The labeling requirements in 110.16 really put the pressure on for electrician to step up their game. In the 2017 NEC^®, in other than dwelling units, service equipment rated 1200 amps or more must be marked with certain items that can only be obtained by an electrician who has an understanding of overcurrent device trip characteristic and available fault current.

When a 1200 amp or larger electric service is installed at other than dwelling units, the following markings must be field or factory applied to the service equipment:

1. Nominal system voltage.
2. Available fault current at the service overcurrent protective devices.
3. The clearing time of service overcurrent protective devices based on the available fault current at the service equipment.
4. The date the label was applied.

Item 1 above is no big deal. It is expected that the installing electrician will know the nominal system voltage for the equipment being installed.

Item 2 has basically been required anyway since the 2011 NEC^® in section 110.24(A) so this is not a major change. To obtain the available fault current (AFC) at the service, most electrician will contact the local utility company to find out the maximum AFC that can be delivered from the transformer secondary to the service.

Item 3 is the game changer. In order to accurately mark the clearing time of the overcurrent device, the electrician will need to understand the time-current curve charts for the specific overcurrent device being used at the 1200 amp or larger service. Take notice that the NEC^® does NOT require an electrical engineer to be involved with this one. When the NEC^® requires an electrical engineer to get involved, the NEC^® states just that. See sections 110.22(B), 110.71, 210.19(B)(2), 215.2(B)(3), 240.86(A), 240.92(C)(1)(3), 240.100(A) for just a few examples of many locations where the NEC^® requires an engineer to be involved.

If multiple overcurrent devices with different clearing times are used at the service, each overcurrent device will need to be labeled with its specific clearing time. This does not mean just taping the time-current curve chart to the service for the inspector to figure out.

The time-current curve chart provides many different clearing times based on the magnitude of the fault condition. The larger the fault, the faster the overcurrent device will operate which means a quicker clearing time.

Even though the code section allows the label to be field or factory applied, the actual clearing time is not known at the factory since it is a variable numeric value based on field installation conditions such as length, size and properties of the service entrance conductors. All of these characteristics have an impact on the final available fault current which is not known at the factory before the equipment is shipped out to the job site.

Item 4 is no big deal. Just mark the date the label is installed. This helps to date stamp the installation should any changes occur in the future.

The exception allows no special marking if an arc flash label is applied in accordance with acceptable industry practice. This is not to be confused with the arc-flash warning label required in NEC^® 110.16(A) which only serves to warn that an arc-flash is possible. The label expected by the exception would be a label with specific values such as the arc-flash boundary, voltage, PPE category, or minimum arc rating of clothing such as those required in NFPA 70E which is the Standard for Electrical Safety in the Workplace.

See the actual NEC^® text at NFPA.ORG for the complete code section. Once there, click on the “free access” tab and select the applicable year of NFPA 70 (National Electrical code).