ABB Contactor Selection Checklist: A110-30, Reversing, and 12V Coil Units

I'm a quality/brand compliance manager at an electrical equipment company. I review every contactor order before it ships—200+ unique items a year, roughly. In our Q1 2025 audit, I rejected about 6% of first-pass deliveries because of mismatched coil voltages or missing documentation. This is the checklist I use when someone specifies an ABB contactor.

Use this if you're ordering, installing, or QC-ing an ABB contactor—especially an A110-30, a reversing contactor, or a 12 volt contactor. It won't replace a full engineering review, but it'll catch the mistakes that cause the most rework.

Step 1: Decide Whether You Need a Contactor or a Relay

People search "what is an electrical relay" because the line between relays and contactors looks blurry. Both are electrically operated switches. The difference is what they're built to switch. A relay is typically for control signals—5A, 10A, signal-level loads. A contactor is built for power loads: motors, heaters, lighting, compressors.

Here's the twist: a 12V contactor still counts as a contactor because the 12V is just the coil voltage. The main contacts can handle real current, often 40A or more. I've seen people try to use a 12V relay for a DC motor pull-in circuit, and it works—until the load spikes and the contacts weld. If the load is above roughly 10A or the inrush is high, a contactor is the safer call.

Step 2: Decode the Model Number, Then Verify It

A common request is the ABB A110-30 contactor. In the ABB-contactor A-line naming, the "A" means the general-purpose contactor series, and the number after it is usually tied to the rated operational current in AC-3 use. The "-30" part tells you the main contact configuration: three main poles and no main normally-closed poles. But don't take my word for it—especially for an order, get the full model number and check the data sheet. "A110-30" alone might be missing an auxiliary contact suffix like "-00" or "-11".

There's a real cost to skipping this. I once rejected a batch of contactors where the model number on the packing list matched, but the actual nameplate showed a different coil configuration. That cost the supplier a redo and cost us a two-week delay. Now every spec I review includes the full catalog number and the corresponding technical data sheet.

Step 3: Confirm the Coil Voltage Before You Wire It

I see the 12 volt contactor requested most often for trucks, marine panels, and control systems running off battery power. It's a sensible choice, but check the coil tolerance first. A DC coil spec might list an operating range like 0.8–1.1 × rated voltage. If your wiring run is long, voltage drop can push the coil below its pick-up threshold. You'll hear chattering or see the contacts bounce.

The upside of a 12V coil is compatibility with low-voltage control. The risk is undervoltage in a long cable run. I've measured 11.3V at a contactor terminal that had a healthy 12.6V at the battery. The solution is simple: verify voltage at the contactor, not just at the source.

Also, don't mix AC and DC coil versions of the same nominal voltage. A 12V AC coil and a 12V DC coil are not interchangeable. The AC version usually has a shading coil in the magnetic circuit; the DC version is often rated for continuous duty. Swapping them without checking the nameplate is a quick way to burn out a coil.

Step 4: For Reversing Applications, Use a Real Reversing Contactor

If you need to reverse a three-phase motor, you need two contactors that swap phase connections. ABB sells reversing contactors as factory-built assemblies with mechanical interlocks. That interlock prevents both contactors from closing at the same time—because if both close, you have a phase-to-phase short.

I have mixed feelings about field-built reversing arrangements. On one hand, they can work if you add a mechanical interlock and careful control logic. On the other, I've opened panels where the only interlock was in the software. When the PLC failed, both contactors pulled in. The arc damage was significant. Bottom line: if the spec says reversing, order a reversing contactor, not just two separate units.

Step 5: Check Auxiliary Contacts and Wiring Diagrams

Auxiliary contacts are the small contacts used for feedback, interlocks, or status LEDs. They're usually rated for a much lower current than the main poles. Confirm whether you need NO or NC, how many, and what the controller's feedback input expects. A lot of documentation errors happen here because the schematic symbol can look similar across different product families.

Ask the supplier for the wiring diagram that matches your exact model. And this is the moment to address a search that lands in the wrong place. If you came looking for a DD15 fuel filter housing diagram, that's a Detroit Diesel engine maintenance drawing, not an electrical contactor diagram. You'll find that in the OEM service portal or a commercial repair manual. A contactor supplier can't give it to you, and I'd rather tell you that than sell you something useless.

Step 6: Inspect the Unit When It Arrives

This sounds obvious, but QC checks are where I earn my salary. Check the nameplate: model number, coil voltage, rated insulation voltage, and production date code. Check that terminals are tight—not torqued down, but not loose either after transit vibration. Look for bent connection lugs or cracks in the arc chambers. I've rejected units where the label was printed correctly but the physical unit had a misaligned spring latch. On a 50,000-unit annual order, that's not a rare event.

Step 7: Bench Test Before Installation If You Can

If the contactor is going into critical equipment, spend ten minutes on a bench test. Use a multimeter to measure the coil resistance; an open or short coil is a red flag. A small 12V DC coil might read anywhere from a few dozen to a few hundred ohms, depending on the coil's power rating. Then apply the rated coil voltage through a fused supply. Confirm the contactor pulls in cleanly and drops out when power is removed. If you have a variable-voltage supply, find the minimum pickup voltage—IEC 60947-4-1 sets requirements for this, and knowing your actual pickup voltage helps troubleshoot later.

Per IEC 60947-4-1, contactors are rated for specific utilization categories—AC-3 for squirrel-cage motors, AC-1 for resistive loads, and so on. The generic "110A" number on a product page is not a substitute for the utilization category rating on the nameplate.

Common Mistakes to Avoid

• Using a relay where a contactor is needed—especially on inductive loads.
• Mixing up AC coil and DC coil versions of the same size. A 24V AC coil and a 24V DC coil are not interchangeable.
• Trusting the model number without the auxiliary contact suffix.
• Skipping mechanical interlock on reversing configurations.
• Ignoring voltage drop on 12V coils in remote panels.
• Forgetting that a contactor alone doesn't provide overload protection; you still need a separate overload relay for motor protection.

That's the checklist. If your application fits standard industrial control, this will probably cover 80% of what you need. If you're dealing with an unusual duty cycle, high ambient temperature, or unusual certifications, this list might not be enough—so reach out to an engineer with the full motor data and nameplate information. Specify the exact model, coil voltage, and auxiliary contacts, and you'll save yourself the rework I've seen more times than I can count.