National Electrical Installation Standards

Standards as High as Your Own

Thursday, March 14, 2019


Re: CQD answer published Wednesday, March 6, 2019 -Existing Installation

I assume that the question is about an older system that is 3 wire 3 phase and no ground. This was used by several local power company's in our area when 480 volt machines were first introduced in our area. We had a 480 volt 3 wire system like this in an egg processing plant. At that time the power company only gave us the 480 volt  3 wire power. One of the machines created a short that did not trip any breakers, but created a corner grounded system. Because the short was in one machine only and they were able to continue to operate they did not call us for several days. One maintenance employee was concerned about the 480 volts to ground on 2 phases, but was ordered to just keep everything running. We were able to correct it by identifying the problem and asked the power company to upgrade the transformers to a 4 wire system. We than added an external ground to every machine in the building. The panels and buses at that time were not designed for grounding. Is this plant in Tuesdays question required to bring every machine up to proper grounding? I would think that any new machine on the line must be properly grounded. 

We have another plant that did not have a neutral on the 480 volt system. Someone started to change the lights to 277 volts and I was called when they had problems. (And ruined several new fixtures.) We installed a dry type indoor transformer to correct this. 480 volt 3 wire primary to 480/277 volt 4 wire secondary. We also installed a new 4 wire panel for the lights.

Tom Inman



Hey Tom thanks for your follow up question and comments. The NEC is generally understood to be not retroactive, it applies to an installation and is usually done as part of a permit process using the NEC edition that is adopted at the time the permit is obtained. Although Annex H is not mandatory, 80.9 gives some guidance for how this works. If new equipment (machine) is installed it needs to comply with the NEC edition that is adopted in that locality. Whether that is enforced or if the AHJ even knows an installation was made also varies.

As we previously mentioned there are retroactive OSHA requirements and liability issues for employers and owners. It might not be specifically required, by the local municipality, to make an existing installation compliant but if an incident occurs explaining why corrections were not made will be difficult.

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