An Amerex A411 and an Amerex 760 are both 20 lb ABC dry chemical extinguishers. Same agent, same 7 in. cylinder, same 803 wall bracket, same 195 PSIG. The A411 is rated 10A:120B:C. The 760 is rated 4A:60B:C — half the Class B coverage from an identical amount of chemical.
That is not a defect. It is the point of Amerex's high flow line, and it is the pair of numbers buyers most often read backward: on a flammable liquid hazard, Class B rating and discharge rate pull against each other, and NFPA 10 sometimes decides which one governs.
What the number in front of the B counts
Amerex lists Class B as "Flammable liquids & gases," with the examples given as "Gasoline, petroleum greases, tars, oils, oil-based paints, solvents, alcohols, propane, and butane." The rating comes from testing, not from cylinder size — Amerex states that its extinguishers are "tested and rated by Underwriters Laboratories or FM Global to UL Standards." We covered what those figures represent in what fire extinguisher UL ratings actually mean.
Two extinguishers holding the same agent in the same capacity can carry very different Class B numbers. Across Amerex's published data the pattern is consistent: hold capacity constant, discharge it faster, and the Class B figure falls.
The Class B ladder in standard portables
In the standard ABC dry chemical line the rungs are 10-B (the 2.5 lb B417T, the 5 lb B500 and B424), 40-B (the 5 lb B402, the 6 lb B443 and B461), 80-B (the 10 lb B456 and B441), and 120-B (the 20 lb A411 and 423). The standard Purple K line runs 2-B at 1 lb, 10-B at 2.5 lb, 30-B at 5 lb, 80-B at 10 lb and 120-B at 20 lb. A 160-B rating appears in the catalog's portable range only in the 30 lb Z-Series Compliance Flow models.
| Model | Agent | Valve | Capacity | UL & ULC rating | Discharge time | Initial range |
|---|---|---|---|---|---|---|
| B456 | ABC | Anodized aluminum | 10 lb | 4A:80B:C | 22 sec | 15–21 ft |
| B441 | ABC | Chrome-plated brass | 10 lb | 4A:80B:C | 22 sec | 15–21 ft |
| B460 | Purple K | Chrome-plated brass | 10 lb | 80B:C | 22 sec | 15–21 ft |
| A411 | ABC | Anodized aluminum | 20 lb | 10A:120B:C | 30 sec | 15–21 ft |
| 423 | ABC | Chrome-plated brass | 20 lb | 10A:120B:C | 30 sec | 15–21 ft |
| A413 | Purple K | Anodized aluminum | 20 lb | 120B:C | 28 sec | 15–21 ft |
| 415 | Purple K | Chrome-plated brass | 20 lb | 120B:C | 28 sec | 15–21 ft |
Two things follow. Class B rating does not move with valve material — the aluminum-valve A411 and the brass-valve 423 are both 10A:120B:C, so that choice is about duty rating, which Amerex states separately as brass "Heavy Duty" against aluminum "Light to Medium Duty." And at 10 lb and 20 lb, ABC and Purple K carry identical Class B numbers, so agent selection turns on the Class A load and on the hazard Amerex assigns each agent. Amerex writes that ABC "smothers and breaks the chain reaction of Class B fires," while Purple K "is particularly effective on Class B flammable liquids, pressurized gases, and Class C electrical fires" and "has been the choice of oil, gas, chemical, and utilities industries as the preferred fire-fighting agent." That is the argument in Purple K vs ABC dry chemical.
Where 40-B starts is not where you would guess
The common sizing error happens below 10 lb. The B402 and the B500 are both 5 lb ABC units with anodized aluminum valves, both at 195 PSIG, both discharging in 14 seconds over 12–18 ft. The B402 is rated 3A:40B:C; the B500, 2A:10B:C — four times the Class B coverage from cylinders that look alike on a shelf. We take that apart in Amerex B402 vs B500.
So "5 lb ABC" is not a specification. If 40-B is your floor, the standard-line options are the B402 or the 6 lb B443 and B461; the 5 lb B424 sits at 2A:10B:C despite its heavier-duty brass valve. Standard Purple K has no 40-B rung at all — it steps from 30-B at 5 lb straight to 80-B at 10 lb in the B460.
