1. Two Standards, One Question
"How far apart do two compressor buildings need to be?" has no single code-mandated number, because two different standards answer two different questions about the same gap:
- NFPA 37 (Stationary Combustion Engines and Gas Turbines) sets the minimum fire separation between detached engine buildings. This is the hard distance requirement.
- API RP 500 (Classification of Locations for Electrical Installations) does not mandate a separation — instead it tells you how the electrical area in and around the gap is classified (Class I, Division 1 or Division 2) at whatever spacing you choose.
So the workflow is: take the NFPA 37 minimum as the floor, then check what API RP 500 classification falls out of your chosen spacing — and whether that classification is acceptable and what conditions it imposes (relief-vent location, ventilation interlock, rated electrical in the gap). Owner/insurer facility-siting standards and, in the U.S., 49 CFR 192 for gas stations can add further spacing on top.
The honest answer is a decision tree, not a lookup
The governing API RP 500 figure depends on whether the gas is lighter- or heavier-than-air and whether the building is adequately ventilated. The standard itself states that vent and relief envelopes are "too diverse to specify distances" and require engineering judgment. A good tool gives the floor, the governing figure, and the conditions — never a fabricated single number.
2. Fire Separation: NFPA 37 & the IBC
NFPA 37 §4.1.2.2.2 (dedicated detached structures) and §4.1.4 (engines located outdoors) set the separation between an engine building and adjacent structures:
| Condition | Minimum separation | Basis |
|---|---|---|
| Standard case — from openings and combustible walls | 1.5 m (5 ft) | §4.1.2.2.2 / §4.1.4.1 |
| Facing wall of either structure has ≥1-hour fire-resistance rating | 0 ft (may be adjacent) | §4.1.2.2.2(1) / (2) |
| The structure is protected by an automatic fire-protection system | 0 ft (may be adjacent) | §4.1.2.2.2(3) |
| Reduced clearance justified by engineering calculation (outdoors) | < 5 ft (case-specific) | §4.1.4.2.4 |
Noncombustible ≠ fire-resistance rated. The 0-ft exception requires a tested ≥1-hour fire-resistance-rated wall assembly (ASTM E119 / UL 263), not merely noncombustible construction or noncombustible insulation (e.g. mineral wool added for acoustics). A standard metal-panel wall with acoustic insulation is noncombustible but is not a rated assembly unless built and documented as one — so the 5 ft minimum still applies.
NFPA 37 §4.1.1.2 separately requires that the combustion-air supply be arranged so it cannot draw flue/exhaust gas from another unit — relevant when two buildings face each other across a narrow gap.
The IBC building-code path — often the real governing driver
In the U.S., the International Building Code usually controls, because a gas compressor building over 1,000 ft² is typically classified Group H-2 (flammable gas above the maximum allowable quantity). Two provisions apply:
- IBC §415.6.4.2: a Group H-2 occupancy over 1,000 ft² (not required to be in a detached building) shall be set back not less than 30 ft. This is a flat requirement — a fire-rated wall does not waive it.
- IBC Table 705.5 sets the exterior-wall fire-resistance rating by fire-separation distance. For Group H, Type IIB (an unprotected steel pre-engineered building): a 1-hour wall at 10–<30 ft, and 0-hour (no rating) at ≥30 ft.
| Fire separation distance | Group H, Type IIB exterior wall |
|---|---|
| < 5 ft | 3-hour |
| 5 – <10 ft | 2-hour |
| 10 – <30 ft | 1-hour |
| ≥ 30 ft | 0-hour (none) |
For a typical U.S. gas compressor building, 30 ft is the governing number — it satisfies the H-2 setback (§415.6.4.2) and lets the plain metal walls go unrated (Table 705.5). NFPA 37 (5 ft) and the API RP 500 classification are less conservative; the most restrictive driver — here the IBC — wins. These provisions are stable across editions (the same 30 ft and wall-rating bands appear in IBC 2009 as §415.3.1 and Table 602). Confirm the occupancy classification and construction type with the AHJ.
3. The Adequate-Ventilation Test
Everything in the API RP 500 classification hinges on one test. Per §6.3.2.4.2, an enclosed area is adequately ventilated when it is provided:
Airflow ≥ 1.0 cfm per ft² of floor area AND ≥ 6 air changes per hour
Evaluate both and take the larger required airflow:
Qreq = max( 1.0 × Afloor , 6 × Vbuilding / 60 ) [cfm]
Afloor = floor area (ft²); Vbuilding = enclosed volume (ft³)
If the airflow available in the worst-case operating mode (for example, one ventilation fan running) meets Qreq, the building is adequately ventilated and its interior is Class I, Division 2. If not, the interior is classified Division 1 to the extent of the building. Because the classification depends on ventilation continuing to run, a loss-of-ventilation alarm/interlock (and often gas detection) is what allows the favorable Division 2 basis to be relied upon.
