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Why is there a 3-phase breaker in a single phase electric panel with only two bus bars?
Thursday, June 14, 2018
A Short History of The High-Leg Delta Service
It’s now just an interesting bit of electrical history, but in the 1950s and ‘60s residential air conditioning was in its infancy and a type of 3-phase service called “HIgh-Leg Delta” was used for homes with larger air conditioning compressors. Three phase motors are more efficient than single phase and use less current. High-Leg Delta provides 240 volts phase to phase, and the third “High Leg” (center-tapped on the transformer) delivers 208 volts. Each of the other two phases are 120 volts to ground, as shown in the transformer diagram below.
The way they accommodated three phases with only two bus bars is by using a “Delta Breaker.” Here’s a description by Schneider Electric, a major manufacturer of electric panel components: “A Delta Breaker is a load-center breaker with 2 plug-in stabs and a wire from the third terminal that goes to the third leg of a transformer. This allowed you to use a single-phase loadcenter and get three-phase power by running a third wire back to the transformer. The breaker is no longer available, and any replacement breaker will require installation of a 3-phase load center.”
High-Leg Delta Banned By NEC
In fact, it is more than just “no longer available.” Delta Breakers have been banned by the National Electrical Code (NEC) for many years now. The 2017 edition of the NEC states at 408.36(C) “Delta Breakers - A 3-phase disconnect or overcurrent device shall not be connected to the bus of any panelboard that has less than 3-phase buses. Delta Breakers shall not be installed in panelboards.”
Replacing a complete panelboard can get pretty expensive. An electrician friend of ours, Craig Eaton of Eaton Electric, says he finds that many High-Leg Delta electric services no longer have a 3-phase condenser connected to them. “The condenser has been replaced with a modern, high-efficiency 240-volt, single phase unit, and the high leg is capped in the outside disconnect, with only the two single phase wires connected,” according to Craig.
This is a once-in-a-blue-moon old house thing that might otherwise leave you mystified, but now you know why. Also, for how to recognize three-phase service, see our blog post What is a reliable way to tell if the electrical service is 3 phase or single phase?
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