Combining two 30-amp circuits
 

If I want to run an appliance that needs 50 amps, is there any problem
with combining two 30-amp circuits of approximately the same length, as
long as the part that carries the full current is using properly-sized
cable? It seems like the circuit would have a 60 amp capacity.

Thanks in advance.

In article . com,
wrote:

If I want to run an appliance that needs 50 amps, is there any problem
with combining two 30-amp circuits of approximately the same length, as
long as the part that carries the full current is using properly-sized
cable? It seems like the circuit would have a 60 amp capacity.

Are the 30 amp circuits single ended or double ended? I have
seen them both ways. The 50 amp appliance is likely a double-ended
unit, which means it needs 220 volts rather than 110. If you
know that the two 30s are single ended and on opposite sides of
the AC mains, you might be able to make it work. It would be far
better to put in the right wiring. You might be able to get by
putting in a 50AMP breaker, and using the 30 amp wiring, assuming
it is properly sized. You are most likely going to need an
electrician in to look at it (and do the work), so bite the bullet
and call one rather than risking killing yourself or burning up
the appliance.

In RV's, the old wiring standard was 30AMP, while the new standard
is 50AMP. RV shops sell 30AMP to 50AMP converters. But these
things do not run 50AMP appliances in the RV. Rather, they have
the main AC on one side of the circuit, and everything else on
the other side of the circuit. The 50AMP conversion is really
two parallel 25 AMP circuits.

-john-

--
================================================== ====================
John A. Weeks III 952-432-2708
Newave Communications http://www.johnweeks.com
================================================== ====================

John A. Weeks III wrote:

Are the 30 amp circuits single ended or double ended? I have
seen them both ways. The 50 amp appliance is likely a double-ended
unit, which means it needs 220 volts rather than 110. If you
know that the two 30s are single ended and on opposite sides of
the AC mains, you might be able to make it work. It would be far
better to put in the right wiring. You might be able to get by
putting in a 50AMP breaker, and using the 30 amp wiring, assuming
it is properly sized. You are most likely going to need an
electrician in to look at it (and do the work), so bite the bullet
and call one rather than risking killing yourself or burning up
the appliance.

In RV's, the old wiring standard was 30AMP, while the new standard
is 50AMP. RV shops sell 30AMP to 50AMP converters. But these
things do not run 50AMP appliances in the RV. Rather, they have
the main AC on one side of the circuit, and everything else on
the other side of the circuit. The 50AMP conversion is really
two parallel 25 AMP circuits.

-john-

--

================================================== ====================
John A. Weeks III 952-432-2708

Newave Communications
http://www.johnweeks.com

================================================== ====================

The two 30 amps runs each have 240 V potential (+120/-120). The 50 amp
appliance also requires 240 V (+120/-120). So far, so good. I've been
told that if the two circuits are running close enough, I might have to
derate the amp capacity per code. I'll have to look into that.

I try to save money on certain projects but electrical is not the
trade to do questionable work.There are safety factors which are
important,also if there is a fire which is traced back to your
questionable work your insurance will be voided. I urge you to get the
proper materials and do an installation that meets code.If you unsure
of your electrical abilities, pull a permit & have it inspected.
Todd

If I want to run an appliance that needs 50 amps, is there any problem
with combining two 30-amp circuits of approximately the same length, as
long as the part that carries the full current is using properly-sized
cable? It seems like the circuit would have a 60 amp capacity.

Thanks in advance.

In article . com, wrote:
If I want to run an appliance that needs 50 amps, is there any problem
with combining two 30-amp circuits of approximately the same length, as
long as the part that carries the full current is using properly-sized
cable? It seems like the circuit would have a 60 amp capacity.

To be Code-compliant, at a minimum, the two 30A circuits would have to share a
common disconnecting means (i.e. throwing *one* handle turns off *both*
circuits simultaneously).

I can't imagine that what you propose is permitted by the NEC, but I can't at
the moment find anything in the Code that explicitly prohibits it. The closest
I can come is:

"In no case shall the load exceed the branch circuit ampere rating." [Sec.
210-23, 1993 NEC -- the most recent one I have at hand.]


--
Regards,
Doug Miller (alphageek at milmac dot com)

Nobody ever left footprints in the sands of time by sitting on his butt.
And who wants to leave buttprints in the sands of time?

In article , "John A. Weeks III" wrote:
In article . com,
wrote:

If I want to run an appliance that needs 50 amps, is there any problem
with combining two 30-amp circuits of approximately the same length, as
long as the part that carries the full current is using properly-sized
cable? It seems like the circuit would have a 60 amp capacity.