4. Classifying the Building
The governing API RP 500 figure — and the size of the classified envelope around the building — depends on the gas density and the ventilation result:
| Gas / vapor | Ventilation | Interior | Exterior Division 2 extent | Figure |
|---|---|---|---|---|
| Lighter-than-air (natural gas) | Adequate | Division 2 | ~15 ft (4.5 m) | Fig 23 (§9.2.2.1) |
| Lighter-than-air | Inadequate | Division 1 | 10 ft (3 m) Div 1 transition, then Div 2 to ~15 ft | Fig 25 / §6.2.3.4 |
| Heavier-than-air (NGL, propane) | Adequate | Division 2 | ~25 ft (7.5 m) + 50 ft (15 m) blanket at 2 ft (0.6 m) above grade | Fig 20/21 (§9.2.1.1) |
| Heavier-than-air | Inadequate | Division 1 | 10 ft Div 1 transition, then Div 2 to ~25 ft + grade blanket | Fig 22 / §6.2.3.4 |
Why density matters. Lighter-than-air natural gas (methane) is buoyant — it rises, so the critical release points are roof ridge vents and high openings, and the upward classified zone matters more than the gap at grade. Heavier-than-air vapors (NGL, propane, rich gas) pool and drift along grade, so their classified area includes a large low-lying blanket; keep trenches, sumps, and pits out of the gap.
These compressor-shelter-specific figures govern over the generic "5 ft past openings" rule of §6.2.3.6, which §6.2.3.6 itself defers to Sections 8–14.
5. Classifying the Gap Between Two Buildings
Each building projects its exterior Division 2 envelope (radius R2) toward the gap. Compare the spacing S to twice that extent:
| Condition | Result in the gap |
|---|---|
| S ≥ 2×R2 | Division 2 extends R2 from each building, leaving an unclassified buffer of (S − 2×R2) in the middle |
| S < 2×R2 | The two Division 2 envelopes overlap — the entire gap is Division 2 |
| S < 2×R1 (inadequately ventilated only) | The Division 1 transition zones overlap — Division 1 reaches into the gap center |
A fully-Division-2 gap between two adequately-ventilated compressor buildings is a normal, code-compliant condition — not a violation. Its practical consequence is that any electrical in the gap (lighting, junction boxes, instruments, conduit, disconnects) must be Division 2-rated, and no non-classified ignition sources may sit between the buildings.
6. Relief & Blowdown Vents
Relief, blowdown, and PRV discharges are separate release sources, not building ventilation openings. Two API RP 500 provisions matter:
- §8.2.3.4.2: a relief vent discharging inside an enclosure classifies the entire enclosure Division 1. Relief/blowdown/PRV must discharge outside and up a stack.
- §8.2.3.4.1: the envelope around an outdoor relief discharge is "too diverse to specify distances" — it requires engineering judgment for the actual discharge (rate, pressure, molecular weight, height), but in no case less than Figure 18.
Locate and elevate the discharge so its plume zone does not reach the other building's openings or air intakes — this, rather than the building-to-building distance, is often the item that actually constrains the layout.
7. Separation to Other Structures
The compressor building also has to sit a safe distance from things that are not compressor buildings. The fire-separation and electrical-keep-out pieces still apply, but what governs changes with the neighbour:
| Neighbour | What governs the separation | What the calculator gives |
|---|---|---|
| Another compressor building | NFPA 37 + API RP 500 gap classification (both are gas sources) | Full answer (Sections 2–5) |
| Occupied building (control room, MCC, manned) | API RP 752 consequence-based siting (explosion / fire / toxic), plus the Division 2 keep-out | Grounded floor only; flags the RP 752 study |
| General building (warehouse, shop) | NFPA 37 fire separation + Division 2 keep-out; NFPA 30 / insurer spacing if it stores flammables | Fire separation + keep-out |
| Production / storage tank | The tank's own classification (vent / thief-hatch Div 1/2) + NFPA 30 tank spacing + dike | Compressor-side keep-out; directs to the tank tools |
The electrical keep-out (every neighbour)
Whatever sits next to the compressor building must keep its ignition sources and air intakes outside the compressor's Division 2 envelope (~15 ft for an adequately-ventilated lighter-than-air gas shelter). That envelope is the universal floor — the minimum any non-source neighbour must clear.
Occupied buildings: why API RP 752 governs
A control room, MCC, or any manned building cannot be sited from a fixed table. API RP 752 requires a consequence-based evaluation: model the explosion overpressure (a vapour-cloud-explosion analysis) reaching the building, then compare it against the building's vulnerability (RP 752 Appendix C — overpressure vs. building damage and occupant injury). The required distance — or the decision to use blast-resistant construction — comes out of that study, not out of this calculator. The tool returns the grounded floor and flags that the RP 752 study still governs on top of it.
