Residental mains wiring questions (USA)
in [Design]
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From: bud-- on 1 Jul 2010 12:43 On Jul 1, 6:08 am, PeterD <pet...(a)hipson.net> wrote: > On Wed, 30 Jun 2010 22:04:59 -0700, Bob E. <besp...(a)invalid.tv> wrote: > >> 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. 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. > > 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... 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. > > >(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. For existing ungrounded circuits see 250.130. > > >> 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? > > You can get and use conduit as large as you want. 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'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. I certainly agree. I see lots of questions. -- bud--
From: bud-- on 1 Jul 2010 13:40 On Jul 1, 6:10 am, "JosephKK"<quiettechb...(a)yahoo.com> wrote: > On Wed, 30 Jun 2010 00:49:16 -0700, Bob E. <besp...(a)invalid.tv> wrote: > >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. > > No. The NEC rule is one neutral per breaker. 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. > > Where to you find non-threaded weatherheads? Easy to find. -- bud--
From: m II on 1 Jul 2010 14:07 PeterD wrote: > 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! 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. A capacity of 15 ampere flow, when derated by 50 percent means that 15/.5 = a 30 amp needed capacity in the NEW wire before it is derated. >> 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! Again, we're NOT derating the existing wire. We are derating a HIGHER guage wire, in this case a 10 AWG, to give us the same ampacity that we had before stuffing these wires in a conduit. You have to go with the existing current requirements divided by the derating factor, then use THAT current capacity to select your new wire size. I'm sorry if I have said this in a confusing manner. mike
From: JosephKK on 1 Jul 2010 23:12 On Thu, 1 Jul 2010 12:07:59 -0400, "Rich." <rcres(a)XXcomcast.net> wrote: > >>"JosephKK" <quiettechblue(a)yahoo.com> wrote in message >>news:m2uo26965hjc6sdg6lv3ns21ci81runl4g(a)4ax.com... >>On Wed, 30 Jun 2010 13:01:20 -0400, "Rich." <rcres(a)XXcomcast.net> wrote: >>> >>>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. > >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.
From: JosephKK on 1 Jul 2010 23:49
On Wed, 30 Jun 2010 22:04:59 -0700, Bob E. <bespoke(a)invalid.tv> wrote: >> 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? Yes it does. 2005 NEC 250.24(C)(1) This [grounding] conductor shall be routed with the phase conductors... > >> 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? 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. > >> 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. 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. > >> 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. 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. > >> 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. > >> mike > > |