National Electrical Installation Standards

Standards as High as Your Own

Tuesday, July 12, 2022


Hello Code Question of the Day.

I have a service cabinet that is fed from the serving utility at 240/480 volt, single-phase (split phase). The 240/480 service entrance conductors feed a 200 A main breaker and 24-space panel board. Inside the service cabinet is a 480-to120 volt transformer that feeds a 6-space 120 volt panel board. The service cabinet also includes a meter base, lighting contactors, photocell, etc. My question regards the grounded neutral service entrance conductor (240/480) and the grounded conductor (120). The service cabinet has only one insulated neutral buss, a grounding buss, a main bonding jumper from the neutral buss to the grounding buss, and grounding electrode conductor to two ground rods. The 120 volt grounded conductor is terminated to the neutral buss as well as the 240/480 service entrance conductor neutral. What specific code (s) prohibits, or restricts, the connection of neutrals and grounded conductors from different systems and voltages?

Thank you in advance.

Ken Morman



Hello Ken,

Thanks for the question. As we understand the question, there appears to be only one grounded conductor bus in the service equipment that contains a transformer for a lower-rated panelboard and transformer. There should be a separate grounded condcutor bus for the panelboard to which the neutral conductors of that system would be connected. The requirements for separately derived systems are provided in Section 250.30(A). If the service equipment contains a transformer separately derived system it should be constructed to meet the requirements for separately derived systems in Section 250.30(A). Also, the manufacturer of the service equipment should have installation instructions and wiring diagrams that guide installers on proper connections of grounded conductors of the two systems within the same enclosure. 

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