Don't use the two-building model for a tank. A tank is a different release source with its own classified zone and its own fire / dike spacing (NFPA 30, API 2610). Use the Electrical Area Classification calculator for the tank's envelope, and tank-spacing standards for the separation distance.
8. Worked Example
Two identical engine-driven natural gas compressor buildings, each 60 ft × 40 ft × 24 ft eave, proposed 30 ft apart. Worst-case ventilation = 12,000 cfm. Standard metal walls (no fire rating, no sprinkler). Relief vents outdoors.
Step 1 — NFPA 37 minimum
No fire-rated facing wall and no sprinkler, so the exception does not apply → minimum = 5 ft. Proposed 30 ft ≥ 5 ft → compliant, margin 25 ft.
Step 2 — Adequate-ventilation test
Afloor = 60 × 40 = 2,400 ft²; V = 2,400 × 24 = 57,600 ft³
Qreq = max( 1.0 × 2,400 , 6 × 57,600 / 60 ) = max( 2,400 , 5,760 ) = 5,760 cfm
Provided 12,000 cfm ≥ 5,760 cfm → adequately ventilated (5.0 cfm/ft², 12.5 ACH)
Step 3 — Classification
Lighter-than-air + adequately ventilated → API RP 500 Figure 23: interior Class I, Division 2; exterior Division 2 to 15 ft.
Step 4 — Classify the gap
2 × R2 = 2 × 15 = 30 ft, exactly equal to the 30 ft spacing → the two Division 2 envelopes meet at the centerline, so the entire 30 ft gap is Class I, Division 2 with no unclassified buffer. Spacing the buildings closer would keep the gap fully Division 2; spacing them farther than 30 ft would open an unclassified buffer of (S − 30) ft in the middle.
Result
30 ft is compliant and acceptable: NFPA 37 (5 ft) is met with a wide margin, and a shared Division 2 gap between two adequately-ventilated compressor shelters is a normal condition. The governing items are the conditions: keep ventilation interlocked, make gap electrical Division 2-rated, and vent relief outside and away from the gap.
9. Good Practice & Conditions
- Provide a loss-of-ventilation alarm/interlock (± gas detection) so the Division 2 basis holds — without it the interior reverts to Division 1.
- Make all electrical in a Division 2 gap Division 2-rated; keep ignition sources out of the gap.
- Vent relief/blowdown/PRV outside and up a stack, located so the plume zone does not reach the other building's openings or intakes.
- Orient intake louvers, ventilation exhausts, and engine-exhaust stacks so they do not face each other across the gap (NFPA 37 §4.1.1.2).
- Check for an owner/insurer facility-siting standard (e.g. FM Global) and, in the U.S., 49 CFR 192 station requirements — these can exceed the code minimum.
- This is a screening tool — confirm on a stamped area-classification drawing and a formal facility-siting study.
Figure dimensions. The 15 ft / 25 ft / 10 ft envelopes here are the API RP 500 (4th ed., 2023) figure values for the stated cases. Confirm the exact geometry (horizontal vs. upward extent, and the point of measurement) against the actual figure before it goes on a stamped classification drawing.
Frequently Asked Questions
How far apart do natural gas compressor buildings need to be?
NFPA 37 §4.1.2.2.2 requires detached engine/compressor buildings to be at least 1.5 m (5 ft) apart, measured from openings and combustible walls. That minimum drops to 0 ft if a facing wall has at least a 1-hour fire-resistance rating or a building is protected by an automatic fire-protection system. API RP 500 does not set a separation distance; it classifies the electrical area in the gap.
What standards govern compressor building spacing?
NFPA 37 governs fire separation of stationary combustion engine buildings, and API RP 500 governs electrical area classification (Division 1/Division 2) around and between them. Owner or insurer facility-siting standards and, in the U.S., 49 CFR 192 for gas compressor stations may impose additional spacing.
Is the gap between two compressor buildings a classified hazardous area?
If the buildings are adequately ventilated, each is Class I, Division 2 with an exterior Division 2 zone of about 15 ft (4.5 m) for lighter-than-air natural gas per API RP 500 Figure 23. Buildings closer than twice that extent have overlapping envelopes, so the whole gap is Division 2 — acceptable, but all electrical in the gap must be Division 2-rated.
What makes a compressor building adequately ventilated under API RP 500?
API RP 500 §6.3.2.4.2 defines adequate ventilation as at least 1.0 cubic foot per minute of air per square foot of floor area AND at least 6 air changes per hour. Meeting it keeps the interior Division 2; failing it classifies the interior Division 1 and adds a 10 ft Division 1 transition zone outside the openings.