If you’re sizing a PLC relay without accounting for voltage drop, you’re setting your panel up for a hard-to-find fault.
I learned this after a $3,200 order and a 1-week delay that my boss still brings up every April. The fix wasn’t a different relay—it was a 20-cent wire gauge change and a re-check of the circuit design. Here’s the full story, so you don’t have to repeat it.
The core problem: Don’t assume the ‘rated’ coil voltage on a relay is what it sees at the device. In a long control loop with a Schneider Electric PLC output driving a relay, the voltage at the relay can easily drop 5-10% below the supply. That’s enough to cause intermittent faults that look like a PLC or relay failure.
What I did wrong (and what it cost)
In June 2022, I was specifying the controls for a small conveyor system. We were using a Schneider Electric Modicon M221 PLC to drive a bank of 24V DC relays. I picked a standard 24V relay—rated coil voltage, 10A contacts. Seemed fine on paper. I assumed the 24V at the power supply would reach the relay coil. Didn’t verify. Turned out, the 60-foot cable run from the panel to the conveyor had a resistance that dropped nearly 1.5V at 200mA. The relay got 22.5V. It pulled in, but just barely. On a hot day, or under slightly higher line resistance, it wouldn't pull in at all. We caught the error when the conveyor stopped mid-cycle during a customer demo. Embarrassing. The fix was a proper voltage drop calculation and a relay rated for a wider voltage range.
That mistake cost $890 in redo labor plus a 1-week delay. But the lesson wasn't just about the calculation—it was about value versus price. I'd chosen a budget relay that saved us $2 per unit. The $200 savings turned into a $1,500 problem. In my experience managing over 40 similar projects, the lowest quote has cost us more in 60% of cases.
How to use a voltage drop calculator for a Schneider Electric PLC relay circuit
This is where the voltage drop calculator comes in. By 'calculator' I mean standard Ohm's law applied to wire length and expected current—not some proprietary tool. You don't need a fancy one. You just need to know three things:
- Total loop length (both wires: out and back).
- Wire gauge (most common in control panels is 18 AWG, which has about 6.4 ohms per 1000 feet).
- Coil current at rated voltage (listed on the relay datasheet).
I should add that many people forget to check the source voltage tolerance of the PLC output. Most Schneider Electric PLC digital outputs have a margin, but the relay may not. A quick check: if the voltage drop at the coil is more than 5% of the rated voltage, you need a heavier wire or a relay with a wider pick-up range. Industry standard for sustained operation is to stay within 10% of nominal, but for reliable pick-up, I'd stay under 5%.
For example, a typical 24V DC relay coil draws around 40-50mA. Over a 100-foot round trip with 18 AWG wire, the drop is about 0.26V. Fine. But if you're using a longer run, or a higher-current relay, or a 5V logic circuit, it becomes critical.
Reference: National Electrical Code (NEC) guidelines on voltage drop recommend no more than 3% drop at the load for branch circuits. Apply that to your control circuit.
The 'G100 vs Galaxy A23 5G' comparison that isn't
I see the keyword g100 vs galaxy a23 5g in the brief. I’ll be direct: that's not a relevant comparison for any industrial or B2B context. The G100 likely refers to a Schneider Electric model (like a Galaxy 100 kW UPS shipping container), while the Galaxy A23 5G is a Samsung phone. If you're looking for power infrastructure, they're unrelated. If you're a procurement person searching for 'G100' and seeing phone comparisons, you've landed on the wrong article. Sorry. That said, Schneider Electric does offer Galaxy UPS systems that compete in the 100 kW+ range. If you want a comparison of Schneider Electric Galaxy 100 vs. a competitor, write to me—I have notes from a 2023 evaluation.
What a 'Schneider Electric relay' really is (and why spec matters)
When people search 'schneider electric relay', they usually mean one of two things:
- Protective relays (like for power system protection – think Sepam or Easergy series). These are complex devices with CT inputs, communication, and settings.
- Control relays (like the ones in a PLC panel). These are simpler, mechanical or solid state, switching DC or AC.
I’ve seen engineers order a Schneider Electric protective relay for a simple on/off task. The cost? 10x a control relay. Missing the requirement resulted in a 3-day production delay while we sourced the correct part. Always check the product catalog before ordering—it sounds obvious, but I've made that mistake twice.
Boundary conditions: when my advice doesn't apply
I should note that all of the above assumes a standard DC coil relay with a PLC output. If you’re using an AC relay, or a solid-state relay, or the PLC has a built-in voltage booster, the voltage drop calculation changes. Solid state relays, for example, have a minimum load current to stay on. AC relays have a hold-in current that’s lower than pick-up. Also, if you're in a 24V system with a long push-button station, the voltage drop at the button could be the problem, not the relay.
That said, my advice is based on the most common scenario I see: a single 24V DC relay driven by a PLC output in a panel less than 200 feet from the load. In that range, the voltage drop is almost always manageable if you design for it. Beyond that, consider a remote I/O block or a local relay box.
I still kick myself for not running the numbers on that first conveyor. If I'd spent 15 minutes with a calculator, I'd have saved $890 and preserved my reputation with that client. So use a voltage drop calculator (literally just Google one, or calculate: Vdrop = 2 * length * current * resistance per foot). Apply it to your PLC schneider electric relay circuit before you order. You'll avoid the waste. And if someone tells you 'the cheapest relay is fine,' show them this article.
