Residental mains wiring questions (USA)
in [Design]
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From: Bob E. on 30 Jun 2010 03:49 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.
From: Tom Horne on 30 Jun 2010 10:59 On Jun 30, 3:49 am, 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. 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
From: Bob E. on 30 Jun 2010 11:43 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! > Tom Horne
From: Glen Walpert on 30 Jun 2010 12:01 On Wed, 30 Jun 2010 00:49:16 -0700, Bob E. 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? 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.
From: John Fields on 30 Jun 2010 12:02
On Wed, 30 Jun 2010 08:43:54 -0700, Bob E. <bespoke(a)invalid.tv> wrote: >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? --- http://www.powerstream.com/Wire_Size.htm JF |