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Onan 12.5jc-18r


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  #1  
Old 02-14-2014, 11:33:02 AM
mscel mscel is offline
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Default Onan 12.5jc-18r

Quick question about amperage. I want to install a female 12 pole military style connector on my generator so that I can plug in either a 480v 3ph or 120/240 Male plug. All of the connections that define the wye wiring or delta/delta would be done between the 12 pole male plug and the standard 480V or 120/240 connector. Doing so will speed the process for changing over from one to the other significantly. But this begs the question, what is the max amperage that any one of the 12 leads from the generator would have to carry? I'll need to know that to size the 12 pole plug properly.

Thanks to those that helped me bring my unit back to life. I converted it to propane with parts from Propanecarbs.com and had it running off of my 250 gallon propane tank when the Arkansas ice killed my power for two days. I'm very pleased with the whole setup. It starts easy, so easy my wife could start it, and makes good power. We are on a well so I have a new appreciation for a hot shower powered by ONAN....
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  #2  
Old 02-14-2014, 11:45:21 AM
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Billy J Shafer Billy J Shafer is offline
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Default Re: 12.5jc-18r

The amp readings should be on the tag.
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Old 02-14-2014, 06:20:31 PM
Gunny Gunny is offline
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Default Re: Onan 12.5jc-18r

If I understand your inquiry, you are wanting to be able to do a voltage change by changing the connecting plug/jack.

There are switches especially designed for that. Sort pf pricey but work well. You will need to make a reconnection at the voltage regulator in addition to reconfiguring the output leads. The voltage regulator sensing leads.

NEVER! Do NOT ever try to change voltage on any genset with the engine running. It WILL destroy the stator windings immediately. Not may or might or stand a good chance of, it WILL! All you'll be able to do is kick yourself. Just so you know the dangers involved in doing this and the consequences of an error.

I've not done that but have second hand experience. Lets the magic smoke out immediately. Might be the reason why the switch I mentioned above has provisions for a padlock/lockout device.

Contact Davidson Sales (800)383-2078 for the switch. You can get them so you can make 2 or 3 different voltage choices with it.
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Old 02-14-2014, 06:20:44 PM
Dave Edmonds Dave Edmonds is offline
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Smile Re: Onan 12.5jc-18r

If your generator's output were wired for 480 volts, 3-phase, 1/3 of its output would be produced by two of the windings in series, another 1/3 by two other windings in series, and 1/3 by the third two windings in series.. If you divide the 12,500 by three, then by 480, you would come up with a current of 8.68 amps per winding.

If the unit were wired for 240 volts, 3-phase, 1/3 of its output would be produced by two of the windings in parallel, another 1/3 by two other windings in parallel, and 1/3 by the third two windings in parallel. If you divide the 12,500 by three, then by 240, you would come up with a current of 17.36 amps. This 17.36 amps, however, is being produced by TWO windings of 8.68 amps each in parallel.

Looks to me like you probably need a 12-pin connector with pins rated for at least 10 amps per pin, unless you have an extended-stack generator that might need a larger-capacity connector. Other, more intelligent people will correct me if I am wrong.

Dave Edmonds
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Old 02-14-2014, 08:23:13 PM
Kpack
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Default Re: Onan 12.5jc-18r

Quote:
Originally Posted by Dave Edmonds View Post
If your generator's output were wired for 480 volts, 3-phase, 1/3 of its output would be produced by two of the windings in series, another 1/3 by two other windings in series, and 1/3 by the third two windings in series.. If you divide the 12,500 by three, then by 480, you would come up with a current of 8.68 amps per winding.

If the unit were wired for 240 volts, 3-phase, 1/3 of its output would be produced by two of the windings in parallel, another 1/3 by two other windings in parallel, and 1/3 by the third two windings in parallel. If you divide the 12,500 by three, then by 240, you would come up with a current of 17.36 amps. This 17.36 amps, however, is being produced by TWO windings of 8.68 amps each in parallel.

Looks to me like you probably need a 12-pin connector with pins rated for at least 10 amps per pin, unless you have an extended-stack generator that might need a larger-capacity connector. Other, more intelligent people will correct me if I am wrong.

