where are you? Which code applies? Lets assume this is in the USA, or at least covered by the NEC.
What sort of loads does this subpanel feed? 120 and 240 or 240V only?
What is the (estimated) age of the original subpanel installation? Look for a permit stapled up somewhere or even a date printed on a panel nameplate or something.
It sounds like you have a subpanel feeding only 240V loads, with no neutrals and what you have is no _neutral_ bus. The bare conductor being a ground. Either that, or this is not in compliance with current code.
But I'm just guessing here. If you can run a ground conductor via 'another route', the you can certainly run the proper feeder to this new panel.
Correct. But being bare, it sounds more like a ground than a neutral. You:
1) Run the correct feeder, with ground and neutral.
2) Serve only 2-wire 240V loads with no neutral from the subpanel.

As noted in my first post, metal conduit is an acceptable equipment grounding conductor if properly installed.
Why?
You'll be running smaller conductors, but a whole lot more of them. What's the point? The first one, hands down. Why not under the roof? Why is it necessary to pierce the roof? AWG3 copper or larger, with insulation rated at least 75 deg C, in 1-1/4" or larger conduit. Individual conductors will be easier to pull than an SE cable. Use wire-pulling lube, too -- it makes the job *much* easier. So does larger conduit. I'd use 1-1/2" or maybe even 2". Why would you even consider doing that?

Incorrect. Shared neutral circuits are permitted by Code (although not seven circuits sharing one neutral). There is also no Code requirement for separate grounding conductors for multiple circuits in the same conduit. Unless all of those circuits will be in use at one time, there's no need for either the box or the feeder to be rated at the sum of the individual branch circuits. 100A over AWG3 wire should be just fine.

Incorrect. It is carrying return current and is therefore an uninsulated neutral. He does not have a separate equipment grounding conductor. Because the National Electrical Code explicitly prohibits that. That's also a Code violation. Neutral is required to be bonded to ground at the service entrance, and is prohibited from being connected to ground at any other point.
He needs a new feeder regardless. But it doesn't necessarily have to be 3+1. Three insulated circuit conductors inside a properly bonded metal conduit will suffice.

JoJo, please listen to the advice above. I like hacking at things that I have never done before too, but this isn't the place for it.

If you read his later reply you will see that the roof top conduit will be exposed to 140 degree heat. That conduit will need one or more expansion couplings in it so a separate equipment grounding conductor is his easiest way out. -- Tom Horne

Yer correct, I stand (OK sit...) corrected.
Regardless his posts trouble me...

Thanks, Doug, for your comments.
Flat roof. This is an "exposed beam" ceiling with tongue-and-groove slats lying on those; there is no attic. Directly on top of the ceiling are 2x6 lumber. On top of that is (probably) plywood, with paper roofing over that. Then sky. This raised portion of the roof provides a sort-of utility space. To run conduit via the roof (either over or under the outermost surface) means puncturing it somewhere.
I've abandoned the idea of running over or through the roof space. The sub-panel is in the laundry area near an outside wall, so I plan to run the conduit through that wall and around the house under the eaves. Being a flat roof, the eaves are equal on all sides which will provide weather protection all the way to the SE panel.
So, minimum 1-1/4 inch conduit, AWG#3 conductors (2 phases, neutral) = 100 amps (the rating of the new sub-panel). The ground conductor can be #8, right?
Since there are going to be 4 wires in the conduit, do I need to derate them? Derating only applies with more than 3 conductors? Does the ground conductor count?
Total cable run (horiz & vert) ~50 ft.
Can EMT be used in this application (with weathertight fittings)? Under the eaves it will be out of the sun and direct rainfall.
It was suggested by a retired electrician. Puts all breakers in one place, albeit outside... Consider the idea abandoned.
JJ

Or bonding jumpers around the expansion couplings. Better still, he should install it on the interior side of the roof, especially as he has already said he doesn't want to pierce the roof.