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

RJsfishin
Explorer
Jan 28, 2016
In this calculator, I just punched in 30 amps, 120 volts, at 4 ft one way......came up 14 ga copper.
12 ga overkill ?
"Battery Salesman", quit snibblin, change the charts, maybe go back to sellin batteries !

http://www.csgnetwork.com/wiresizecalc.html
dougrainer
Nomad
Jan 28, 2016
MEXICOWANDERER wrote:
Try as I might, I have looked at hundreds of Onan generators, and not a one had umbilical wire sticking out of it. Must be a conspiracy between Onan and the vehicle manufacturer, huh?

Onan used to use a phenoic terminal block for OEM to connect to right inside the control box. Big gensets like >1MW used Kerny Nuts that did not come with the genset. Onan's liability ended at these terminations. Yotta try confronting a 12-wire connection that runs through a Scott Connection pair of transformers.

New RV manufacturers would be glad to hear from you if you could reach someone who speaks English.

Just for MECHANICAL STRENGTH alone I would hesitate to use 12 gauge on an 30-amp circuit. Even six-inches.

RV Onan Gensets come with the 120 wire harness attached from Onan. The length of that harness (some are 3 wire and some are 4 wire depending on the Genset), is about 6 foot. THIS 120 wiring is then connected to the RV at a 4x4 or 6x6 junction box. Then the RV OEM runs the 120 wiring from THAT box to the RV connections. In some installs the Genset is located close to the Transfer box or the 30/50 amp receptacle if no transfer box is used. In this case, the Onan is wired direct with no junction box. The wire Onan installs is within Code. Doug
Chris_Bryant
Explorer II
Jan 28, 2016
Just looked at the cutoff from the last Onan 7kw I installed- it is marked:

12 AWG (3.31mm2) E137925-L (UL) SIS 600V AND AWM 3173 600V OR 3237 1000V -- CSA CL1251 125C 600V OR AWM T A/B 125C 600V FT2 -- MADE IN USA

Other markings are LOAD CB1 | LOAD CB2 | NEUTRAL |and GND

This wire is from the factory- the only gensets I have run in to with lug hookups are the 33kw Kohler and the 45kw Onan I used to use.
RJsfishin
Explorer
Jan 28, 2016
Doesn't matter what the wannabees and "the battery salesman" want to believe. according to the chart I saw (link won't post) 14 ga wire will carry 30 amps to 4ft long.
If ya don't like it, quit snibblin like little kids, and change the charts
MEXICOWANDERER
Explorer
Jan 27, 2016
Try as I might, I have looked at hundreds of Onan generators, and not a one had umbilical wire sticking out of it. Must be a conspiracy between Onan and the vehicle manufacturer, huh?

Onan used to use a phenoic terminal block for OEM to connect to right inside the control box. Big gensets like >1MW used Kerny Nuts that did not come with the genset. Onan's liability ended at these terminations. Yotta try confronting a 12-wire connection that runs through a Scott Connection pair of transformers.

New RV manufacturers would be glad to hear from you if you could reach someone who speaks English.

Just for MECHANICAL STRENGTH alone I would hesitate to use 12 gauge on an 30-amp circuit. Even six-inches.
enblethen
Nomad
Jan 27, 2016
An engineered system must have proper maintenance.
It is not to be used to circumvent the NEC.
NinerBikes
Explorer
Jan 27, 2016
enblethen wrote:
A full engineered system must also have proper maintenance personnel. It is not just a blanket engineered design so it is OK not to follow the NEC.

Please don't use double negatives in one sentence... try rephrasing the last sentence with no negatives, so we know what you mean.
enblethen
Nomad
Jan 27, 2016
A full engineered system must also have proper maintenance personnel. It is not just a blanket engineered design so it is OK not to follow the NEC.
RCMAN46
Explorer
Jan 27, 2016
john&bet wrote:
RJsfishin wrote:
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
This will probably get me booted, but here goes. You are completely wrong. The NEC says that you can not use a 30amp c/b on 12awg. Period. And 10awg can not have a c/b larger than a 30amp. Oh, and by the way I am not a "wannabe electrician" as you call some people. Statements like yours get me rialed up.

"The NEC says that you can not use a 30amp c/b on 12awg. Period."

Not correct. As I stated previously an engineered system does not fall under the requirements of the NEC. It is not uncommon to find wire sizing and circuit breaker sizing that does not meet NEC requirements in engineered/designed systems.
NinerBikes
Explorer
Jan 27, 2016
john&bet wrote:
This will probably get me booted, but here goes. You are completely wrong. The NEC says that you can not use a 30amp c/b on 12awg. Period. And 10awg can not have a c/b larger than a 30amp. Oh, and by the way I am not a "wannabe electrician" as you call some people. Statements like yours get me rialed up.

Rj's fishing alright... he's trolling. I don't waste my time quoting him.

IMHO... some folks posts around here, I heavily discount. Apply information from outliers suggestions from most posts at your own peril. Or ask for their professional qualifications.

Critical thinking skills should be applied to all posts, and all posters, for trend analysis.