Need Help Trouble Shooting

[ZT9510]

Hello All,
I am having an issue with the power in my shop and am left scratching my head pretty hard over it. First I'll describe the set up then my issue. The shop is attached to the main panel with a dual pole 50A breaker fed with 3-3-3 directly buried aluminum. The run to the shop is about 200 feet. In the shop is a 100 amp sub panel that is currently housing 2 20A breakers on a single side of the panel. The first breaker uses all 12 gauge cu and feeds a single plug, three 4 T-12 florescent lights, one 300 watt halogen, and 2 incandescent fixtures. The second breaker uses 14 gauge cu and feeds a single plug and an exterior 2 light flood fixture. When I moved into this house and started using the shop the second breaker was on the right hand post in the panel but any load other than the light would cause a dramatic loss of voltage but not trip the breaker in the sub panel, but the first breaker on the left hand post would power any and all my tools no problem. My solution was to switch the second breaker to the left hand post and that worked for a while. Now the problem has escalated so that any time I use any power on the second breaker including just the flood light causes the breaker in the main panel in the house to trip. The breaker in the house will also trip with moderate use on the first breaker. Moderate use in this case would be a 15 amp tool for longer than 10 min or repeated use in the course of an hour. When I inspect the breaker in the main panel in the house they are very warm and suspect that is their cause of tripping. I should note that not once has either breaker in the shop tripped when any of these issues has occured.

What I have thought so far is that one of the hot wires has a break somewhere underground, most likely the one on the right hand post in the sub panel. I plan on doing a complete rewire of the shop with appropriate wiring and plugs for my needs and also installing new 3-3-3 copper from the main panel to the sub panel in conduit. So my two main questions are: does it sound like there is a break in one of the hot supply wires and is it possible to run the sub panel with only one hot wire to run 2 20 amp breakers?

Sorry for the long vomitous post, but I would really like to be able to work in my shop for longer than an hour at a time. Thank you for any help or insight.

Zach

[akaothermick]

Have you pulled the breakers out to check the cleanliness and tightness of the connections in both panels?, then coat them in a anti oxidation compound, sounds like a high resistance problem on a leg, aluminum will oxidize and do that, the underground feeder could be damaged if incorrectly installed, but go with the easy solutions first and clean, retighten, and coat the connections with compound, and see if it corrects the problem

[ZT9510]

I received that same advice from some other sources and gave it a shot. I'm cautiously optimistic since the breaker didn't trip with some pretty heavy use after the touch up. Thanks for the reply.

[HayZee518]

first of all, whomever installed the sub panel in the garage should have used a four wire feed, not a three wire. within the cable assembly is two hots, a neutral and an equipment ground. at the house panel, the sub-panel neutral and equip ground goes on the neutral bus in the main panel. in the sub panel, the neutral bus is NOT bonded to chassis ground. An aux ground bar receives the ground wire NOT the neutral although technically they are both at the same potential in the house panel neutral.
the reason for not bonding the neutral in the sub panel is to prevent objectionable current from charging the ground of a tool being used which creates a hazardous condition. The neutral in any line is a return to ground. in a multi wire circuit, the neutral carries the unbalanced load to ground. Ideally the neutral's potential should be zero volts. At the sub panel you should also have a driven ground rod with a minimum #6 bare stranded copper going to the aux ground bar. get hold of a clamp around ammeter and with loads connected in the sub, measure the current on each hot leg from the main panel. any deviation more than 20 volts would indicate an open or reduced wire size in the subject cable. code says no more than a 3% voltage drop in a cable length of 100 feet. but what people don't understand when doing the voltage drop calculation, they only measure the length going out to the load and don't even consider the return leg. so if your sub is 200 feet from the house, the combined length is actually 400 feet. so your voltage drop = 12 volts.
when your voltage goes down, the current goes up by 2.