Residental mains wiring questions (USA)

 

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
non-threaded type?

This is in N. California.

Thanks.

 


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!

 




---
http://www.powerstream.com/Wire_Size.htm

JF

 

On Wed, 30 Jun 2010 00:49:16 -0700, Bob E. wrote:


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.