Capacitors, contactors, and 24V transformers are three of the cheapest parts on your truck and three of the most common reasons a routine call turns into a second trip. None of them cost much, all three fail constantly, and none of them come in just one size. Carrying the right spread of ratings is what turns "I'll have to order that and come back" into a same-visit fix.
Capacitors: The Sizes That Cover Almost Every Call
Most residential AC and heat pump systems use a dual run capacitor in the outdoor unit, which is really two capacitors in one can sharing a common terminal. The label shows both values, 45+5 uF for example, where the larger number feeds the compressor (terminal HERM) and the smaller one feeds the condenser fan motor (terminal FAN), sharing a common terminal (C).
The sizes below cover the large majority of residential tonnages:
- 1.5 ton: 30/5 uF
- 2.0 ton: 35/5 uF
- 2.5 ton: 40/5 uF
- 3.0 ton: 40/7.5 or 45/5 uF
- 3.5 ton: 45/5 or 50/5 uF
- 4.0 ton: 50/7.5 or 55/5 uF
- 5.0 ton: 60/5 or 70/5 uF
Almost all of these are rated 370/440 VAC, and a 440V-rated capacitor can always replace a 370V original, never the other way around. Round out the bin with single run capacitors (5, 7.5, 10, 15, 20, and 25 uF) for blower and fan-only motors, and stick to parts within +/-5% of their rated microfarad value.
Contactors: Poles, Coils, and Amp Ratings to Keep On Hand
Contactors fail almost as often as capacitors, usually from pitted or welded contacts after years of arcing, and they're just as cheap to carry in bulk.
- 1-pole (single pole), 24V coil, 30-40A: the most common residential condensing unit contactor
- 2-pole (double pole), 24V coil, 30-40A: used on many heat pumps and some straight-cool condensers
- 3-pole, 24V or 240V coil, 40-60A: for light commercial rooftop and package units
Stock double what you think you'll need in 1-pole and 2-pole, 24V-coil, 30-40A contactors, that combination clears the vast majority of residential service calls. Keep one or two 3-pole units on hand if you run any light commercial routes.
Transformers: VA Ratings and the 24V Load That's Growing
Every 24V control circuit runs through a step-down transformer, and undersizing one is a callback waiting to happen. A basic single-stage system draws light current, but smart thermostats, zone board relays, communicating controls, and Wi-Fi-connected accessories all pull from the same 24V leg, which is why 50VA and 75VA transformers are showing up more often than the old 40VA standard.
- 40VA: basic single-stage furnace or air handler, no accessories
- 50-75VA: systems with a smart thermostat, humidifier, or zone panel
- 100VA: multi-zone systems with several relays and dampers
Carry both 120V-primary and 208/240V-primary versions (or dual-primary units that cover both) stepping down to 24V secondary, and check the VA rating on the equipment nameplate or the failed transformer before swapping. A 40VA replacement on a zoned system will nuisance-trip the moment the second zone calls.
A Sample Truck Stock List
This is a reasonable starting point for a residential-focused service van; adjust quantities to your call volume and route mix.
- Dual run capacitors (370/440V): two each of 35/5, 40/5, 45/5, and 50/5 uF; one each of 55/5 and 60/5 uF
- Single run capacitors: two each of 5, 7.5, and 10 uF
- Contactors: three 1-pole 24V/30A, three 1-pole 24V/40A, two 2-pole 24V/40A, one 3-pole 24V/40A
- Transformers: two 40VA (120V/24V), two 40VA (208-240V/24V), two 75VA multi-tap
Keeping Truck Stock From Turning Into Truck Clutter
- Buy UL-recognized parts with a tight tolerance (+/-5%). A capacitor that's "close enough" shortens compressor life instead of extending it.
- Cross-reference OEM part numbers (Carrier, Trane, Goodman, Rheem, Lennox) before you restock, so one bin covers multiple brands instead of five separate SKUs.
- Restock capacitors and contactors weekly, they're your fastest movers, and check transformers monthly since they turn over more slowly.
- Label the bin by rating, not by brand, so a rushed pick between calls doesn't mean grabbing the wrong microfarad.
CoralVolt stocks dual run and single run capacitors, 1-, 2-, and 3-pole contactors, and 24V transformers with OEM cross-references built into every listing, so restocking the truck is one order instead of three.