Dave Edmonds
I couldn't have have figured it better great job . laugh I'm going to call max on those calculations.
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Old 02-14-2014, 08:36:47 PM
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Default Re: Onan 12.5jc-18r

I think the switch will be a mistake. I have spent to many hours digging through burned and melted wires to trust them
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Old 02-14-2014, 10:40:17 PM
Jim Rankin Jim Rankin is offline
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Default Re: Onan 12.5jc-18r

The amp calculation needs to be done by dividing by the phase to neutral voltage, not the line to line voltage. Also need to start with the kVA rating of the generator end, not the kW rating of the engine.

So, for a typical 3 phase reconnectable power factor would be 0.8 resulting in a kVA of 15.625.

15.625kVA x 1000 VA/kVA=15625 VA.

15625 VA/ 277 V/3 phases=18.8 A per phase.
or
15625/1.732/480=18.79 A per phase

120/208 Volts 3 phase usually has the highest amp rating of all the reconnection possibilities.........
Reconnecting for 120/208 Volts yields
15625 VA/120/3phases=43 A/phase
or
15625VA/1.732/208V=43A with two windings in parallel, so 21.5 amps per winding maximum.
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Old 02-14-2014, 11:17:50 PM
krc002 krc002 is offline
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Default Re: Onan 12.5jc-18r

It sounds like others have the answers, i am more curious as to the why? not many folks on the stack are searching for 480v three phase on a regular basis.

Is there an end game to this switch-ability?

just out of curiosity...
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Old 02-18-2014, 07:59:04 PM
mscel mscel is offline
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Default Re: Onan 12.5jc-18r

I need the generator both for standby power for the house and also to run a mill and lathe. There is no 480V three phase where I live. It's kind of a pain to rewire it for applicaition. That's all, however, the warnings about switching while on are well heeded. I'll have to error proof that.

I have 480V/3phase and 120/240 single phase. How would the amp calculations look for the 120/240 Single phase? Thanks a lot...
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Old 02-18-2014, 10:43:47 PM
krc002 krc002 is offline
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Default Re: Onan 12.5jc-18r

it all makes sense now. glad to hear others are using onans to run machinery.
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Old 02-18-2014, 10:48:33 PM
John Ward John Ward is offline
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Default Re: Onan 12.5jc-18r

Quote:
Originally Posted by mscel View Post
I need the generator both for standby power for the house and also to run a mill and lathe. There is no 480V three phase where I live. It's kind of a pain to rewire it for applicaition. That's all, however, the warnings about switching while on are well heeded. I'll have to error proof that.

I have 480V/3phase and 120/240 single phase. How would the amp calculations look for the 120/240 Single phase? Thanks a lot...
Is your equipment dedicated 480v? Or is it 208/230/480v 3phase?
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Old 02-18-2014, 11:26:37 PM
Jim Rankin Jim Rankin is offline
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Default Re: Onan 12.5jc-18r

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Originally Posted by mscel View Post
I have 480V/3phase and 120/240 single phase. How would the amp calculations look for the 120/240 Single phase? Thanks a lot...
You should be able to read the amperage off the tag of the generator itself.

But for a typical reconnectable generator, not extended stack, with a power factor of 0.8, you would have a little over 10kW after reconnection to 120/240 single phase or about 43 amps per line.

If it was an extended stack, then amperage would be 52.
12,500 VA / 240 volts= 52A

Would you have 120/240 cord connecting to a transfer switch for the house and a 277/480 3 phase cord connecting to a 3 phase panel for your machines? Seems simple enough.
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Old 02-20-2014, 03:55:42 PM
mscel mscel is offline
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Default Re: Onan 12.5jc-18r

It is dedicated 480V 3phase

As for connections, yes I would only have one 12-pin connector on the genset, I would then plug my harness in for 480V or for 120/240 single phase. Each harness on the opposite end would then have the proper connection for my house or for the shop equipment.
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Old 02-20-2014, 06:48:36 PM
grif grif is offline
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Default Re: Onan 12.5jc-18r

Quote:
Originally Posted by Gunny View Post
NEVER! Do NOT ever try to change voltage on any genset with the engine running. It WILL destroy the stator windings immediately
So,,, 15JC 12 lead. It's ok to run it with several of the leads isolated,,, just not switched during power generation, right?
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