Are the 30 amp circuits single ended or double ended? I have
seen them both ways. The 50 amp appliance is likely a double-ended
unit, which means it needs 220 volts rather than 110. If you
know that the two 30s are single ended and on opposite sides of
the AC mains, you might be able to make it work.

False and dangerous. What you propose would provide a 30A 220V circuit, *not*
a 60A circuit.

It would be far
better to put in the right wiring. You might be able to get by
putting in a 50AMP breaker, and using the 30 amp wiring, assuming
it is properly sized.

False and dangerous. If the wiring is "properly sized" for a 30A circuit, it's
dangerously undersized for 50A.


--
Regards,
Doug Miller (alphageek at milmac dot com)

Nobody ever left footprints in the sands of time by sitting on his butt.
And who wants to leave buttprints in the sands of time?

Doug Miller wrote:
.... To be Code-compliant, at a minimum, the two 30A circuits would
have to share a
common disconnecting means (i.e. throwing *one* handle turns off
*both*
circuits simultaneously).

I can't imagine that what you propose is permitted by the NEC, but I
can't at
the moment find anything in the Code that explicitly prohibits it.
The closest
I can come is:

"In no case shall the load exceed the branch circuit ampere rating."
[Sec.
210-23, 1993 NEC -- the most recent one I have at hand.]

That's a good point about having one breaker turn off the circuit.
I'll be sure to do that. Perhaps I'll have to run a single cable
rated for 50 amps rather than having the conductor split, so to speak.
The only other alternative would be to have a 50 amp breaker at the
service box that then splits into the two 30 amp runs, and the
recombines at the end to the 50 amp cable, taking into account proper
derating factors.

In article . com, wrote:


That's a good point about having one breaker turn off the circuit.

Not only is it required by Code, it's very dangerous *not* to do that: killing
only one breaker would still leave the unit fully energized. *You* would know
about the situation, but what about the next person who buys your house?

I'll be sure to do that. Perhaps I'll have to run a single cable
rated for 50 amps rather than having the conductor split, so to speak.

Always best to do it the right way.

The only other alternative would be to have a 50 amp breaker at the
service box that then splits into the two 30 amp runs, and the
recombines at the end to the 50 amp cable, taking into account proper
derating factors.

That's prohibited by the Code, though: the conductors are rated for only 30A,
but they're protected by a 50A breaker, and that's not permitted. What if one
conductor on one side comes loose? The stove (or whatever it is that you're
powering) will still pull its full 50A draw through the other side that's
still connected, overloading and overheating the wires, and possibly
[probably?] starting a fire.

Not safe. Don't do it.

--
Regards,
Doug Miller (alphageek at milmac dot com)

Nobody ever left footprints in the sands of time by sitting on his butt.
And who wants to leave buttprints in the sands of time?

wrote:
If I want to run an appliance that needs 50 amps, is there any
problem
with combining two 30-amp circuits of approximately the same length,
as
long as the part that carries the full current is using
properly-sized
cable? It seems like the circuit would have a 60 amp capacity.

Thanks in advance.

Lots of problems, including that it's probably contrary to code.
At least, the inspectors I've run into would be highly unlikely
to approve it.

I'm assuming you're talking about taking two existing 30-amp
circuits and connecting the corresponding phases together
in a 50-amp receptacle.

1. Load balancing. Since the resistance of the wiring is
low, small (absolute) differences in resistance can cause
one or the other parallel wire to carry much more than the
other. While you probably won't have 50 amps in one and 0
in the other, you might get 35 and 15.

The NEC does have provisions for parallel conductors, but
there are a lot of rules about it. It's not for the home
handyman.

2. Circuit breakers in parallel.

I've seen descriptions of circuit breakers wired in parallel,
but I believe they have to come from the factory that way.
Again, two random 30-amp breakers in parallel probably won't
trip the same way a 60-amp break will.

Also note that you need 50-amp "overcurrent protection", not
60,
since you have a single 50-amp load. (I don't think the NEC
has provision for 50-amp multiple-load circuits in residences,
except for "split" stoves.)

It might be possible to do something like what you're planning
if you really know what you're doing, but I think those of us
who need to ask for advice on the net (and many of us who don't)
would be better off running a fresh 50-amp circuit with
its own breaker.

-- Alan McKenney

Yet another good point about one of the paths coming loose and
overheating the remaining line. Thanks for the wise input. I'll go
with one path.