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NEC in the Facility, July 10, 2012

Apply the 125% rule to any branch circuit that has one or more continuous loads

In our previous issue, we noted the NEC requires you to apply the 125% rule to any branch circuit that has one or more continuous loads. It's acontinuous load when "the maximum current is expected to continue for three hours or more" [Art. 100]. That rule applies for both conductor sizing [210.19(A)1] and overcurrent protective device (OCPD) sizing [210.20(A)]. We also mentioned that the ampacity tables address only the minimum ampacities required for safeguarding personnel and property [90.1]. This leads to an important consideration for facilities.

Unlike residential structures, commercial and industrial facilities tend to undergo reconfiguration. Over time, loads may change dramatically. Machine moves are common in many manufacturing facilities because of changing production demands. Office buildings rearrange cubicle farms or in some other way reconfigure branch circuit loads.

Consider an office where a large meeting room is repurposed because the business needs to run DVD copier/printers to take advantage of a lucrative new opportunity. If all of the conductors and OCPDs already meet or exceed the 125% rule requirements, the physical plant won't impede the business needs of the organization. But what if you had "saved money" by designating some circuits, and those in particular, as noncontinuous load circuits? You must either derate or rewire.

Planning ahead for operational flexibility prevents confusing user restrictions and expensive rework.

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