When NFPA 10 calls for high flow
Amerex builds a separate high flow line and is direct about why. These extinguishers are "for applications where discharge rates exceeding one pound per second are required as specified in NFPA 10. These extinguishers utilize the same hardware and are manufactured to the same UL specifications as Amerex compliance-rated products." The chart heading is blunter: "HIGH-FLOW—EXCEEDS 1 LB. PER SECOND DISCHARGE RATE." Amerex credits the line with "Twice the flow rate of standard models," says it "Meets DOT requirements," and rates it -65°F to 120°F.
| Model | Agent | Capacity | UL rating | Discharge time | Discharge rate | Discharge range |
|---|---|---|---|---|---|---|
| 720 | ABC | 10 lb | 1A:20B:C | 10 sec | 1.22 lb/sec | 15–25 ft |
| 722 | Purple K | 10 lb | 20B:C | 12 sec | 1.15 lb/sec | 15–25 ft |
| 760 | ABC | 20 lb | 4A:60B:C | 19 sec | 1.14 lb/sec | 15–25 ft |
| 762 | Purple K | 20 lb | 60B:C | 20 sec | 1.08 lb/sec | 15–25 ft |
Read the two tables together. The 720 holds the same 10 lb as a B456 at 20-B against 80-B — lower, in fact, than a 5 lb B402. The 722 is 20-B against the B460's 80-B. The 760 and 762 reach 60-B against 120-B at 20 lb. Each empties far faster than its standard-flow counterpart — the 720 in 10 seconds against the B456's 22, the 762 in 20 against the A413's 28 — and Amerex publishes a 15–25 ft discharge range for all four, against a 15–21 ft initial range for the standard 10 lb and 20 lb models.
That is not a downgrade; it is a different instrument. Amerex splits its range on this line: "'Compliance' extinguishers are designed to meet and exceed state or local fire protection requirements. 'Specialty' extinguishers are intended for use on particular types of hazards." Note that the high flow text says these units are manufactured to the same UL specifications "as Amerex compliance-rated products" — a comparison, not a synonym. Buy high flow because a specification called for the discharge rate, not because it sounds stronger.
Fast Flow, Compliance Flow, and the models that also clear a pound per second
High flow is not the only place Amerex publishes rates above 1 lb/sec. The Z-Series — "the ultimate choice in fire extinguishers for high hazard, frequent use, and extreme environment applications" — splits into Fast Flow, credited with "Quicker knockdown" and "More operator protection," and Compliance Flow, with "Long range" and "Highest UL ratings."
| Model | Agent | Build | Capacity | UL & ULC rating | Discharge time | Avg. discharge rate |
|---|---|---|---|---|---|---|
| 714 | ABC | Compliance Flow | 20 lb | 10A:120B:C | 30 sec | 0.65 lb/sec |
| 717 | Purple K | Compliance Flow | 20 lb | 120B:C | 29 sec | 0.67 lb/sec |
| 791 | ABC | Fast Flow | 20 lb | 4A:40B:C | 18 sec | 1.20 lb/sec |
| 794 | Purple K | Fast Flow | 20 lb | 40B:C | 15 sec | 1.44 lb/sec |
| 795 | Purple K | Fast Flow | 20 lb | 20B:C | 10 sec | 2.00 lb/sec |
| 715 | ABC | Compliance Flow | 30 lb | 10A:160B:C | 34 sec | 0.95 lb/sec |
| 718 | Purple K | Compliance Flow | 30 lb | 160B:C | 34 sec | 0.81 lb/sec |
| 792 | ABC | Fast Flow | 30 lb | 4A:40B:C | 21 sec | 1.60 lb/sec |
| 796 | Purple K | Fast Flow | 30 lb | 40B:C | 16 sec | 1.88 lb/sec |
Three things follow. Every 20 lb and 30 lb Fast Flow model publishes an average discharge rate above one pound per second — the 795 tops the range at 2.00 lb/sec — at the Z-Series 240 PSIG rather than the high flow line's 195 PSIG. But Amerex names only the HIGH-FLOW line as built for NFPA 10 discharge-rate requirements, so where a written specification drives the purchase, confirm which line satisfies it with the specifier or your authority having jurisdiction.
