How to choose electrical cable type for subfeeder?

[Wayne Whitney]

Hello,

A little background: my exterior combo meter base/panel with an underground feed is right by my front door, which I think is an ugly place for it. Since it would be trouble to move the service entrance, I would like to replace it with a meter base/disconnect, which is smaller. I plan to do this in three steps: first add a subpanel in my basement, then move all the circuits over to it (replacing all the knob-and-tube wiring), then swap out the main panel to a disconnect.

As to the first step, it's 35-40 ft from the exterior 100A main panel to the basement: down and into the crawl space, 30 ft through the crawl space, then into the adjoining basement and down to the subpanel. My understanding is that I will be using # 4 copper for the conductors to get to 100A ampacity, and of course I'll need 3 conductors plus EGC.

What's the appropriate cable type for this application? One electrician I spoke with suggested using NMB, as the run is basically all inside. Presumably there would be some pipe or something for the short exterior part. Another electrician suggested running conduit and individual THHN conductors. And a book I'm reading suggests using SEU cable in this case, since the basement subpanel will be the primary panel. Also, most (all?) of the run will be above grade, but it is a crawl space/basement, so should I be using cable rated for wet areas?

Thanks, Wayne

[SQLit]

NMB is not service rated, branch circuit only. SEU does not have a ground conductor, SER does. Right track wrong train. As long as the cable is not subject to physical damage then the SER is fine. Otherwise you will need to run conduit and conductors. This is overhead right? The disconnect will need to be service rated and that may cost you more than an all in one meter socket and panel. Better check with the AHJ, authority having jurisdiction, about what they require. When you move the conductors why not move the meter location to the edge of the home and re-use the panel as is. Your issue is looks, I am trying to save ya a buck. As long as you get a permit and the overhead conductors will reach it is a simple matter to cut them off us a rope to prevent them from reaching the ground. Connect the neutral first, and then the hots, use a strain relief on the neutral, Be cautious about the neutral sometimes the utility uses ASCR, aluminum with a steel core. Reconnect on the roof with split bolts and your done.

[RB]

If you will have a main breaker in the meter base/disconnect I'd be comfortable with NMB for the "outside" run. If not I'd run in conduit (not EMT). Many inspectors won't permit a 30 foot interior run before the main breaker unless it is in conduit.

RB

Wayne Whitney wrote:

[Wayne Whitney]

No, underground feed. The feed comes up through a pipe, goes across through a box, and down to the meterbase, then down to the main panel. I can't open that box or the meterbase, they have PGE seals on them.

Looking at SquareD's online catalog and the list prices there, that doesn't seem to be the case. Also, if I have a disconnect, doesn't that count as the "main panel", so that the line to the (sub)panel is a "branch circuit", which would allow NMB? On the other hand, is there any downside to using SER over NMB?

Well, the layout is this:

-----------------Property Line-------------------------- ! S F HouseHouseHouseHouseHouseH T e o o R n u u E c s s E e e # e T !--! H*useHouseHouseHouseHouseH Sidewalk | Porch | !--Fence------------

-----------------Property Line--------------------------

And the * is the location of the underground feed and current meterbase/panel. I don't really see any way to move the service entrance. # (in the basement) is where I plan my (sub)panel.

Well, it is not only looks, I am redoing the bath and kitchen and the new subpanel location will be more convenient. Plus the current panel does not seem like the best.

BTW, the current panel has a 100A main breaker, but what I can read on the wires coming from the meter base into the panel is: ". . . E-13056 W Type THW 1 AW . . ." and ". . . 600 Volt Oil-Resist . . .". I assume that the "1 AW . . ." is "1 AWG" and these wires are #1 (hopefully copper). Should I run #1 to the (sub)panel for maximum flexibility?

Thanks, Wayne

[Tom Horne]

Read the OP again and you will find he is planning on using a meter base service disconnect combination so the cable used to supply the feeder to the new lighting and appliance panelboard need not be "service rated". Type SE cable in the R configuration having three insulated and one bare condutor will be the easiest to install and the most suitable cable for the damp location.

-- Tom H

[Mark or Sue]

You can use NM-B or UF-B cable because what you are running is a feeder, but these cable types are limited to 60C ampacity even though they have 90C conductors. They are also not allowed to use the ampacity values in table 310.15(B)(6), so you'd have to use #2 copper NM or UF cable. SER is 75C rated, and permitted to use the 310.15(B)(6) ampacity (which says #4 copper for

100 amps).

Are most appliances on your house gas? If not, you may want to consider a 200A service upgrade if the conduits can allow it.

-- Mark Kent, WA

[Wayne Whitney]

Yes: water heater, stove, dryer, furnace.

OK, SER sounds like the way to go. I assume in my crawl space I can just attach the cable to the underside of the floor joists, with either non-conducting staples or perhaps plastic plumber's tape.

Does the cable need extra protection for the brief run in the basement (over a foot, down 4 feet through the cripple wall, out slightly to the panel mounted on the concrete) and for the exterior run (down a couple feet before going into the crawl space)? If I want to leave a couple feet of slack near the main panel in the crawl space, to aid the future replacement of the combo main panel with a combo meter base/disconnect, what's the best way to do that?

As I mentioned, what I can read on the wires from the current meter base to the main panel is "E-13056 W Type THW 1 AW". I'm thinking that's 1 AWG. If so, is it reasonable to assume that my service conductors are #1? I gather I'm not supposed to open the meter base, as there is a PG&E seal on it. Should I consider running something bigger than #4 Cu cable? I'd like to have enough capacity to add a subpanel for a garage woodworking shop.

