National Electrical Installation Standards

Standards as High as Your Own

Friday, January 19, 2018


Hi Charlie,

We have a customer who is feeding an 800 Amp 208/120 V panel with two 400 Amp fused disconnects. The switches are in the same panel, but separate buckets and each is feeding a set of 500 kcmil. This appears to be a violation of 240.8 because the fused disconnect switches are not factory assembled in parallel and listed as a unit. My question is, was this ever legal under an older code? Thanks for your time and for this service you provide.

Regards, Edward M. Barr, P.E.



Hey Edward thanks for your question. In the 1920's, and possibly before, overcurrent devices were only allowed to be installed "in multiple" if the "ampacity" exceeded the largest available overcurrent device. Back then it is also possible that overcurrent devices were not available (or not available as a tested product) in 800 ampere sizes because the standard cartridge fuse size in the NEC did not exceed 600 amperes. Larger sizes appear to have been added in the 1950's. The wording was changed to "in parallel" for the 1968 NEC and eventually resulted in the wording we have today in 240.8. Depending on when the example you provided was installed, the NEC edition that was adopted there, and what "approved" overcurrent device sizes were available it seems it would need to be somewhere around 1950 for it to be considered acceptable but exact dates are difficult to confirm.

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