Residental mains wiring questions (USA)

[Bob E.]

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.

 

[Bob E.]

Thanks, Tom.

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.

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.

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.

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)?

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.

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?

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.

Right! Thanks!

 

[Glen Walpert]

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?
No.

How do I size it?

In accordance with your local codes. Not all jurisdictions use the same
version of the NEC, and some have additional requirements.

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.

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.

 

[Rich.]

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?

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.

 

[Bob E.]

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.

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?

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.

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?

IF the feed to this panel is underground, the following will not apply.

Service feed is arial, from the pole to a mast on the roof.

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 are no existing grounds, but nonetheless, I will separate the box
ground and neutral in the old panel (now sub panel).

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.

Thanks. I just wanted to know if an unthreaded could be used at the top.
Looks straightforward.

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.

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.

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.

It has to go through the roof, so 2" threaded according to the utility
(PG&E).

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.

I'm a big boy. Just ask my GF. ;-) No worries, mate. Thanks.

 

[PeterD]

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

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...

(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?

Yes, they do.

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?

You can get and use conduit as large as you want.

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).

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.

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.

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.

 

[JosephKK]

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 bea
non-threaded type?

This is in N. California.

Thanks.

No. The NEC rule is one neutral per breaker.

Where to you find non-threaded weatherheads?

 

[PeterD]

Bob E. wrote:



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.

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!

Then divide
by the temperature derating percentage, if applicable. Your 14 AWG has
become at least a 10 AWG.

Again, totally flawed math. The 14 AWG would be (roughly) the equal to
18 AWG. Not 10 AWG!

 

[JosephKK]

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.

That is just asking for multiple code violations.

 

[Rich.]

That is just asking for multiple code violations.

Um, it's completely legal and approved. Why are you suggesting it's not?

 

[JosephKK]

Um, it's completely legal and approved. Why are you suggesting it's not?

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.

 

[JosephKK]

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?

Yes it does. 2005 NEC 250.24(C)(1) This [grounding] conductor shall be
routed with the phase conductors...

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 heatingin
the conduit and that if you oversize the conduit (EMT) that you will avoid
approaching the heating limit. No?

Give me each of the run lengths and i will calculate it up for you. Also,
i will need to know how the conduit is mounted and against what material.
I also will want to know what conductor insulation you are considering, i
may ask you to change it.

Service feed is arial, from the pole to a mast on the roof.

Bonding the neutral at the service entrance is an NEC requirement.
Bonding the rest of the neutrals is a separate and somewhat twitchy
matter. I was party to a 3 hour meeting in the workplace trying to
determine correct policy on this a month or two back.

There are no existing grounds, but nonetheless, I will separate the box
ground and neutral in the old panel (now sub panel).

You likely will have to install a grounding electrode. If you just
follow NEC you certainly will.

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.

Don't be afraid to use that phone number. They would much rather that
you get it right. It is definitely in their interest that you do.

 

[Rich.]

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.

What the heck are you talking about. All he's doing is relocating the panel
and extending the homeruns over to the new location. There's no back-up or
alternate power sources being used here.

 

[JosephKK]

I guess I should have been exact to the minutest extreme, but I don't
feel the original poster was mislead.

If you have a wire that is allowed to carry a specific current and you
have to keep feeding the same load, you don't derate the existing
wire. You find the next size up, that, when derated, allows you to
maintain the present load.

If your new wire is capable of 30 amperes, when derated because of
pipe fill, will become a 15 amp capacity wire, hence my phrase "Your
14 AWG has become at least a 10 AWG." That means your new conductor in
the pipe has to be a 10 guage. Then it connects to the existing 14
guage in the junction box.

Wire ampacity is not derated due to conduit fill. The derating for
operating temperature is usually minor. (pretty much for small
conductors #6 AWG and smaller)

 

[JosephKK]

Zinsco is one of the wonders of the electrical industry.

You may be able to make a cover made for the old box if that would
make the installation easier - a question for the inspector. That is
what I would try to do if the enclosure is in good condition.


Existing ungrounded wiring can be grounded by adding a ground wire
which does not have to be run with the power wires. It is in 250.130,
which also details where the added ground wire is to be connected at a
'source'. I believe the ground wires does not necessarily have to
connect through boxes on the way back to the grounding 'source' , but
the connections probably have to remain accessible.

Over the roof? Doesn't sound like a good idea, but minimal
information has been given. I might try running a #4 bare copper
ground wire, which is relatively immune from abuse, or a ground wire
in PVC. Could use one ground wire for all the receptacle boxes you are
adding grounds to. Not obvious now the wires over the roof connect to
the receptacle boxes.

Another thing to clear with the inspector.


For existing ungrounded circuits see 250.130.

For the level and kind of change contemplated it is near certainty that
upgrading to current code can be required. It is an Authority Having
Jurisdiction thing and if they say upgrade you upgrade.

I would split into multiple conduits so the derating is reasonable.

