API RP 500 / 505 · Hazardous locations

Hazardous area classification: API RP 500 vs RP 505 explained

Division vs Zone, gas groups, temperature codes, how the API figures turn into an area-classification drawing, and where the NEC, RP 540, and RP 14F/14FZ fit. Explained in our own words, with a worked example.

Need the code text? Read NEC Articles 500–505 in the free NFPA viewer → and get API RP 500/505 from the API standards store →

Division vs Zone — how the two systems line up (concept only)

How often is a flammable atmosphere present?	Division system (NEC 500/501, API RP 500)	Zone system (NEC 505, API RP 505)
Continuously or for long periods	Division 1	Zone 0
Likely during normal operation, repair, or maintenance	Division 1	Zone 1
Only under abnormal conditions (leak, failure), and then briefly	Division 2	Zone 2
Not expected at all	Unclassified	Unclassified

Division 1 spans both Zone 0 and Zone 1, which is why the Zone system allows finer-grained equipment choices. Our own summary, not the text of API RP 500/505 or the NEC. Classification is an engineering decision — follow the stamped area-classification drawing for your facility and verify against the NEC edition adopted by your AHJ.

How the API classification figures are used, step by step

Step	What you do	Watch out for
1	List the flammable materials handled and their properties (SDS / process data)	Vapor density matters — heavier-than-air vapors spread low and far
2	Identify each source of release: seals, vents, drains, flanges, sample points, relief outlets	Normal-operation releases (vents, sample points) drive Div 1 / Zone 1
3	Judge the ventilation: open-air, adequately ventilated enclosure, or inadequately ventilated	The same pump gets a very different envelope indoors vs outdoors
4	Find the RP 500 (or RP 505) figure that matches the equipment, fluid, and ventilation	The figures are recommended starting points — the engineer may extend or reduce with justification
5	Draw the envelopes on plot plans / elevations as the area-classification drawing	Pits, trenches, and below-grade areas often get a more severe rating than the air above
6	Assign gas group and temperature class for each area	Use the worst-case material present in that envelope
7	Select equipment and wiring methods per NEC 501 (Division) or 505 (Zone)	Seals, boundaries, and conduit crossing an envelope edge are where inspections fail

Our own summary, not the text of API RP 500/505 or the NEC. Classification is an engineering decision — follow the stamped area-classification drawing for your facility and verify against the NEC edition adopted by your AHJ.

Gas groups: Division lettering vs Zone lettering (representative gases)

Division group	Zone group	Representative gas	What it means for equipment
Group A	IIC	Acetylene	Most demanding — IIC equipment also covers IIB and IIA
Group B	IIC (or IIB + H2)	Hydrogen	Hydrogen service often needs IIC or a specific "+H2" marking
Group C	IIB	Ethylene	IIB equipment also covers IIA
Group D	IIA	Propane (methane and most hydrocarbons land here too)	Most common group in oil & gas production and refining

The lettering runs in opposite directions: Group A is the most severe, IIA the least. Assign the group from the actual material (NFPA 497 lists material data). Our own summary, not the text of API RP 500/505 or the NEC. Classification is an engineering decision — follow the stamped area-classification drawing for your facility and verify against the NEC edition adopted by your AHJ.

Worked example — electric motor on a hydrocarbon transfer pump, outdoors

Input / step	Decision	Why
Fluid	Light hydrocarbon liquid, vapor heavier than air	Process data / SDS
Source of release	Mechanical seal — leaks only on failure	Abnormal-condition release
Ventilation	Open-air, adequately ventilated	Outdoor pad, no walls
Classification at the motor	Class I, Division 2 (Zone 2 if the site uses RP 505)	Envelope extents come from the matching RP 500/505 figure — read it in the standard
Gas group	Group D (IIA)	Hydrocarbon of the propane family
Temperature class chosen	T3 = 200 °C max surface temperature	Stays well below the autoignition temperature on the SDS
Motor selection	Div 2-suitable non-sparking motor, or explosion-proof if the drawing says Div 1	NEC 501 equipment rules; confirm the nameplate marking

Illustrative example described generally — not a classification for any real facility. Envelope distances are intentionally omitted; take them from the stamped drawing and the standard.

Which document applies when?

Document	What it covers	Where you run into it
API RP 500	Classifying petroleum-facility locations as Class I, Division 1 or 2	Onshore refineries, gas plants, pipeline stations, tank farms using Divisions
API RP 505	The same job using Class I, Zone 0, 1, and 2	Facilities that adopted the Zone system (common with IEC-marked equipment)
API RP 540	Electrical installations in petroleum processing plants — power, grounding, motors, lighting	Refinery and petrochemical electrical design beyond classification
API RP 14F	Offshore fixed and floating facility electrical systems, Division-classified	Production platforms; referenced in federal offshore rules
API RP 14FZ	The offshore companion for Zone-classified locations	Offshore facilities designed to the Zone system
NEC Art. 500 / 501	Division definitions; equipment and wiring for Class I locations	Every onshore US installation in a Division area
NEC Art. 505	Zone definitions and protection techniques (flameproof, increased safety, intrinsic safety, etc.)	US installations using Zones

API documents tell you where the hazard is; the NEC tells you how to wire it. Get the current API edition from the publisher. Our own summary, not the text of API RP 500/505 or the NEC. Classification is an engineering decision — follow the stamped area-classification drawing for your facility and verify against the NEC edition adopted by your AHJ.

