The general question: In residential electric wiring, is it okay (code
and safety-wise) to use a double run of one wire gauge to get the
equivalent benefit of the next higher gauge?

More specifically, my situation is thus: I have a 240 volt clothes
dryer that is fed from an old fusebox subpanel (with buss fuses). When
we moved into the house, the two circuits feeding the dryer had 20 amp
fuses installed in the box. This worked fine for a year, but the dryer
has started blowing fuses now as it gets older. The dryer's rated for
30 amp, so that's not too surprising.

I'd like to replace the fuses in the box with 30 amp fuses to keep
them from blowing, but I don't want to do anything unsafe. I've
examined the wiring in the outlet box at the dryer, and it's 10-3.
I've traced this wiring down the hall to a junction box in the
ceiling. There, each hot wire connects to _two_ 12 gauge wires that
run down to the fuse box. So, each circuit in the fuse box has two 12
gauge conductors screwed down next to it.

The dryer was obviously added after this fusebox was, and maybe the
doubled 12 gauge runs were repurposed from other circuits, or maybe
the electrician doing the work just didn't have enough 10 gauge wire
to go the whole way. Either way, I think from ohm's law and all that,
it should be okay, and should be the equivalent of at least 10 gauge
cable. So, I should be okay doing 30 amp fuses on these circuits. But,
I'd like to know for sure, and I'd like to especially know if it's
okay code-wise just in case it's spotted during any inspection on
nearby work.

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Clarification of Question by adavidw-ga on 08 Mar 2005 14:13 PST

I don't know if my area has any local codes, but I'm in the US, so I'm
mainly interested in this from an NEC perspective.

This is not a common situation so I would be suprised if it's
specifcly mentioned in the code. But from an electrical point of view
having two 12g wires should more than equal the cross-sectional area
of a single 10g. Cross-sectional area, along with temperature and
material are what determins a wire's current carrying capiblities.

I am woried about the the connectors that join up the wires, as they
could be rated for 12g and thus not have the capacity of 10g
connectors. Also, the two phases comming from the fuse box must
_never_ be crossconnected withought a load (your dryer) between them.
This could happen accidentally when dealing with 4 wires comming into
the box instead of 2.

Because the dryer is rated at 30 amps you do need to upgrade the fuses
and possibly the fuse box to handle that load. I would also suggest a
thurogh cleaning of the dryer (compleatly disassemble it). The exaust
could be pluged, putting extra strain on the motor (and creating a
fire hazzard) or you could have a shorted heating coil. If the unit
consistantly blows the fuse right when the motor turns on you should
get slow-blow fuses. They are desinged to allow the very short over
current surge that happens when a motor starts without blowing.

adavidw-ga:

This is just an opinion, as I do not know where in the world you are
located and therefore cannot reference your local building code to
give a conclusive answer.

Fuses and circuit breakers are intended to be the 'weakest' link in an
electrical circuit, to protect the rest of the circuit from burning up
in the case of a problem. When you look at your layout, what you
should be looking for is the potential of a single failure elsewhere
in the circuit causing a problem that the fuse won't protect you from.
In this particular case, it sounds like the part you should be looking
at is the ability of one of the two 12-gauge wires to carry 30 A
without any problems, should the other 12-gauge wire either break or
comes loose. 12-gauge is nominally rated to a max of 20 A (plus any
derating required by the code due to the ambient conditions), 10-gauge
is rated to 30 A. So, in a worst-case scenario, if one of the 12-gauge
runs is cut, breaks, or comes loose, then you'd be potentially trying
to draw 30 A through the remaining 12-gauge run, leading to the
potential of a melt-down and fire since the fuse won't blow at less
than 30 A.

Best advice is to have a local electrician take a look to see if it's
possible to pull a new cable through to that junction box from the
fusebox.

Anyway, just my opinion, hope it helps.

aht-ga

anotherbrian-ga:

In the junction box that connects the double 12ga to the 10ga, the
wire nuts are plenty big enough to make a secure connection between
the wires. From there, it's 10ga all the way to the dryer, so in the
dryer outlet box, there's only the three wires + ground from the 10ga
bundle.

I did the complete teardown and vacuuming of the dryer when it first
started blowing fuses, hoping that would solve it. I got a ton of crap
out of it, but the fuses still blew. I also made sure I was only using
the slo-blo fuses, just in case that was it. My guess is that the
resistance of the heating element is changing as it ages, and just
pulling more current. But since the dryer's rated for 30A, it's still
within design spec, so I can't complain too much.

aht-ga:

All this time I've been wondering about the load carrying capacity of
the doubled 12ga, but have never even considered the possibility of
one of those conductors breaking or becoming dislodged. Yeah, if it
was 10ga all the way and the cable came loose, the circuit would be
dead. But, with the double 12ga circuit, if one of those comes loose
you still have 30A going across the other one, with my whole house in
flames as a possible result. Thanks for pointing out that possibility.
For that reason alone, I'll look into pulling 10ga from the fuse box
to the junction box.

As of right now, I've got 25A fuses in the two legs of the dryer
circuit, and the dryer's running fine without blowing them. And, like
I suspected, the wiring is more than adequate for that load. However,
because of the possibility of one of the 12ga wires becoming
disconnected, I'm not going to consider that a permanent solution, and
find a way to pull 10ga wire instead.

Thank you both for your help.

This is not code in any state.
The fuse panel will not handle the load

elitehomeinspectors-ga:

What do you mean, the fuse panel won't handle the load? My question is
about the wires. Are you implying that even with 10ga wire to the
panel, the panel itself can't handle a 30amp circuit? Why wouldn't it
be able to?