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Posted by on November 7, 2009, 4:45 am
wrote:
> > Existential Angst wrote:
> >> Awl --
> >> A two-part Q:
> >> Due to some remodeling, I'm forced to splice/lengthen some old bx cabl=
e
> >> that has #14 wire in it, and re-connect to the fuse box.
> >> Should I splice the #14 with #14, or splice with #12?
> >> The reason I would continue with #14 wire is just to remind me and fut=
ure
> >> people that Hey, this old wire is a little lighter than modern #12, an=
d
> >> to not take liberties with 20 or 30 amp fuses or breakers. =A0Other th=
an
> >> that, #12 would be fine with me.
> > There's nothing "unmodern" about 14 for 15A circuits.
> Really? =A0Does the NEC state that?
> I thought #12 was the de-facto "standard" for wiring, but mebbe that was
> just NYC in its oppressive heyday, before it relented and went with the N=
EC.
> In that heyday in NYC, =A0 20 A breakers were illegal on #12 wire! =A0Alt=
ho
> after inspection, few people adhered to that limit.
> What is the max breaker allowable on #14 and #12 wire with modern
> insulation, nowadays?
> Ditto, with cloth insulation?
> > I'd recommend sticking w/ same gauge as the original circuit for the
> > reason mentioned.
> > Remember to not bury connections in inaccessible places.
> Indeed!
> >> Next, =A0is there a rule of thumb for how many hot wires can share one
> >> neutral of the same gauge? =A0If using different gauges, is there a "g=
auge
> >> ratio", ie, some formula for cross sectional area between total hots a=
nd
> >> total neutrals?
> > Don't follow this question. =A0Each hot needs its own return neutral.
> > If you're talking about 3-wire ("Edison") circuits, if properly wired t=
he
> > return is in effect a neutral w/ a balancing currents from the two side=
s
> > if both are loaded equally; hence only the same size conductor is requi=
red
> > for a single-sided load.
> > If that isn't it, I have no clue what you're thinking.
> I think you answered the Q: =A0If each hot =A0in a 3-wire bx =A0is connec=
ted to a
> different leg in the panel, then they can share a common equal-sized
> neutral. =A0So the "ratio" would be 2:1.
> Which is why the neutral in main service can be "de-rated" rel. to the ho=
ts,
> because the assumption is that there will in fact be some balancing of th=
e
> load between the two legs in "Net usage", reducing the demand on the
> neutral.
The neutral is not derated in the service. Let's take a 200 amps
service. ALL the conductors in the service cable are rated for 200
amp capacity because that is the max current you can have flowing in
them. If it's a pure 240volt load of 48KVA or a balanced 120volt
load of 48KVA, then 200 amps is flowing in the two hots, zero in the
neutral. If it's a 120volt unbalanced load, then 200 amps is flowing
between one hot and the neutral.
Gee, seems I recall having this discussion here before.....
> In a worst case scenario -- full load occurring on only one leg -- then t=
he
> neutral proly should be equal in gauge to the hot gauge. But statisticall=
y,
> esp. in large buildings, this is unlikely.
> =A0iirc, the svc neutral to large buildings can be smaller than the gauge=
of
> either hot leg, but I wouldn't bet the farm on my memory.
> --
> EA
> > --- Hide quoted text -
> - Show quoted text -- Hide quoted text -
> - Show quoted text -
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>
> A two-part Q:
>
> Due to some remodeling, I'm forced to splice/lengthen some old bx cable that
> has #14 wire in it, and re-connect to the fuse box.
> Should I splice the #14 with #14, or splice with #12?
>
> The reason I would continue with #14 wire is just to remind me and future
> people that Hey, this old wire is a little lighter than modern #12, and to
> not take liberties with 20 or 30 amp fuses or breakers. Other than that,
> #12 would be fine with me.