Ok, have a quick question for you electrical guys. I had to pull my Onan 4000 generator for some landing gear work. Noticed that the AC feed wires are 12 gauge and about 6' long before a junction box where they tie into 10 gauge wire to run back to the converter. I understand that the generator has a 30A breaker on the control panel, so it won't ever put out more than 30A. I thought that 12 ga wire was too small for a 30A circuit, but that is what came on it from Onan. Should I rewire with 10 gauge or just trust that Onan knows what they're doing??

Wire size is not "fixed" for a certain amp rating. Wire size can vary for distance. The shorter you go, the less wire you can get away with. The longer you go, the thicker.Many "calculators" out there..... But I suspect for a 6 foot run, 12AWG is fine.

Something sounds funny!It should be minimum 10 gauge wire for 30 amp. NEC requires 310 for 30 amp.The AC unit should not be running directly off genset.The genset should go through a transfer switch or on some rigs a receptacle where the shore power cord plugs in.It then should go to a 120 volt power distribution panel. There a 20 amp breaker would feed the AC unit on #12 wire.Onan only wire what is inside the genset. Connection to the rig is done by the rig's manufacturer. I would be checking with them!

14 ga will carry 30 amps up to 4 ft12 ga will carry 30 amps up to 8 ft.Don't need no wannabe elect tech to tell you,...just look at a chart w/ ga and amps, and length of wire,Oops, excuuuuuuuuse me, 12 ga will carry 30 amps for only 6 ft

he said AC not A/C connection power junction box NOT Air Conditionerenblethen wrote:Something sounds funny!It should be minimum 10 gauge wire for 30 amp. NEC requires 310 for 30 amp.The AC unit should not be running directly off genset.The genset should go through a transfer switch or on some rigs a receptacle where the shore power cord plugs in.It then should go to a 120 volt power distribution panel. There a 20 amp breaker would feed the AC unit on #12 wire.Onan only wire what is inside the genset. Connection to the rig is done by the rig's manufacturer. I would be checking with them!

#12 is probably fine but still I would pull #10 just because.If the wire feeds a single appliance it is fine. Does it only feed the converter or the whole electric panel?

30 amp generator with 12 gauge wire????? | Good Sam Community

39 Replies

KRumm
Explorer
Jan 25, 2016
I think the "junction box" the OP refers to is his transfer switch...
The 12 gauge should be ok... but if it were me I would replace with 10 gauge while I have generator out.
RCMAN46
Explorer
Jan 25, 2016
enblethen wrote:
Something sounds funny!
It should be minimum 10 gauge wire for 30 amp. NEC requires 310 for 30 amp.
The AC unit should not be running directly off genset.
The genset should go through a transfer switch or on some rigs a receptacle where the shore power cord plugs in.
It then should go to a 120 volt power distribution panel. There a 20 amp breaker would feed the AC unit on #12 wire.
Onan only wire what is inside the genset. Connection to the rig is done by the rig's manufacturer. I would be checking with them!

I believe the wiring in question is part of an engineered system. Engineering calculations determined the wire size and temperature rating of the wire required.

As such the system does not have to comply with NEC requirements.

NEC covers systems that have not been engineered.
enblethen
Nomad
Jan 25, 2016
Why would they have spliced wire?
Confused! Thought they had tied a separate air circuit directly into genset.
joebedford
Nomad II
Jan 25, 2016
I'm pretty sure he missed the step where the generator feed goes to a transfer switch then to the breaker panel then to the converter.
time2roll
Nomad
Jan 25, 2016
#12 is probably fine but still I would pull #10 just because.

If the wire feeds a single appliance it is fine. Does it only feed the converter or the whole electric panel?
MrWizard
Moderator
Jan 25, 2016
he said AC not A/C connection
power junction box NOT Air Conditioner

enblethen wrote:
Something sounds funny!
It should be minimum 10 gauge wire for 30 amp. NEC requires 310 for 30 amp.
The AC unit should not be running directly off genset.
The genset should go through a transfer switch or on some rigs a receptacle where the shore power cord plugs in.
It then should go to a 120 volt power distribution panel. There a 20 amp breaker would feed the AC unit on #12 wire.
Onan only wire what is inside the genset. Connection to the rig is done by the rig's manufacturer. I would be checking with them!
RJsfishin
Explorer
Jan 25, 2016
14 ga will carry 30 amps up to 4 ft
12 ga will carry 30 amps up to 8 ft.
Don't need no wannabe elect tech to tell you,...just look at a chart w/ ga and amps, and length of wire,

Oops, excuuuuuuuuse me, 12 ga will carry 30 amps for only 6 ft
enblethen
Nomad
Jan 25, 2016
Something sounds funny!
It should be minimum 10 gauge wire for 30 amp. NEC requires 310 for 30 amp.
The AC unit should not be running directly off genset.
The genset should go through a transfer switch or on some rigs a receptacle where the shore power cord plugs in.
It then should go to a 120 volt power distribution panel. There a 20 amp breaker would feed the AC unit on #12 wire.
Onan only wire what is inside the genset. Connection to the rig is done by the rig's manufacturer. I would be checking with them!
SolidAxleDurang
Explorer II
Jan 25, 2016
Wire size is not "fixed" for a certain amp rating. Wire size can vary for distance. The shorter you go, the less wire you can get away with. The longer you go, the thicker.

Many "calculators" out there..... But I suspect for a 6 foot run, 12AWG is fine.

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30 amp generator with 12 gauge wire?????

39 Replies

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