Note that if you are using #12 THHN wire, the table ampacity (310.16)
is 30A. If derated to 70% the allowable ampacity is 21A. (The wire can
not be used at over 20A.)

For #14 THHN the table ampacity is 25A. If derated to 70% the
allowable ampacity is 17.5A (and can only be used at 15A).

If some of this is over the roof the wire would, I believe, be a wet
rating and THHN would use the THWN rating that all that wire (that I
have seen) also has. THWN wire has different table ampacities.

Derating is in 310.15.

If wiring is going above the roof you will likely have to derate it
for the higher temperature it will be at.


I certainly agree. I see lots of questions.

I third that.

 

[JosephKK]

If i am fully informed about what is existing and what is contemplated i
can give "official" advice. Use the local electric company and
inspectors as much as you can first, though.

Nope, but the NEC may not allow a single neutral as proposed.
Multiwire branch circuits (1 neutral for 2 or 3 hots) are allowed and
have been widely used in the past.

The NEC has cut that back pretty sharply recently. A single neutral for
a single multipole breaker is allowed and very common. It may be allowed
in some other cases, provided none of the load current is presented to
the neutral conductor.

 

[Rich.]

Last I heard, a multipole breaker was only required if the hots went
to the same device. Where two or three hots, on alternate phases, feed
their own individual loads, a single neutral wire was allowed for that
group of breakers.

If this has been changed, when did it happen? I haven't opened a Code
book in a couple of years.

Look at office receptacle circuits. You can have three receptacles in
a row, on phases a, b and c. A single white goes back to the panel. If
only one receptacle is being used, the white is certainly 'presented'
with the load current.

If TWO of the receptacles are being used, the neutral is STILL
carrying load current. Only when all three hots are carrying an equal
current is the neutral current free.

Multiwire circuits are now required to have a common trip (multi-pole)
breaker feeding all circuits that share a neutral. In your example of 3 (20
amp) outlet circuits in an office, yes you can run 3 hots and one neutral
back. But now, instead of 3 1-pole 20 amp breakers you are required to
install one 3-pole 20 amp breaker. The reasoning behind this is if you turn
off one of the 1-pole 20 amp breakers to work on the circuit. Yes that one
hot wire is dead, but if you were to break the neutral splice, the other two
circuits sharing the neutral can backfeed the white wire and kill you.

BTW, your load carrying neutral example is incorrect. The neutral carries
current anytime there exists an imbalanced load between any two hot wires
sharing a neutral. If you have two hots and one neutral with one hot
carrying 10 amp and the other carrying 13 amp, then the neutral carries the
difference of 3 amps. Assuming the 2 hots are correctly install to not be on
the same phases. If they were on the same phases, then the neutral carries
the combined load of 10 and 13 amps, or 23 amps. It's basically the same
with a 3-phase setup except you throw the third hot into the mix and the
unbalance load calculations are a bit more complex. If all hot carried the
same current load, then the neutral is carrying nothing.

 

[Fester Bestertester]

All neutrals are white, but not all whites are neutrals. You have to

have 2 (or 3) hots, from different phases, sharing the white in order
for it to be a neutral.

Ah, it's beginning to make some sense to me. Thanks!

A real neutral carries only the *difference* in amperage between the
hots. **

** could be called sum. It depends how you mark the vectors in the
diagram. (+120 v at 0 degrees) plus (+120 v at 120 degrees) plus (+120
at 240 degrees) equals zero.

Your example is for 3-phase, right? For residential split-phase, the vectors
would be 180 degrees apart, right?

 

[JosephKK]

What the heck are you talking about. All he's doing is relocating the panel
and extending the homeruns over to the new location. There's no back-up or
alternate power sources being used here.

So i check the thread and i can't find any such either. I must have
wigged over he new panel and j-box.

 

[JosephKK]

Multiwire circuits are now required to have a common trip (multi-pole)
breaker feeding all circuits that share a neutral. In your example of 3 (20
amp) outlet circuits in an office, yes you can run 3 hots and one neutral
back. But now, instead of 3 1-pole 20 amp breakers you are required to
install one 3-pole 20 amp breaker. The reasoning behind this is if you turn
off one of the 1-pole 20 amp breakers to work on the circuit. Yes that one
hot wire is dead, but if you were to break the neutral splice, the othertwo
circuits sharing the neutral can backfeed the white wire and kill you.

BTW, your load carrying neutral example is incorrect. The neutral carries
current anytime there exists an imbalanced load between any two hot wires
sharing a neutral. If you have two hots and one neutral with one hot
carrying 10 amp and the other carrying 13 amp, then the neutral carries the
difference of 3 amps. Assuming the 2 hots are correctly install to not be on
the same phases. If they were on the same phases, then the neutral carries
the combined load of 10 and 13 amps, or 23 amps. It's basically the same
with a 3-phase setup except you throw the third hot into the mix and the
unbalance load calculations are a bit more complex. If all hot carried the
same current load, then the neutral is carrying nothing.

Thanks, i was about to try expressing that clearly. You already did it
well.