I am installing a new meter & load panel. The old panel is 50 feet from the
new location. I'm going to install a junction box at the old location and
splice new wires to run in conduit to the new panel. The existing circuits
are a mix of 120v 15A & 20A circuits, plus a 220v 20A (weird, I know), a
total of 10 circuits.
Can I use a single neutral wire between these 2 boxes? How do I size it?
The mast for the service conductors must be threaded at the bottom end (where
it mates with the meter box). How about the top end (where the weatherhead
mounts to the mast)? Must it also be threaded, or can the weatherhead be a
This is in N. California.
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.
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
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?
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.
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
That will have you running #10 for your 20a circuits and #12 for your
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.
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.
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.
How about that there is not appropriate bus transfer? Nor is the
occupancy and some other special applications properly addressed. There
were big changes between the 2005 and the 2008 NEC for all classes of
backup and alternative power systems.
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
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
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
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.
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
It has to go through the roof, so 2" threaded according to the utility
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
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
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?
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