Compliance Flow is where the highest Class B numbers live: the 30 lb 715 at 10A:160B:C and the 30 lb 718 at 160B:C are the highest Class B ratings in the catalog's hand-portable range. Above that, the Z-Series is "AVAILABLE IN 125-AND 250-LB. WHEELED EXTINGUISHERS."
And the "Long range" bullet deserves a footnote: in the published 20 lb and 30 lb data both builds share the same 40–50 ft initial range. At those sizes the difference shows up in the rating, not the reach.
What changes physically at 20 lb
Cylinder diameter is the hinge. Amerex's extinguishers at-a-glance chart puts every 10 lb dry chemical portable — B456, B441, B460, 720, 722, and the Z-Series 711 and 712 — on a 5 in. cylinder, and every 20 lb dry chemical unit on 7 in. That is why the A411, 423, A413, 415, 760 and 762 all list the 803 wall bracket as included and the 810 heavy-duty box-type vehicle bracket as optional, and why the 864 rubber-strap bracket and 859 dolly option first appear at 20 lb in the dry chemical range. Fitment by cylinder size is in which Amerex bracket fits your extinguisher.
The high flow four also share one parts list. Amerex's Parts Book covers the 720, 722, 760 and 762 on a single page — "10 - 20 lb. High Flow Dry Chemical Brass Valve Models" — with valve assembly 11952 for all models, a hose and horn assembly unique to each (.312 on the 720, .250 on the 722, .266 on the 760, .228 on the 762), and hanger loop and downtube changing with capacity. We stock all four hoses in hoses, nozzles and horns and the valve in valves, gauges and downtubes.
Choosing between the tiers
Work backward from the requirement. If a rating is specified, match the rating and ignore capacity: in the standard ABC line 40-B means a B402, B443 or B461, and 80-B means a 10 lb unit in either agent. A specified minimum discharge rate sits outside the rating entirely and points at the high flow or Fast Flow models. If nothing is specified and you are sizing a fuel or solvent exposure yourself, the 20 lb tier at 120-B is the workhorse — and Amerex's Hazard Analysis chart places Purple K at fuel storage, oil-based paint storage, paint spray booths, auto repair shops, and aviation tugs and fueling carts.
Amerex's own note is the right closing caution: "The most current issue of NFPA-10 should be consulted for minimum recommended fire extinguisher types, placement, and travel distances." Placement and travel distance sit alongside type in that sentence: they govern how many units a site needs, not only how large each one is.
What we stock
40-B — the 5 lb B402, the 6 lb B443 and the 6 lb brass-valve B461, all three 3A:40B:C.
80-B — the 10 lb B456 in ABC and the 10 lb B460 in Purple K, with the brass-valve B441 alongside them in ABC dry chemical.
120-B — the 20 lb A411 and brass-valve 423 in ABC; the 20 lb A413 and brass-valve 415 in Purple K.
High flow — all four models and their service parts: 720 and 760 in ABC, 722 and 762 in Purple K.
Z-Series — the 160-B 715 and the fast 795, with the full range in high performance ABC and high performance Purple K. Above 30 lb, see wheeled units.
Sizing a flammable liquid hazard and want a second opinion on the tier or quantities? Send us the hazard and the specification you are working to and we will come back with options and lead time — request a quote.
Sources
Every rating, capacity, discharge time and rate, range, operating pressure, cylinder diameter, bracket listing and quotation above comes from Amerex Corporation's Portable Fire Extinguisher Catalog — the "where to use" and "classes of fire" pages, the "Hazard Analysis" chart, the ABC dry chemical, Purple K dry chemical, Z-Series high performance and HIGH-FLOW dry chemical pages, and the "extinguishers at-a-glance" chart — and from the Amerex Parts Book list for 10–20 lb high flow dry chemical brass valve models. The catalog lists these extinguishers as conforming to "CAN/ULC-S504 - UL 299 & CAN/ULC-S508 - UL 711." Confirm any critical rating or discharge rate against the extinguisher's own label and Amerex's current documentation, consult the most current issue of NFPA 10, and confirm hazard classification and required coverage with a qualified fire protection professional and your authority having jurisdiction.
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