Common field mistakes

Mistake	What to do instead
Wiring from memory instead of the area-classification drawing	Get the current stamped drawing before ordering fittings — envelopes change with revisions
Mixing Division and Zone equipment without checking marking rules	The NEC allows some crossover, but only as it permits — verify nameplate markings
Missing a seal where conduit leaves a classified area	Locate boundary seals at the envelope edge on the drawing, then walk it
Choosing the T-code from the pipe temperature instead of the autoignition temperature	The T-class limits equipment surface temperature relative to the material’s autoignition temperature
Assuming a trench or pit shares the classification of the air above it	Below-grade spaces collect heavy vapors and are often classified more severely

📋 Embed this chart on your site — freeGet the code →

Paste this where you want the live chart to appear. It works on any site that allows iframes, and includes a link back to Field PM.

<iframe src="https://www.field-pm.com/embed/charts/hazardous-area-classification-api-500-505" title="Hazardous Areas: API RP 500 vs 505 — Field PM" width="100%" height="620" loading="lazy" style="border:1px solid #e5e7eb;border-radius:12px;max-width:560px;width:100%"></iframe>
<p style="font:13px/1.5 system-ui,sans-serif;margin:8px 0;color:#6b7280"><a href="https://www.field-pm.com/charts/hazardous-area-classification-api-500-505" target="_blank" rel="noopener">Hazardous Areas: API RP 500 vs 505</a> by <a href="https://www.field-pm.com" target="_blank" rel="noopener">Field PM</a></p>

Where the hazard is vs how you wire it

This page explains the standards; it does not reproduce them. API RP 500, 505, 540, and 14F/14FZ are sold by the American Petroleum Institute. NEC Articles 500, 501, and 505 can be read free in the NFPA free-access viewer. Check the editions your AHJ and the facility owner have adopted.

Two questions, two documents. The API practices answer where a flammable atmosphere can exist and how often. The NEC answers how to install equipment and wiring in that space. At a refinery, gas plant, compressor station, or tank farm, the owner’s engineer uses RP 500 (Divisions) or RP 505 (Zones) to produce the area-classification drawing. The contractor builds to that drawing using NEC 501 or 505.

How the figures work. RP 500 and RP 505 contain many figures for common equipment: pumps, compressors, vents, drains, tanks, loading racks, and more. Each shows an envelope around the source of release, adjusted for whether the vapor is heavier or lighter than air and whether the location is open-air or enclosed. The engineer picks the figure that fits, justifies any deviation, and draws the result. Treat the figures as a starting point. They don’t replace engineering judgment.

Gas groups and T-codes. Equipment has to match both the group (how easily the gas ignites and how hard an explosion is to contain) and the temperature class (how hot the equipment surface can get). Hydrocarbon service is usually Group D / IIA, while hydrogen and acetylene are the demanding cases. In the worked example, a T3 motor (200 °C max surface) works because the fluid’s autoignition temperature is well above that.

Beyond classification. API RP 540 covers electrical design in processing plants more broadly: power distribution, grounding, motors, lighting. Offshore, RP 14F (Divisions) and RP 14FZ (Zones) cover platform electrical systems and are referenced in federal offshore regulations. For static and lightning protection at the same facilities, see our API inspection codes guide (RP 2003).

Related field references: NEMA enclosure types (Types 7 and 9 are the hazardous-location ratings), ISA instrument tag letters, and our hot work permit and electrical JSA templates. Field PM keeps the area-classification drawing, RFIs, and seal inspection photos with the job.

FAQ

What is the difference between API RP 500 and API RP 505?+

Both classify locations at petroleum facilities where flammable gas or vapor may be present. RP 500 uses the Class I, Division 1 / Division 2 system that matches NEC Articles 500 and 501. RP 505 uses the Class I, Zone 0 / 1 / 2 system that matches NEC Article 505. Both give recommended envelope figures for common equipment, and the facility engineer applies them to produce the area-classification drawing.

Is Division 1 the same as Zone 1?+

Not exactly. Division 1 covers places where the hazard is present continuously or during normal operation, so it spans both Zone 0 (continuous or long periods) and Zone 1 (likely in normal operation). Division 2 lines up closely with Zone 2 (only under abnormal conditions, and briefly).

How do gas groups A–D compare to IIA, IIB, and IIC?+

Group A (acetylene) and Group B (hydrogen) fall under IIC, Group C (ethylene) corresponds to IIB, and Group D (propane and most hydrocarbons) corresponds to IIA. The lettering runs in opposite directions: Group A is the most severe, while IIA is the least severe. Hydrogen service may call for IIC or a specific IIB+H2 marking.

What does a temperature code like T3 mean?+

The T-code is the maximum surface temperature the equipment can reach. A T3 marking means 200 °C maximum. That temperature has to be lower than the autoignition temperature of the gas or vapor in the area, which you get from the SDS or NFPA 497. Read the NEC temperature-class rules in Article 500 in the free NFPA viewer at nfpa.org/freeaccess.

Who is allowed to classify a hazardous location?+

Area classification is an engineering decision, normally made by the owner or its engineer and documented on a drawing. The electrical contractor installs to that drawing. If what you see in the field (a new vent, a drain, an enclosure wall) does not match the drawing, send an RFI before you install.

Built by a contractor, for the field

Field PM runs daily reports, job costing, QA/QC, and billing in one platform — $99/month with unlimited foremen, QA/QC, and safety free. 30-day trial, no credit card.

Start free trial →

The Field PM platform

Construction project management software →Electrical contractor software →Daily field reporting app →GPS construction time clock →AIA G702 / G703 billing software →Weld mapping & tracking software →

More reference charts

Rated by contractors on