Why NFPA 70E Labels Aren’t Always Consistent

Question:
Some arc-flash labels list the arc-flash boundary in inches, others in millimeters or meters, and some even show odd values like 3.4 feet. On top of that, IEEE 1584 uses joules per square centimeter (J/cm²), but PPE ratings are in calories per square centimeter (cal/cm²). Why is there so much inconsistency on NFPA 70E arc flash / equipment labels?

Answer:
It looks messy, but there’s a logical reason behind it. The short version is this: IEEE 1584 is based on metric units, and the end user prefers imperial units (inches etc.) for NFPA 70E labels. However, the units used on arc flash labels still seem to vary widely

IEEE 1584 Is Metric by Requirement

IEEE as well as many other standards requires the use of the metric system in its equations. As a result, IEEE 1584:

• Arc-flash boundary distances are calculated in millimeters
• Incident energy is calculated in joules per square centimeter (J/cm²)

This isn’t optional. IEEE standards are written to be internationally applicable, and the metric system is required for consistency. It should be noted that IEEE 1584 is a global standard and some countries or regions do prefer metric units.

So when an arc-flash study is performed, the raw calculation results coming out of IEEE 1584 are always metric — even if you never see those units on the final report or label.

Why Labels Show Inches, Feet, or Decimals

NFPA 70E does not mandate a specific unit system for arc-flash labels. What NFPA 70E 130.5(H) does require is that electrical equipment be labeled with meaningful arc-flash information so workers can recognize the hazard and take appropriate protective measures.

To meet that requirement, arc-flash software typically converts:

• Millimeters into inches, feet, or meters
• Based on user preference, company standards, or regional practice

That’s why you may see:

• 42 inches on one NFPA 70E label
• 1.1 meters on another
• 3.5 feet on a third

The math is correct — it just wasn’t rounded to a “nice” number after conversion. From an NFPA 70E standpoint, that’s acceptable as long as the information is accurate and clearly communicated.

Incident Energy: J/cm² vs cal/cm²

The same issue shows up with incident energy.

IEEE 1584 is used to calculate incident energy in J/cm², where:

• One joule equals one watt-second

However, PPE Ratings defined by standards such as ASTM and IEC and referenced in NFPA 70E are based on cal/cm², and arc-rated clothing and face shields are tested and labeled in those units.

Because NFPA 70E requires PPE selection based on incident energy exposure, arc-flash software converts:

• J/cm² into cal/cm²
• So the value on the NFPA 70E label matches PPE ratings and hazard categories

Again, the inconsistency isn’t an error — it’s a necessary translation between calculation standards and safety requirements.

What NFPA 70E Actually Cares About

NFPA 70E 130.5(H) doesn’t care whether a distance is shown in inches, feet, or meters. It cares that:

• The arc-flash boundary is clearly identified
• The incident energy or PPE information is accurate
• The label supports the arc-flash risk assessment

As long as the label addresses the requirements of NFPA 70E 130.5(H) and aligns with the arc-flash study, the units are technically a secondary concern.

The Real Problem: Confusion, Not Conversion

Where this becomes an issue is worker interpretation. A label that mixes unfamiliar units or odd decimal values can:

• Slow down decision-making
• Increase the chance of misreading a boundary
• Reduce confidence in the NFPA 70E labeling program

That’s why many companies standardize:

• One distance unit system
• One rounding convention
• One label format across the facility

This isn’t required by NFPA 70E — but it’s often a very good idea.

Bottom Line

The unit differences on arc-flash labels exist because:

• IEEE 1584 requires metric units
• NFPA 70E requires usable safety information
• Software bridges the gap by converting units for labels

As long as the label meets NFPA 70E 130.5(H) requirements and accurately reflects the arc-flash study, the units are technically correct — even if they don’t always look consistent.