Arc Flash Field Marking Under NFPA 70E and NEC 110.16

An arc flash is not an electrocution. It is an explosion, and the label on the door is the only thing that tells a qualified worker how big.

When current arcs across air, it produces temperatures hotter than the surface of the sun, a pressure wave, molten metal, and a fireball, in a few milliseconds. The hazard is thermal and blast, not shock, which is why arc flash has its own boundary, its own PPE, and its own label separate from everything else on the panel.

Shop arc flash and electrical hazard signage

Two documents, two jobs

NEC 110.16 is the code requirement. It requires switchboards, switchgear, panelboards, industrial control panels, meter socket enclosures, and motor control centers that are likely to require examination or servicing while energized to be field-marked to warn qualified persons of potential arc flash hazards. For larger service equipment, it requires additional information on the label.

NFPA 70E is the standard for electrical safety in the workplace. It tells you what the label must say, how the boundaries are calculated, and what PPE the number on the label requires. OSHA does not adopt 70E directly, but enforces electrical safety under its general duty clause and its own electrical standards, and cites 70E as the recognized practice.

What the label has to contain

Under NFPA 70E, equipment likely to require work while energized must carry a label with:

• The nominal system voltage
• The arc flash boundary
• At least one of: the available incident energy and the working distance it was calculated at, the arc flash PPE category, the minimum arc rating of clothing, or a site-specific PPE level

The label comes from an arc flash risk assessment, which must be reviewed at least every five years and whenever changes to the electrical distribution system could affect the results. A label that predates a service upgrade is not a label. It is a guess with a date on it.

The boundaries

The two inner boundaries are about shock and are set by voltage. The outer one is about thermal energy and is set by the available incident energy at the equipment. They are calculated separately, and the arc flash boundary can fall inside or outside the limited approach boundary depending on the system. The diagram shows the common case. Yours may differ, which is the whole reason the label exists.

The number that matters

1.2 cal/cm² is the incident energy at which unprotected skin sustains a second-degree burn. The arc flash boundary is the distance at which energy falls to that level. Everything inside it requires arc-rated clothing and PPE matched to the energy shown on the label.

What gets cited

• Equipment with no arc flash label at all
• Labels from a risk assessment that predates changes to the system
• Labels stating a PPE category without the information needed to select it
• Working space in front of the equipment obstructed, so the boundaries cannot be maintained
• Unqualified persons inside the limited approach boundary

Where signage fits

The arc flash label is a calculated engineering document, and it belongs on the door of the specific equipment it was calculated for. Area signage does a different job: identifying switchgear rooms, disconnect locations, and equipment yards as arc flash hazard areas at the door or the gate, so the person who did not come to work on the panel knows not to lean on it.

Aluminum matters here because these signs live on panel doors in mechanical rooms and on outdoor enclosures, and a peeling label is a label that will be ignored on the day it matters.

Browse arc flash signs or high voltage signs

Sources: NFPA 70E, Standard for Electrical Safety in the Workplace, including equipment labeling requirements; NEC 110.16 (NFPA 70), arc flash hazard warning; OSHA 29 CFR 1910 Subpart S. Boundary distances and incident energy are site-specific and come from an arc flash risk assessment performed by a qualified engineer.