Thanks, Wayne

P.S. Is there any reason to use NMB 14-2 for 15A branch circuits, rather than just use NMB 12-2 and make all branch circuits 20A? The cost difference in the cable seems small.

[HA HA Budys Here]

You can use steel staples. Tape is no good.

The cable needs to be placed so that it is not likely to be subject to physical harm.

Wayne, the cable size now entering your main panel should match the size of your service equipment and the service entrance cable from the utility - and all this determines the size of your service. If you have a 100a service now, you'll have a 100a service after you relocate your main panel unless you upgrade everything from the utility to the panel and the panel itself.

There are issues concerning workability, box fill, standard practice, competitive bidding, and economics that cannot be learned through a USENET post.

If you're referring to your existing 15a branch circuits, you cannot make them

20a branch circuits unless you rewire the entire circuit with #12 awg. NM.

If you're referring to completely new additional circuits you're free to use #12 NM and make them 20a circuits.

[Takoma Park Volunteer Fire Dep]

comfortable with NMB for the "outside" run. If not I'd run in conduit (not EMT). Many inspectors won't permit a 30 foot interior run before the main breaker unless it is in conduit.

The main breaker in this case is at the meter. So there is no issue with the length of conductor prior to the main breaker.

-- Tom H

[Mark or Sue]

You can get plastic staples in rectangle or round depending on the type of cable you choose. Don't use plumbers tape!

Cable needs protection where it is exposed to physical damage. The inspector is the judge on what is exposed to physical damage. Generally, outside runs delow about 6' above the ground need to be run through a conduit stub to protect it. PVC conduit is usually accepted for this. If the basement cable is up against the wall and above the panel, you may not need to protect it. You can leave more slack by running along the ceiling joists and loop it back to the panel.

#1 AWG aluminum is good for a 125A service. If your meter base is limited to

125A, then I'd run #2 copper because I don't like pressing the amp limits allowed by 310.15(B)(6). If the power company says its good for 125A, then I'd look for a panel with a 125A breaker if it doesn't cost much more (but most around here are either 100A or 200A). Typically, buying just the 125A breaker to put in the panel costs as much as the panel and breaker combo kits at the home centers. Since most your stuff is gas, a 100A service is probably fine unless you have a huge air conditioner and an all electric hot tub.

Usually, you can open the meterbase if you coordinate with the power company. Typically, the just want to be notified that you messed with stuff and not find out on their own. They may allow you to pull the meter so you can add a replacement main panel. However, they may not reseal it without looking things over and seeing an inspection stamp on your new main panel.

Yes - box fill. If you have old metal boxes, they usually aren't too big. If you're replacing them all, then #12 is the way to go for receptacle circuits. I prefer #14 for light circuits that have

3-way and 4-way switches, because the boxes may get too crowded and too difficult to push all those wires into the back if they are #12. Lighting only load are also well known, so you can load them to about 10A before starting a new circuit.

-- Mark Kent, WA

[Wayne Whitney]

So do you suggest that for new circuits I segregate receptacles from lighting? Presumably lighting here means "hardwired devices of known maximum amp draw", e.g. it includes hard-wired smoke detectors. [My jurisdiction indicated that they do not interpret the codes as requiring a separate circuit for hard-wired smoke detectors.]

I'll also be replacing the old knob-and-tube wiring, so for those circuits, I will check the size of the old boxes, and use #14 if necessary. For new boxes, I'm inclined to use metal boxes, as I have an environmental bias against PVC. I assume new metal boxes and new plastic boxes are the same size? So in terms of box fill, the only downside of metal boxes would be the need to pigtail a ground to the box?

Cheers, Wayne

[Wayne Whitney]

I opened up the box that is between the underground feed and the combo meter box/100A main panel; this box provides an overhead connection to the combo meter box. It turned out to contain splices from the Al service conductors to Cu conductors going to the combo meter box. The Cu conductors are marked 2 AWG. The Al conductors were unmarked on the sections I could see, but they are bigger than #2. There was a sticker saying the lugs are approved for #1/0 through #4, so I assume the Al conductors are #1/0 or #1. I believe that NEC 310.15(B)(6) specifies that #2 Cu is good for 125A, that #1 Al is good for 110A, and that #1/0 Al is good for 125A.

The upshot is that I will size my new work for possible future 125A service, so I will use Cu #2 SER cable for my feeder cable. If the Al service conductors are #1/0, then do I understand correctly that to upgrade my service to 125A, all I need to do is replace the combo meter box with a 125A rated meter and disconnect?

Thanks, Wayne

[zxcvbob]

It's been my experience that the plastic boxes have about 10% more cubic inches than an equivalent metal box. I am biased toward metal boxes, but I used one plastic box recently for an outlet box that has three #12-3 cables going to it; I couldn't find a metal box that would fit with enough cubic inches. Not having to ground the box was nice when the box was that crowded.

-Bob

[Mark or Sue]

You can argue either way for separate lighting. It is nice to be able to see when working on receptacle circuits that have been de-energized. Conversely, if you trip a receptacle breaker, you may not know because the lights still work. Since I rarely trip breakers, especially 20A ones, I prefer separated lighting and receptacle circuits. Some circuits must be done this way (e.g. kitchen). Smokes on light circuits is a good idea, because you'll know what those circuits are dead.

Typically, the only way to get metal boxes with enough capacity is to use the 4" square by 2" deep ones and put a single gang mud ring on the front. Then, you have gobs of room in the box. You can also get nonmetallic boxes made from a bakelite material if you like that better than PVC.

-- Mark Kent, WA