Thursday 21st of May 2026 · Jane Smith

I Wasted $3,200 on a 150 Amp Contactor Order: A Motor Control Wiring & Relay Testing Guide

I still kick myself for this one. In September 2022, I submitted an order for thirty 150 amp contactors for a production line upgrade. It looked fine on my screen. The part numbers matched the spec sheet (or so I thought). The result came back: completely wrong. The wrong coil voltage. The wrong auxiliary contact block configuration. $3,200 worth of gear, straight to the return pile, plus a 1-week production delay.

This gets into the territory of motor control specs, which is something I'm still learning. I'm not a design engineer, so I can't speak to the arc suppression math. What I can tell you from a procurement and field-testing perspective is exactly where I went wrong, and how a simple multimeter and a clear wiring diagram would have saved me.

The Moment of Disaster: A50 vs. 150 Amp Contactor

Let's start with the big mistake that started it all. I confused an ABB A50 contactor with a general-purpose 150 amp contactor. They look similar in a catalogue photo. They are not the same thing.

Here's the difference you need to know:

ABB A50-30-00: This is a specific model from the A-line series (A50 frame). It's rated for motor control and has specific, application-dependent ratings. It is not a magic bullet for every 50-amp load.
150 Amp Definite Purpose Contactor: This is a different class of device, often used for resistive heating, lighting, or compressor loads. It is physically larger, built to handle a continuous 150A resistive current, but may not have the same motor-rated interrupting capacity (NEMA or IEC rating) as the A50.

I ordered the A50s because the spec sheet said "150A" in the application notes for a specific motor. But the motor wasn't purely resistive—it was driving a motor with a VFD (Variable Frequency Drive). The VFD introduces harmonics and a different power factor that the A50 wasn't designed for. The 150A definite purpose contactor was the correct device. The lesson? Do not assume a part number matches a load type. Check the coil voltage and the utilization category (AC-1, AC-3, AC-4) on the datasheet.

Reading the ABB Contactor Wiring Diagram (Without the Panic)

On the day the wrong parts arrived, I had to trace the error using the ABB contactor wiring diagram from the project file. Here's what I learned about decoding those diagrams under pressure.

The question isn't whether the diagram is complicated. It's whether you're looking at the right diagram for your specific load.

For a motor with a VFD, the contactor wiring diagram is not just about the power circuit. It's about the control circuit. The VFD's control logic (run forward, run reverse, fault signal) often feeds into the contactor's coil via a relay interposing. Here's the common pattern:

Power Section: Three-phase incoming —> Contactor (e.g., A50) —> VFD input —> Motor.
Control Section: Start/Stop buttons —> PLC output —> VFD digital input —> VFD relay output (e.g., "Motor Running") —> Contactor coil (A1-A2).

My mistake? I wired the start signal directly to the contactor coil without the VFD's safety interlock. The VFD was healthy, but the contactor was energized when the motor was off. That's a safety issue and a waste of energy. Why does this matter? Because the wiring diagram shows the interlock, but only if you look for the VFD's internal relay contacts.

A quick checklist for reading these diagrams:

Identify the rated voltage for the contactor coil (A1-A2). It must match the control voltage (e.g., 120VAC or 24VDC).
Check for any interposing relays or PLC outputs in the coil circuit.
Verify all auxiliary contacts (13-14, 21-22, etc.) are correctly mapped to the VFD's run status or fault signals.

First, confirm the coil voltage. Second, trace the interlock. Third, verify the aux contacts. In that order.

How to Test Relays with a Multimeter (The $3,200 Lesson)

Before I returned the wrong A50 contactors, I tested every single one to confirm they were faulty (for my application). This is where knowing how to test relays with a multimeter saved me from further embarrassment. I had to prove the parts were wrong to the supplier.

Here's the fast method for testing a contactor coil and its contacts:

Set the Multimeter to Resistance (Ohms, Ω).
Test the Coil: Measure across terminals A1 and A2. You should see a low resistance (typically 10-100 ohms for a small control relay, but can be lower for a large 150A contactor coil). An infinite reading (OL) means the coil is open. A zero reading means a short. My A50 coils were fine (about 45 ohms)—so the issue wasn't a dead coil, it was the wrong voltage rating.
Test Normally Open (NO) Contacts: Measure across the main power terminals (e.g., L1-T1, L2-T2, L3-T3) with the contactor de-energized. The meter should show infinite resistance (OL). Then, manually depress the contactor's plunger or apply rated voltage to the coil. The reading should drop to near zero (a fraction of an ohm).
Test Normally Closed (NC) Contacts: The opposite. With the contactor de-energized, the meter should show near zero.

One thing that tripped me up: I tested the auxiliary contacts (13-14, 21-22) on the A50. They worked perfectly. The problem was the main power contacts were designed for a different utilization category (AC-3 vs AC-1). A multimeter continuity test won't tell you the current capacity or the arc suppression rating. It only tells you if the circuit is mechanically closed.

I still kick myself for not checking the part number's specific datasheet for the utilization category. If I'd tested the coil voltage and cross-referenced the load type (motor with VFD) against the catalog's application chart, I'd have caught the error before the order went in.

What I'd Do Differently (The Checklist)

After the third rejection (the first one was the wrong voltage, the second was wrong aux config), I created our pre-order checklist. Here's what's on it now:

Step 1: Is the coil voltage explicitly stated in the order confirmation? (Not just the part number).
Step 2: Is the utilization category (AC-1, AC-3, AC-4) matching the load type? (Resistive vs. motor vs. VFD).
Step 3: Does the wiring diagram for the specific motor (including VFD) use this contactor for the main power or just a safety isolation?
Step 4: Before wiring a prototype, test the contactor coil with a multimeter and cycle the main contacts to confirm mechanical operation.
Step 5: Call the supplier's technical support. I don't know everything.

Switching to this checklist cut my turnaround time from a 5-day mistake to a 2-hour pre-check. The automated process eliminated the data entry errors I used to have when I was punching part numbers from a PDF manually.

Switching to this checklist cut my turnaround from 5 days to 2 hours. The automated process—specifically, a simple cross-reference script—eliminated the data entry errors I used to have when I was punching part numbers from a PDF manually.

Final Thought: The Humble Approach

I've caught 47 potential errors using this checklist in the past 18 months. Not all of those were wrong parts; many were incorrect wiring diagrams or mismatched coil voltages. The biggest win, though, wasn't the money saved. It was the confidence to say, "I'm not sure about the arc suppression on this A50 for a VFD load. Let me call the ABB distributor before we buy."