Main breaker

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trojans4

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Semi -retired master electrician. Fully retired math and physics teacher.
100A main breaker (bolt on with lugs) in Square D Homeline box. I am sure the main breaker is defective but thought I would check to see if there were any other possibilities before I bought a new one. All breakers powered by one hot wire read 122V from load side to ground and all of those circuits are working. The breakers powered by the other hot wire all read around 30V and those circuits obviously are not working. When you turn the main breaker off and on several times the 30V breakers will read 120V for a time and those circuits are working(sometimes for several hours) and then go back to anywhere from 0 to 30V. I don't think there could be a loose/broken neutral problem since that usually means the other half of the circuits are reading high like 180V to 200V. Any other ideas besides a bad main breaker.
 
trojans4 said:
100A main breaker (bolt on with lugs) in Square D Homeline box. I am sure the main breaker is defective but thought I would check to see if there were any other possibilities before I bought a new one. All breakers powered by one hot wire read 122V from load side to ground and all of those circuits are working. The breakers powered by the other hot wire all read around 30V and those circuits obviously are not working. When you turn the main breaker off and on several times the 30V breakers will read 120V for a time and those circuits are working(sometimes for several hours) and then go back to anywhere from 0 to 30V. I don't think there could be a loose/broken neutral problem since that usually means the other half of the circuits are reading high like 180V to 200V. Any other ideas besides a bad main breaker.
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Do you have a Wiggy or Volt-Con ? Check your two hots on the line side of the Main & on the Load side of the Main. If you have two hots on the Line side & only one hot on the Load side, then your Main is defective. Turn off any two pole breakers as you can get back feed.
 
trojans4 said:
100A main breaker (bolt on with lugs) in Square D Homeline box. I am sure the main breaker is defective but thought I would check to see if there were any other possibilities before I bought a new one. All breakers powered by one hot wire read 122V from load side to ground and all of those circuits are working. The breakers powered by the other hot wire all read around 30V and those circuits obviously are not working. When you turn the main breaker off and on several times the 30V breakers will read 120V for a time and those circuits are working(sometimes for several hours) and then go back to anywhere from 0 to 30V. I don't think there could be a loose/broken neutral problem since that usually means the other half of the circuits are reading high like 180V to 200V. Any other ideas besides a bad main breaker.
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Here are my thoughts:
I am understanding that your reading have been taken from the load side. If so check the voltage from the line side to the load side of each pole. You shoukd read basically close to zero volts across the pole that measured 122v and the other pole would measure essentially 122 less 30v or 90v.
As such you have failed contacts on the pole that measures 30vv. Not that this will correct the issue with exercise the breaker by turning it on and off numerous times and you may see that the voltage on that pole will increase.
When closing a breaker the moving contact will not simply close against the stationary contact but it rubs against it. As such the resistance of the compromised contact will decrease and decrease the voltage drop across the contact. It is also not that unusual for that pole to be noticably warmer also.
Exercising the breaker may improve the issue but would only be a temporary fix.
 
Thanks templ. That is basically what is happening and what I was thinking was going on. The owners of the house don't have a lot of money so I didn't want to buy a 100A main breaker if there could be some other problem I wasn't thinking of.

BTW on another related subject the 100A main for that SQ D box is hard to find around my area and is more expensive than the 200A main breaker (in some places twice as expensive) The supply house said they assumed because most new residential construction was 200A service that they make more of the 200A breakers therefore the 200A are less expensive.
 
hurk - Thanks. I hadn't thought of that. I will check on that before I buy the new breaker.
 
trojans4 said:
... The supply house said they assumed because most new residential construction was 200A service that they make more of the 200A breakers therefore the 200A are less expensive.
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The machines that make the breakers don't know or care what size parts go inside, and there isn't 25 cents worth of copper difference inside. The price is higher for one and only one reason;

Because it can be...

Here's what I would do.
Check the bolt tightness, that's cheap. Then if that's not it, order a breaker on line and do the exercise trick if it acts up again to get a little more time out of it while you wait for it. Someone will sell it for less. But tempdl is right, it's only a stop-gap measure. You have a bad breaker and unlike heart disease, exercise will not make it go away.
 
trojans4 said:
BTW on another related subject the 100A main for that SQ D box is hard to find around my area and is more expensive than the 200A main breaker (in some places twice as expensive) The supply house said they assumed because most new residential construction was 200A service that they make more of the 200A breakers therefore the 200A are less expensive.
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I still use a lot of panels with 100 amp mains - not necessarily for residential construction though. It all depends on what they buy larger quantities of and what they actually stock. Places that normally stock and have turnover in both sizes will sell the 100 amp unit for less then the 200 amp unit.

Also as suggested you may find it to cost less or at least about the same to buy a panel and rob the main out of it.

If you want a harder time finding a breaker try finding a 60, 70, 80, 90 or 110 amp in the same series, or a 175 amp in the QOM2 series.
They are standard sizes and are made, but you don't find them factory installed in a loadcenter either.
 
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