ABB Contactor Buying Checklist: How to Spec, Verify, and Test the Right Magnetic Contactor

I'm the office administrator for a mid-sized industrial maintenance group. I handle the controls and electrical ordering—roughly $120k a year across a mix of eight vendors. I report to operations and finance, which means I get questions from both sides when a part doesn't work. I'm not an engineer. I'm the person who places the order and then has to explain why the machine didn't start.

The hard lesson took a while. It took me three years and about sixty contactor orders to understand that a magnetic contactor is not just a heavy relay you can swap by appearance. It's a circuit component with ratings. At least, that's been my experience with panel replacements.

This checklist is for people like me. It applies whether you're replacing an ABB magnetic contactor in an existing cabinet or ordering one for a new control panel. Here are the six checks I run before I sign a purchase order.

When This Checklist Applies

Use this list when you need to replace an existing ABB contactor, when you're ordering a spare from a drawing, or when someone tells you to match the NEMA size 4 contactor that's already there. It also applies if you're cross-referencing an old unit to a current ABB A63-30 contactor and the catalog page doesn't line up with the nameplate.

If a machine is down, the temptation is to skip straight to the part number and hit buy. I've done that. In a 2024 rush order, I typed the same number from a worn nameplate and missed a superseded auxiliary contact revision. The maintenance crew lost half an hour while we returned the wrong part. This list stops that from happening.

The Six-Step Checklist

1. Write down the load, not just the old contactor

Start with the motor nameplate: full-load amps, voltage, phase, and horsepower. If it's a lighting or heating load, write down the total amps and the control voltage. Don't start with same as what's in the panel. The old contactor might be oversized, undersized, or from an older duty rating.

If someone says it's a NEMA size 4 contactor, treat that as a starting point. NEMA size is a category, not an exact ampere rating by itself. A NEMA size 4 contactor has a range, and that range changes with voltage and service factor. For an ABB A63-30 contactor, the 63 points to the 63A frame, but the actual AC-3 rating depends on the specific version and the coil and auxiliary arrangement. The catalog data is the only safe answer.

2. Check the coil voltage before you search

The coil is still the part that burns up most often, and it's specified by voltage and frequency. Look at the A1-A2 terminals. Common options for ABB contactors are 24V AC, 110V AC, 220V AC, and 24V DC, but there are many others. If the coil is DC, confirm the exact DC voltage and whether there is a suppressor module in the wiring.

I want to say one out of every five wrong orders I've reviewed involves the coil voltage, but don't quote me on the exact percentage. It's a lot. It also happens when someone assumes the old one ran on 120V and the new panel actually uses a 24V control transformer.

3. Count the auxiliary contacts and the enclosure rating

Most ABB magnetic contactor frames include at least one auxiliary contact along with the main poles. But that doesn't mean you won't need a separate auxiliary block. Check the schematic for NC feedback contacts, fault contacts, and reversing interlocks. If you miss this, you'll get a contactor that physically fits but cannot complete the circuit's logic.

While you're in the drawing, confirm the enclosure or the mounting environment. A small difference, like IP20 versus IP56, can change the part code. I learned this when a replacement for a washdown station arrived without the sealed coil cover. It was my fault for not looking at the drawing first.

4. Read the wiring diagram before you order

I know this one sounds like the engineer's job. It isn't. If you can read a simple relay diagram, you can read a contactor diagram.

Take a common search like 4 wire fuel pump wiring diagram. People looking that up are trying to figure out which wire is power, which is ground, which is signal, and which is the switched output. A contactor circuit is the same logic: line, load, coil, and auxiliary feedback. Trace the line that feeds the coil. Does it come from a PLC output? Does it pass through another device's NC contact? If so, your contactor needs to match that control scheme.

This step has saved me more than once. If you don't have the drawing, get it from the panel manufacturer before you place the order.

5. Test it with a multimeter before installation

New parts can sit in a warehouse for years, or they can get damaged in transit. So I test before the electrician is standing by. The quick way is the same as answering the question, How to test starter relay with multimeter?

Put the meter in ohms mode. Measure the coil between A1 and A2. You should see some resistance—not zero, not open. The exact value varies by coil voltage and manufacturer, so I use the catalog page or an identical known-good contactor as a reference. Then check the main contacts. With the coil de-energized, a normally-open pole should read open or very high resistance. If it reads near zero, the contactor is stuck or welded.

If you are trained and working safely, you can apply rated coil voltage to the coil and listen for the pull-in. Then test continuity through the main poles and the aux contacts. The same bench procedure is what you'd use to test a starter relay: energize the coil and confirm the switched terminal changes state. Don't measure live circuits unless you're qualified to do that work.

6. Verify the current ABB catalog data

Part numbers get superseded. I keep that in mind every time a maintenance supervisor gives me a number from memory or an old spreadsheet. For example, the ABB A63-30 contactor in the current line might look identical to a version from earlier catalogs, but the auxiliary block compatibility and the full ordering code can change. According to ABB's contactor documentation (ABB.com, accessed January 2025), ratings and connection accessories can vary by catalog revision.

If you are replacing a NEMA size 4 contactor, double-check that the replacement is actually rated under NEMA rules. NEMA size classifications are defined in NEMA ICS 1. Do not assume an IEC 63A contactor is automatically equivalent just because the terminals look similar. Different standards, different testing, different application data.

Common Errors to Avoid

I still make mistakes, but these are the ones I repeatedly catch before the machine is down:

• Ordering by an old part number without checking the current catalog revision.
• Forgetting the auxiliary block. The main frame and the auxiliary contacts are often separate line items.
• Skipping the coil voltage check because the replacement should be the same as the old one.
• Wiring something new into an old panel without reading the updated diagram.
• Planning to test it when it arrives and then not doing it before install.

The checks that have saved me the most phone calls are the first, fourth, and fifth: know the load, read the drawing, and test with a multimeter. Do those three, and the chance of ordering the wrong ABB magnetic contactor drops dramatically.