The last delta breaker, I installed, was in the 60's. I think the code first prohibited the use in 1984.
This was a very economical method to operate a water well pump, on farms, and a limited number of 3 phase motors, from a single phase panel board.
The code section read" a three pole breaker could not be installed in a panel with less than three busses". This eliminated the delta breaker.
I don't remember all the reasons for discontinuing this concept. I heard one reason was; the utility workers were installing these in existing services. This was against union work agreements. The last one I installed, was supplied by the utility company.
This procedure started when running water was first in homes. Well pumps with a 3 phase motor worked better than a single phase.
The existing single phase 240/120 transformer could remain in place, and connected. A small transformer, sized by the 3 phase load would be added. The primary was connected open wye, using two phase legs and the MGN. The secondary would be open delta.
This created a high leg, which was run to the feed through pole of the delta breaker.
This high leg would often be only a #10 wire.
This was the first time I used Orange for color marking.
There was some problem with metering this system, I don't know all the details.
I think phase converters caused this systems demise, prior to any code rules.
The decision to prohibit the delta breaker, was either for selling more converters, more 3 phase panels, a jurisdiction dispute, or maybe all the above. I can see no logical safety problem with their use.
I am sure there is no existing services with a delta breaker. Maybe on an old farmhouse somewhere.
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