Definite purpose contactors are one of the easiest HVAC parts to cross — and one of the easiest to get wrong, because two of the numbers printed on the side mean completely different things. Below: 572 interchange rows from the Supco, MARS and Packard published cross-references, plus the selection rules that actually matter.

What has to match

Why your “1-pole” has two legs on it

This confuses people at the counter every week, and it is not their fault.

What everybody calls a 1-pole HVAC contactor is actually a 1½-pole, or “1-pole with shunt.” It has one switched pole plus a permanently jumpered bar for the other leg. Both L1 and L2 land on it — one gets switched, one does not. That is why it drops straight into a 2-pole footprint and why the wiring looks the same at a glance.

Different brands print it differently for the same part: MARS says “1 Pole with Shunt,” Supco says “1.5 Pole,” Jard says “1-1/2 Pole,” Totaline says “Single Pole with Shunt.”

Can a 2-pole replace a 1-pole?

Yes, and it is the better part. It does strictly more — it opens both legs instead of one — it fits the same holes in the 30–40 A sizes, and it costs a few dollars more. The only difference is where the wires land: on the 1½-pole both legs go to the same side with one on the shunt bar; on a 2-pole you land one leg per pole.

The reverse is not an equivalent. Do not put a 1½-pole where a 2-pole was unless you know the equipment was designed for it.

Copeland goes further and recommends against the unswitched-leg approach for reliability as well as safety — they note evidence that unexplained spring-startup compressor failures traced back to winter lightning finding a path through the unbroken line. Two-pole contactors are cheap insurance against that.

FLA vs resistive amps — the number people get wrong

Look at the side of a contactor and you will usually see two current ratings, something like 30 FLA / 40 Res. They are not two ways of saying the same thing.

Published rating families run like this. Note that LRA falls as voltage rises — a definite purpose contactor holds roughly 6× its FLA at 230 V, 5× at 460 V and 4× at 575 V, which is one of the real differences between a DP contactor and a general purpose one.

Two of those resistive figures are published differently by different manufacturers — MARS lists 63 at 50 FLA where Square D and Titan Max list 65, and Titan Max lists 90 at 75 FLA where the others list 94. Use whatever the brand in your hand publishes.

Picking the right amp rating

The manufacturer method is nameplate-based, not tonnage-based: take the compressor’s rated load amps off the data plate, add the condenser fan motor amps, pick the next amp rating above that total, then check that the contactor’s locked-rotor rating clears the compressor’s LRA.

You will find tonnage charts all over the internet — roughly 30 A for 2½ to 3 ton, 40 A for 3½ to 4 ton, 40 to 50 A for 5 ton. No manufacturer publishes one. They are a reasonable starting point for what to pull off the shelf and nothing more. Confirm against the data plate before you buy.

Is going bigger harmful? No manufacturer document says a larger contactor damages anything. The limits on oversizing are physical rather than electrical — above 40 A the body gets bigger, the mounting changes and the lugs change. Match or exceed; one size up is fine if it fits and your wire lands.

Decoding the part number on your old contactor

Most of these tell you the spec if you know how to read them.

Lennox is the exception — their contactor numbers are short opaque codes with no consistent prefix, so there is nothing to decode. Bring the part.

Brand interchange grid 39 Supco parts × 19 brands

From Supco’s published cross-reference. Find any number in any column and read across for the equivalent in every other brand.

Swipe the table sideways to see all brands →

MARS ↔ Furnas / Siemens with full specs 112 rows

From the MARS 780 catalog. Every row carries the poles, both amp ratings, locked rotor, coil voltage and terminal type — so you can confirm the match rather than trust it.

Swipe the table sideways to see all columns →

Titan Max ↔ MARS ↔ Furnas 90 rows

Packard’s Titan Max line, now under DiversiTech. Note that Packard, DiversiTech and Titan Max are one family rather than three independent brands.

Swipe the table sideways to see all columns →

Ants: the Dallas failure mode

If you have pulled a contactor apart in North Texas and found it packed with dead ants, you are not imagining a local problem.

The tawny crazy ant is established across Texas, Louisiana, Mississippi and Florida. They are under an eighth of an inch and they get into contactor housings. Texas A&M AgriLife describes the mechanism: an ant gets shocked crossing the contacts, releases an alarm pheromone that calls in more ants, and those get electrocuted too. The carcasses build up between the contacts and physically stop them closing. Fire ants do the same thing.

An open-frame contactor has nothing to keep them out. If this has happened to you once at a given address, it will happen again — and a sealed or enclosed contactor is the part to ask for. Manufacturers are blunt that half-measures do not work: arc covers, dust covers and plastic noise shields reduce the problem but ants still get in.

Worth knowing before you buy the same open contactor a third time.

Testing a contactor

Contactor, capacitor, or something else?

The clean way to think about it: a contactor is a switch. If it is passing full line voltage downstream and the compressor still will not run, the contactor did its job. Look at the capacitor.

Looking for CNT09648 or CNT05482?

This trips people up constantly because CNT and CTR look alike on a parts list. If the number on your part starts CNT, you are holding a board, and a contactor cross-reference will not help you.

Frequently Asked Questions

Sources: Supco definite purpose contactor cross-reference (F155-040417); MARS 780 catalogue; Packard Titan Max cross-reference; Square D 8910 catalogue; Copeland application engineering bulletin AE10-1244 on contactor selection; White-Rodgers SureSwitch literature; Texas A&M AgriLife and ACHR News on tawny crazy ants. Manufacturers publish these as functional equivalents rather than exact matches — verify poles, coil voltage, amp rating and terminal type before installing. Compiled July 2026.

Related: How to replace an AC contactor · Capacitor cross-reference · AC clicking but not starting · Motor cross-reference