Arc Flash Warning Labels: Why the Confusion?
- Brian Hall & Jeff Kershner
- 4 days ago
- 5 min read
If you work around electrical equipment, you’ve probably noticed that arc flash labels don’t all look the same. Some have orange headers with the word “WARNING” in black. Others have red headers with the word “DANGER” in white.
So which one is correct? Are there codes or standards that require one over the other?
The short answer is yes – but which label to use is not always obvious. In the article below, we will explore some of the reasons for this uncertainty.
Where Arc Flash Labels Come From
Arc flash warning labels are one of the key outputs of an incident energy analysis, commonly called an arc flash study.
The labels are applied to equipment so workers know how to protect themselves from shock and arc flash dangers.
An incident energy analysis calculates how much thermal energy a worker could be exposed to during an arc flash event. That information is used to:
Determine appropriate arc-rated PPE
Establish approach boundaries
Populate equipment warning labels with critical safety data
Professional arc flash studies—and the labels that result—are developed by electrical engineers trained in power system modeling and protective device coordination using software such as SKM, ETAP, or EasyPower.
Sources of the Public’s Confusion
When we visit a facility to conduct an arc flash analysis or talk to customers, we often get questions about arc flash labeling. This is because there is no single checklist for what the “correct label” must look like. Instead, several organizations, some governmental, some not, are involved in electrical safety compliance; only by carefully studying the relevant rules and guidelines (and staying abreast of the periodic changes) can we know the way forward.
Here’s a quick, hierarchical overview of the organizations involved in electrical safety (and consequently label design). Each organization answers a different question; none of them control label design outright:
OSHA electrical safety standards: Stipulates employers must keep employees safe from electrical risks.
→ Reading a hazard label, workers can stay safe
NEC: The installation standard, now has a requirement for applying arc flash hazard labels; developed by NFPA and updated every 3 years; legally binding when adopted by local jurisdictions
→ Experts interpret and understand where to apply a label and where not to
ANSI: Develops consensus standards.
→ Experts can narrow down exactly how to apply rules, guidelines, and best practices
One thing is clear: knowing which label to apply to which equipment in which circumstance is not by any means a simple matter. Rather, it’s a result of a detailed electrical engineering study.
Now, let’s look at the requirements and guidance standards for label placement and design.
What Does the NEC Require?
The National Electrical Code (NEC) addresses arc flash warning labels in Section 110.16, which requires equipment to be field-marked to warn of arc flash hazards. For design requirements, NEC 110.16 refers to code 110.21(B).
NEC 110.21(B) — Field-Applied Hazard Markings
When caution, warning, or danger markings are required, they must:
Be durable for the environment
Use effective words, colors, symbols, or a combination
The key point:
The NEC does not directly mandate specific colors or signal words.
Instead, it includes informational notes referencing:
ANSI Z535.2 – Environmental & Facility Safety Signs
ANSI Z535.4 – Product Safety Signs & Labels
The NEC makes references to hundreds of other Standards throughout the code in the Informational Notes.
Informational notes are not enforceable as requirements of the code, but they often refer to the other Standards used as the basis of the enforceable code language located above them.
This is the source of most of the inconsistency you see in the field.
What Is ANSI Z535?
ANSI Z535 is a family of U.S. standards that defines how safety signs and labels should be designed so hazards are communicated clearly and consistently.
In plain language:
ANSI Z535 is the guidebook for safety colors, signal words, symbols, and layouts.
It is often used as a benchmark for label design.
Relevant ANSI Z535 Standards
ANSI Z535.2 – Facility and environmental safety signs
ANSI Z535.4 – Product and equipment safety labels
ANSI Z535 also standardizes signal words:
DANGER – Will result in death or serious injury
WARNING – Could result in death or serious injury
CAUTION – May result in minor or moderate injury
Voltage Hazards: Danger vs. Warning
There is not a specific voltage threshold to use a DANGER signal word for electrical hazard labels referenced in any standard. However, there are specific situations where there is a specific voltage level requirement for alerting by the NEC and OSHA.
NEC Requirements
For systems exceeding 1,000 volts, the NEC requires permanent signage:
“DANGER — HIGH VOLTAGE — KEEP OUT”
NEC 110.34(C) Locked Rooms or Enclosures, states:
The entrance to all buildings, vaults, rooms, or enclosures containing exposed live parts or exposed conductors operating at over 1000 volts, nominal, shall be kept locked unless such entrances are under the observation of a qualified person at all times.
OSHA Requirements
For systems exceeding 600 volts, OSHA requires permanent signage:
“DANGER — HIGH VOLTAGE — KEEP OUT”
OSHA 1910.303(h)(5)(iii) states:
The following requirements apply to the entrances to all buildings, rooms, or enclosures containing exposed live parts or exposed conductors operating at over 600 volts, nominal:
1910.303(h)(5)(iii)(B)
Permanent and conspicuous warning signs shall be provided, reading substantially as follows:
“DANGER - HIGH VOLTAGE - KEEP OUT.”
It’s evident that OSHA and NEC agree (consistent with ANSI Z535 definitions) that voltages above 1000V clearly fall into the “Danger” category.
We use orange "WARNING" up to 1000V, anything beyond that is red "DANGER". Shock hazards are proportional to voltage; 4160V would supply 9x more current to cross your body vs 480V, 12470V would supply 26x more current vs 480V (the threshold choice between 600V or 1000V likely won’t affect you since there are few common voltages between 480V and 1000V).
How We Apply Rules and Guidelines at Guidant Power
At Guidant Power, we align our arc flash labels with ANSI Z535 principles to create clear, defensible hazard communication.
A Time-Tested Approach
Equipment ≥ 1,000 volts:
→ Red and white DANGER labels
Arc flash incident energy < 40 cal/cm²:
→ Black and orange WARNING labels
While serious injury or death is possible at lower values, this aligns with common industry practice.
Arc flash incident energy ≥ 40 cal/cm²:
→ Red and white DANGER labels
Why 40 cal/cm²? Because 40 cal/cm² without arc-flash PPE is not an injury scenario—it is a fatal exposure scenario.
At this level, PPE is no longer a reliable control, and the hazard clearly meets the ANSI Z535 definition of “Danger.”
The Bottom Line
The NEC requires arc flash warning labels
The NEC references ANSI Z535
ANSI Z535 is the recognized best practice
Consistent use of ANSI Z535 improves clarity, defensibility, and worker understanding
The variety of arc flash labels you see in the field isn’t accidental—it’s the result of standards that reference guidance rather than mandate design. Understanding that distinction helps cut through the confusion.
Have questions about arc flash labeling or would like to schedule an arc flash analysis? At Guidant Power we've conducted over 4,000 arc flash analyses around the country and we're dedicated to keeping your facility and people safe from electrical hazards.
70E®, Standard for Electrical Safety in the Workplace®, NFPA 70®, NEC®, and National Electrical Code® are registered trademarks of the National Fire Protection Association, Quincy, MA. All rights reserved. This informational material is not affiliated with nor has it been reviewed or approved by the NFPA.
