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Onan 12.5JC-3R Wiring Question

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Old 01-09-2018, 11:39:50 PM
Reeves Nut Reeves Nut is offline
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Default Onan 12.5JC-3R Wiring Question

So I recently picked up an Onan 12.5 JC-3R generator and I have a few questions. It has vacuflow cooling and has a gaseous fuel carburetor. It is currently set up for 120/240 output. It cranks over and has oil pressure and brushes look good. Magneciter coils also look good. It was not runnable when I bought it but the owner claimed it made power.

Model: 12.5JC-3R/10210S
Serial: 0371290645

I have the operators manual 967-500 and the T-015 Tech Bulletin so far.

It at some point had the optional mouse nest in the control box. The previous owner had removed the nest and occupant(s) and I am cleaning up the mess. I plan on replacing the 2 original ignition relays with Allen Bradley relays via part numbers I found on here. The start relay is also getting replaced. My question is what is the insulation class for the wires in the control box? It's obviously not THNN or THWN wire. Is there a source for original wire as I fix the wiring and update it? I am also in search of an original 80 amp ammeter for the output cabinet since one is damaged and I would like to keep it looking as original as possible.

My other question is about the fuel supply. I plan on feeding it vapor from a 500 gallon LP barrel. I'm in central Iowa and have seen temperatures as low as -28°F. The table for sizing the tank shows for a draw of 100 CF/H at -30°F I need a 1500 gallon tank. Is this realistic? This unit will be used for backup power in the event of a power failure.

I need to build a skid, acquire the correct motor mounts, and fix the wiring mess then we'll see if it works. I also have the original 100 amp automatic transfer switch but that's a whole different can of worms.

I read quite a bit of the archives and there is a great wealth of information here.


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Old 01-09-2018, 11:56:55 PM
JohnnyC JohnnyC is offline
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Default Re: Onan 12.5JC-3R wiring question

Congrats on your JC. Based on your calculations, don't forget to add in anything else that runs on propane while the JC is running if sizing of the tank is critical in those extreme cold climates.

In the past I completely rewired a JC and many RV gensets using wire similar to the original wire in color and gauge. I bought it at an electronics specialty store on a large spool. I don't know what the insulation is made of, but since the wires are not exposed to oil, THHN and similar is not required. I'm sure it will stand up to the Mouse Option if they move in at a later date.

If you lived close to me I'd give you a skid made up of 3/8 steel plate that will easily fit a JC and make it movable. It is freakin heavy like the JC

New Jersey
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Old 01-10-2018, 08:58:27 PM
Reeves Nut Reeves Nut is offline
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Default Re: Onan 12.5JC-3R wiring question


Thanks for the advice. The 500 gallon barrel also feeds liquid propane to my grain dryer but it and the JC won't be running at the same time because the dryer pulls about 80 amps at full load. The JC is good but it's not that good!

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Old 01-10-2018, 09:00:34 PM
Ray Lynch Ray Lynch is offline
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Default Re: Onan 12.5JC-3R wiring question

Check out this link regarding control wire.
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Old 01-10-2018, 09:06:49 PM
Max Thompson Max Thompson is offline
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Default Re: Onan 12.5JC-3R wiring question

14 AWG E125352 (P) (UL) BC-5W2 OR MTW OR THHW OR AWM 1011/1015/1230/1335 600V OR 1032 1000V MOISTURE RESISTANT VW-1 --156016 CSA 105C 600V FT1 OR AWM 1 AB 105C 600V FT1

This is what is printed on the wire I have been using. I suppose the only pertinent info is 14 ga. MTW or THHW or AWM.

Maybe one of the electric gurus can decipher all this for you (and for me).
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Old 01-11-2018, 08:13:28 AM
armandh armandh is offline
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Default Re: Onan 12.5JC-3R Wiring Question

just thinking about location and cold weather

extreme cold limits in tank gasification
vacuflow generator has a nice waste heat output

alternate summer/winter ducting


or if co-location is inconvenient
liquid to the generator "dog house" and
a winter ducting arrangement to the liquid/gas transition
by using this free advise, you, your heirs, etc
absolve, save, and hold harmless the advisor.

Last edited by armandh; 01-11-2018 at 11:18:32 AM.
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