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Can Someone Refresh my Memory?

Feb 1, 1999 12:00 PM, Edited by Robert J. Lawrie

Q. One of my electricians asked if it was a Code violation to install a 200A-rated, 42-circuit-load center, main lugs only, with protection consisting of a 100A switch/circuit breaker. I know I have read a good, sound, and reasonable response against this practice; however, I can’t find it in the NEC or elsewhere. Can someone “refresh” my memory? —R.M.B.

A. I see no NEC violation with this arrangement, although it might not be good practice. Sec. 384-16 (a) says to protect the panelboard with not more than two main circuit breakers or two sets of fuses, with a combined rating not more than the rating of the panelboard. Exception No. 1 says the overcurrent protection for the panelboard may be in the feeder supplying the panelboard, if the overcurrent rating is not more than the rating of the panelboard. Since the 100A switch/CB is rated less than the 200A panelboard, it meets NEC requirements. —G.C.

A. There’s no Code violation in connecting a 200A panelboard to a 100A feeder. The feeder’s protective device, consisting of a fused switch or circuit breaker, will properly protect the 200A-rated bus bars in the panel. In fact, it will “overprotect” them. Although such arrangements “pass Code,” they are not advisable, because the nameplate on the panelboard may mislead someone looking for capacity to serve new loads. This can be a booby trap for the inexperienced. Good engineering practice would be to match feeders to panelboard nameplate rating, even though the load can be served adequately by a feeder rated below the panelboard—and still pass Code. When price prevails over good engineering design, such skimpy feeders are often chosen. From the first page of the NEC: “compliance...will result in an installation essentially free from hazard but not necessarily efficient, convenient, or adequate for good service or future expansion...” —J.F.F.

A. It’s not a Code violation to protect a 200A, 42-circuit panel with a 100A fusible switch or breaker. Protection for the panel is usually accomplished with this arrangement. Having a smaller feeder than panel bus is a common situation because manufacturers have standardized bus sizes having large gaps in amperage ratings. Standard sizes are usually 100A, 225A, and 400A, for most branch-circuit panels. As engineers, we generally design our feeders, based on actual load. We might have to greatly oversize feeders if we were required to match the bus and feeder sizes. This could have a major economic impact on our projects. One exception might be a large office or industrial plant. Engineers at these facilities usually don’t want to have to worry about changing feeder sizes as the load expands.

A project I did recently is a good example of the large-panel, small-feeder situation. The project had a large room full of individual personal computers, with a separate panel for the room. The owner wanted each computer on an individual (dedicated) circuit. As a result, we needed about 30 computer circuits plus a few general light and power circuits. The total ampere draw was about 65A. Putting in a 200A feeder would have added an unnecessary cost to the project. —J.C.H.

A. It’s not a Code violation to install a protective switch or CB for a panelboard or load center (rated for lower ampacity than the load center), provided the overcurrent device is properly sized to protect the wire serving the board. In fact, there may be cases where you must use a board with higher ampacity, because of varying manufacturer’s requirements, availability of standard bus sizes, or getting 42 spaces (when smaller ampacities are available only with a smaller number of circuits). This arrangement isn’t only legal—it’s quite common. It is similar to oversizing feeders, acceptable even if not always economically prudent.

The value of the overcurrent protection should be based on the load served; the number of circuits based on the quantity needed; and the ampacity of the bus bars based on what is commonly available—but still meeting the minimum ampacity requirements. The rating (ampacity) of the overcurrent devices really has nothing to do with the ampacity of the bus bars, so long as their rating is equal to or greater than the overcurrent device. —M.H.L.

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