HVAC transformer wiring confusion

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Old 11-24-17, 12:39 PM
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HVAC transformer wiring confusion

Hi guys, I am trying to replace my transformer. Long story short is that I have had to replace fan motor, contactor, start-run cap etc. I have the unit running again but it blows the 5a fuse after about 10 minutes of running. The confusion is due to the current wiring of the existing transformer. It's a 240 primary to 24 secondary. As you can see in the pic they jumpered the 208 and common together and then came off the 240 wire. Why would there not be a common wire going to common? I have a new transformer, would just wire it up the same they have now? 240 to the red wire and then connect the common and 208 wires together?
 
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Old 11-24-17, 02:11 PM
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Wow. Someone really didn't know what they were doing. Do you have a picture of the wiring diagram? Or model number of the unit?

I do remember hearing about some mis-marked transformers. Maybe this was one of them or they thought that it was? Wired like that won't work so is there another transformer nearby?

Also - Do you have 208 or 240 vac in the house? 208 usually in condos and such.
 
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Old 11-24-17, 02:52 PM
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Glad I am not the only one confused as to why it's wired up this way. It's a trans X12 heat pump with air and emergency heat in a residential home so it would have to be 240. Lightening hit the unit and took out the variable speed fan motor. I replaced that with the exact same part and serial. After doing that it would start but only if you helped the outside fan start moving. I noticed the cap top was swollen so I replaced that and it started and ran but would not shut off even after the tstat told it to. The contactor had welded together so I replaced that and now it starts and runs great both cool and heat but after a few minutes it blows the 5 amp fuse. I had read about the transformer possibly having a short so decided to replace it just to see. When I went to replace it I couldn't figure out why it would be wired the way it is. It has ran since 2004 wired like this but I can't figure out why. I will get more pics tomorrow of the wiring diagram and to show where the other wires are running.
 
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Old 11-24-17, 03:32 PM
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Here is the model number picture.
 
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Old 11-24-17, 04:55 PM
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Here is a picture of how the transformer is hooked up in someone else's system. Note different transformer and rotated. https://f01.justanswer.com/jINOm7Sx/..._161006364.jpg

And typical wiring diagram: https://i2.wp.com/carlplant.me/wp-co...size=665%2C744
 
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Old 11-24-17, 06:32 PM
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Oh thanks. The does make more sense and looks correct. Now I have to figure out where the heck my 240 common wire is and I should be fine. I'll update tomorrow when I take a look at it. I do appreciate all the help!
 
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Old 11-24-17, 08:41 PM
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There should be two wires on the transformer primary. Since it's 240v there is no common.... just two legs of power. There should not be a jumper from 208 to 240. The two wires should be on common and 240v. The 208 terminal should have nothing on it.

You posted the model number for the condensor. Isn't the transformer inside the air handler ?
 
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Old 11-25-17, 10:25 AM
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Just to make sure I am interpreting this correctly, this is how I would hook up the new transformer?
 
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Old 11-25-17, 10:40 AM
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Correct..... nothing on the 208v tap.
 
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Old 11-25-17, 02:59 PM
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Good news, I believe the problem is solved after the transformer replacement. Ultimately it was allot of things but the transformer was the last bad part. I appreciate all the help. A total of $200 in parts to get the system going again. I am sure a technician would have recommended a brand new system.
 
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Old 11-25-17, 03:49 PM
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That's good news.

Although.... I feel you may not have solved your problem fully. You said the 5A fuse was blowing. The transformer would have no affect on that. That fuse blows on a 24vac short..... after the transformer. I don't like using 5A fuses as a short is hard on a transformer. I use 3A or 4A.
 
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