Residental mains wiring questions (USA)

Cite that

Nope. The "ampacity" is derated 50%

14ga wire is rated at 25a (in the 90c column) half of that is 12.5 so it can't be used at 50% derating. That bumps you up to 12ga, 30a in the 90c, half if that is 15 and off you go. Same is true in your 20a circuit. You bump up to 10ga, 40a in the 90c, half is 20 and off you go.

Cite them

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

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.

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.

-- bud--

Sure. I would about always use separate neutrals or multiwire (Edison) branch circuits. With a multiwire you need a common disconnect, which could be a handle tie or 2-pole breaker.

Group (tape, tie wrap, ...) the wires for each circuit at each end of the conduit.

If you use either GFCI or AFCI breakers you have to have separate neutrals on that circuit connecting to the breaker.

Not obvious whether AFCIs will be required (ask the inspector), but with wiring as old as ungrounded you might want them anyway. For new wiring they are required for 15/20A 120V circuits that, in general, are not required to be GFCI protected (210.12).

-- bud--

I was surprised there seems to be no definition of a multiwire branch circuit - not in 100 or 210.

Suppose you run a common neutral as the OP originally proposed - say A- B-A-B-N.

Is that a multiwire branch circuit? Seems to be. Does it have to have a simultaneous disconnect of all hot wires (210.4- B)? Would it then have to use a 4-pole breaker or a listed 4-breaker handle tie? Does that then mean you can't do a common neutral as proposed?

I never thought about it until this thread.

-- bud--

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.

Easy to find.

-- bud--

Bud Does the current version of 210.4B still read "(B) Dwelling Units. In dwelling units, a multiwire branch circuit supplying more than one device or equipment on the same yoke shall be provided with a means to disconnect simultaneously all ungrounded conductors at the panelboard where the branch circuit originated."? I only have the 2002 handbook here at home.

-- Tom Horne

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.

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

That was moved to 210.7-B - didn't look up when - and now applies everywhere, not just dwelling units. (Looks like you can still do multiple circuits to a common yoke if they are separate branch circuits with separate neutrals.)

In the 2008 NEC the requirement was extended to *all* multiwire branch circuits. "210.4 Multiwire branch circuits. (B) Disconnecting means. Each multiwire branch circuit shall be provided with a means that will simultaneously disconnect all ungrounded conductors at a point where the branch circuit originates."

The rationale was the hazard of 'hot neutrals' if you just disconnect the branch circuit you are working on.

(IMHO this essentially kills the use of multiwire branch circuits, which as you have likely seen, were extremely common - like a 3-phase 4-wire set.)

Article 100 Branch Circuit, Multiwire. A branch circuit that consists of two or more ungrounded conductors that have a voltage between them, and a grounded conductor that has equal voltage between it and each ungrounded conductor of the circuit and that is connected to the neutral or grounded conductor of the system.

Interesting catch ... Kudos. A half dozen inspectors missed that in a conversation we had about the same thing. I agree.

Just remember we are using the 90c column of 310.15 not 240.4(D) so a

12 guage wire is OK for 30a when you are derating.

Why would it stop people from using multiwire circuits? You just need handle ties or common trip breakers. Note the 2008 also requires these conductors to be grouped where they enter the panel enclosure

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.

overhauled) to=20

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

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.

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.

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

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.

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.

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)

=46or 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 third that.