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I have a 240 volt 225 amp sub-panel. Both lines attached to the bus bar have 120v.

When I turn on any breaker connected to one of the bus bars, the voltage drops to 80-108v. If I remove the load and then turn on the breaker, I have 120v.

All breakers on the other bus bar when turned on do not affect the voltage. It remains at 120v.

Any idea what the issue is, as it doesn't seem to be the wire, the panel or the breakers.

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    Sounds like you have a bad connection somewhere, and a potentially very dangerous situation -- if you have a 10A load and a 40V voltage drop, that means that 400W is being dissipated somewhere, possibly in a junction box somewhere in your house, the meter, or somewhere in the power company's circuit, so it's a serious fire hazard. So I'd keep the panel off until you find the problem. – Johnny May 9 '16 at 18:15
  • Possible duplicate of How can I replace an I-T-E panelboard? – Ecnerwal May 9 '16 at 18:19
  • similar problem, but this post is on trouble shooting not replacing. – Ed Beal May 9 '16 at 18:24
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    When you apply this one load, and hot-neutral voltage on its bus bar drops, what happens to hot-neutral voltage on the other bus bar? Does it exceed 120V by a similar amount? That's a neutral problem and extremely serious, fix it immediately. – Harper May 9 '16 at 21:46
  • When voltage drops on one bus bar, the voltage remains at 120v on the other bus bar. The good bus bar isn't affected by the voltage drop and is always at 120v, even if breakers are on or off on the bus bar with the voltage drop. – Bruce May 10 '16 at 14:20
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It sounds like a bad connection to the main breaker or a bad hammer in the main breaker. Try making the measurement at the main prior to the breaker. If your voltage is normal above the breaker the hammer in the breaker is not making a good contact. If it is the breaker turn your small breakers all off. With all the small breakers turned off flip the main breaker 10-20 times on and off and see if this cleans the contact. Sometimes this fixes the problem with a bad contact. but be prepared to replace the main if it happens again. If your voltage is low prior to the main you need to call the power company and have them check the connections at the transformer / line fuses on the pole for a loose connection. I just realized you said this was a sub panel if this is a sub measure the voltage coming out of the main and verify the breaker feeding the panel is not the problem other than that same procedure for checking the main in a sub. The next thing to try is swap L1 & L2 your 2 hots feeding the sub and see if the problem follows the wire.

  • It's a sub-panel that does not have a main in the panel. Voltage coming off the 225 amp breaker in the main panel is 120v on each feed. Voltage coming into the sub-panel is 120v on each feed. Voltage on the bus bar is 120v. As soon as I turn on any breaker that has a load and is connected to one of the bus bars, the voltage drops. If I remove the load from the breaker, the voltage doesn't drop. – Bruce May 9 '16 at 21:31
  • Can you check the breaker at the main there is most probably a loose connection there or on your lugs in the sub. Are there any junctions between the main and sub? (If the wire size is correct for the distance and load). What size wire is feeding the sub? How far away is the sub from the main? Do you know how many amps the load is? – Ed Beal May 9 '16 at 21:58
  • The main panel is a 600 amp old ITE panel in the garage, about 100 - 120 feet from the sub panel. Power leads are 3-0, neutral is 2-0. Wires look good. – Bruce May 10 '16 at 14:06
  • The main panel is a 600 amp old ITE panel in the garage, about 100 - 120 feet from the sub panel. The sub-panel is also an old ITE 225 amp panel. Power leads are 3-0, neutral is 2-0. Wires look good. I have 120v after the main and 120v on the wires coming into the sub-panel. Lug nuts are tight, and there's no corrosion or evidence of overheating. Loads are 20 - 30 amps. A typical circuit can be a few lights and receptacles. As soon as I turn on any one breaker, the voltage drops on just one of the bus bars, which drops the voltage to all breakers on that bus bar. – Bruce May 10 '16 at 14:15
  • Ok the wire size should not be a problem unless heavily loaded and then only a few % drop. I know you said you verified the connections were tight and had voltage at the wires in the sub but Try swapping the 2 hots at the main panel 225A breaker. There may be a break in the feeder wire. Other than this You have checked the lugs at the main& sub with no overheating / damage visible. If the problem stays on the same leg the panel has the problem if it changes legs the problem is in the wire or breaker / breaker connection to the main panel. This simple test will help pinpoint the problem. – Ed Beal May 10 '16 at 15:54

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