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Onan 7.5JB-3cr/13173ac Questions


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  #1  
Old 11-13-2017, 08:14:46 PM
rmassey rmassey is offline
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Default Onan 7.5JB-3cr/13173ac Questions

Hello Gentlemen, I recently acquired a non working JB in a rusty factory enclosure. PO used it for home backup for 5 days a few years back then let it set for a few years, when he tried to crank it this year it would do nothing. PO put it on craigslist for fifty bucks. Having another JB I bought it on the chance that I could fix it or use it for parts if not. After getting it home I found that the flexible exhaust had broken and rain water had gone into engine locking it up. I pulled the head and found a garden growing in one cylinder. I was able to break pistons loose with rope around flywheel cleaned and honed cylinders, hand lapped valves and reassembled. After changing the oil I hooked up a battery and propane tank and it fired up and runs fine. The gen is putting out 120/240 as it should. Today I went into the connection box [beneath the control box] and found the two hot legs were tied to t1 and t4 and the neutral tied to t2 and t3 there is a wire coming off the ground stud in the box but it is not tied to anything it obviously worked for PO that way but in my other JB that ground stud is also tied to the neutral. Not being an EE I am wondering which way is right. Thanks Robert
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  #2  
Old 11-13-2017, 11:32:35 PM
nothingbutdarts nothingbutdarts is online now
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Default Re: 7.5JB-3cr/13173ac Questions

I am not going to open a can of worms here, sometimes T2 & T3 are bolted together to the stud inside the junction box, sometimes not depending on how the generator is used because of grounding/bonding issues.

I know when I hooked up my 240v. generator to feed my house as a back up unit, my inspector said I had to disconnect the T2 and T3 leads from the junction box.
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Old 11-13-2017, 11:33:26 PM
JohnnyC JohnnyC is offline
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Default Re: 7.5JB-3cr/13173ac Questions

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Originally Posted by rmassey View Post
Hello Gentlemen, I recently acquired a non working JB in a rusty factory enclosure. PO used it for home backup for 5 days a few years back then let it set for a few years, when he tried to crank it this year it would do nothing. PO put it on craigslist for fifty bucks. Having another JB I bought it on the chance that I could fix it or use it for parts if not. After getting it home I found that the flexible exhaust had broken and rain water had gone into engine locking it up. I pulled the head and found a garden growing in one cylinder. I was able to break pistons loose with rope around flywheel cleaned and honed cylinders, hand lapped valves and reassembled. After changing the oil I hooked up a battery and propane tank and it fired up and runs fine. The gen is putting out 120/240 as it should. Today I went into the connection box [beneath the control box] and found the two hot legs were tied to t1 and t4 and the neutral tied to t2 and t3 there is a wire coming off the ground stud in the box but it is not tied to anything it obviously worked for PO that way but in my other JB that ground stud is also tied to the neutral. Not being an EE I am wondering which way is right. Thanks Robert
My JB was used for backup lighting in a school and connected to an ATS. The neutral wires were tied to a ground stud inside the wore box. The picture below is what I seen the first time I opened the box. The arrow points to the ground stud which the neutral wires are attached to.

JohnnyC
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  #4  
Old 11-14-2017, 12:05:44 AM
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Jim McIntyre Jim McIntyre is offline
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Default Re: 7.5JB-3cr/13173ac Questions

Robert,

We'd have to know what you'll be connecting the genset to in order to answer your question. If a transfer switch is involved, we'd at least need to know if it switches the neutral.

Otherwise, we'd just be guessing.
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Old 11-14-2017, 02:28:36 AM
len k len k is online now
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Default Re: 7.5JB-3cr/13173ac Questions

for 120/240 output t2/t3 tied together sounds right for a 3cr.

Deciding whether to tie that t2/t3 neutral to frame depends on if neutral/ground bond is to be in gen or house. That depends on if transfer switch switchs the neutral
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Old 11-14-2017, 10:25:42 AM
rmassey rmassey is offline
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Default Re: 7.5JB-3cr/13173ac Questions

Hello again, First I want to thank you all for all the information.This is a great site that I have enjoyed reading for the last year. This morning I went out and looked closer at the existing wiring in the box on the generator. There is a wire tied to the ground post in the junction box that I thought went into the generator I see now that it is the green wire on the output wires, there is also a pigtail off of the ground post that is tied to nothing and taped up. It looks as if the pig tail may have been tied to the neutral at one time. On my house I have a 200 amp manual switch that ties my house to the grid or to the gen the switch does not switch the neutral. does the pigtail mentioned above need to be tied to the neutral or left isolated? Thanks Robert
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Old 11-14-2017, 02:00:34 PM
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Default Re: 7.5JB-3cr/13173ac Questions

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...On my house I have a 200 amp manual switch that ties my house to the grid or to the gen the switch does not switch the neutral. does the pigtail mentioned above need to be tied to the neutral or left isolated?
The neutral should not be connected to ground at the genset.

You should 'ground' the genset itself by connecting a ground wire between it and your house service panel ground. Thus, you need 4 wires between the genset and the transfer switch - L1, L2, Neutral, and Ground.
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Old 11-14-2017, 02:10:10 PM
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Default Re: 7.5JB-3cr/13173ac Questions

Overview.......NEC code allows only have ONE active neutral/ground bond ( N/G) in the system. Since your transfer switch doesn't switch the neutral that means N/G bond needs to be at the house. So can't have another N/G bond in gen. And you do as Jim posted above.
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Old 11-15-2017, 09:16:25 AM
rmassey rmassey is offline
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Default Re: Onan 7.5JB-3cr/13173ac Questions

Thanks everyone for the information. I guess I need to go back into my house backup JB and disconnect from the ground stud. I will be back later with more questions. Thanks Robert
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