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How to select the rated current of a soft starter?

by joy•

Hey there, if you’re here, odds are you’re staring at a soft starter spec sheet and that “rated current” line is making you go, “Wait, do I size this for my motor’s full load, peak startup, or some random number in between?” Don’t worry—we get it, this is the kind of thing that can turn a simple motor upgrade into a headache, especially when you’re worried about blowing the starter mid-first startup or wasting money on an oversized unit that sits around collecting dust. As a soft starter supplier who’s talked to a hundred maintenance managers, electricians, and plant engineers, I’m gonna break this down like we’re geeking out over coffee, no stuffy jargon (okay, a little jargon, but I’ll explain it). Soft Starter

First off, let’s clear up the basics: rated current for a soft starter is NOT the same as a motor’s nameplate full load current (FLC). That’s the most common mistake we see, and it’s why folks end up with a starter that either dies fast or doesn’t work right. Wait, let’s define FLC quick—your motor’s nameplate has that number, usually labeled as “FLA” (full load amps) sometimes, yeah, that’s FLC. It’s how much current the motor pulls when it’s running at normal operating speed, no extra load.

Now, soft starters have two key current ratings you’ll see: the continuous rated current (that’s the one we’re talking about, printed big on the front or spec sheet) and the short-time overload current. The continuous one is non-negotiable—this is the max current the starter can handle 24/7 without overheating, even if the motor’s running at full load all day. The overload is for startup spikes, which we’ll get to in a sec.

So step one: start with your motor’s FLC. But here’s the catch—you can’t just match them 1:1. Why? Because every startup is a little different. If you’re driving a conveyor belt or a fan that doesn’t have a lot of inertia (stuff that starts quick, no heavy load hanging around), the startup current spike is 5-6x FLC, same as direct online (DOL) start. But if you’ve got a heavy load—think a crusher, a large pump, a mixer with a full vat of material—startup spikes can hit 7-10x FLC, and that’s where sizing matters.

Let’s do a real example that came to us last month. A guy called up saying his old 110kW motor kept blowing soft starters. His motor nameplate said FLC was 140A. He’d been buying a starter rated 140A, but every time he started the crusher, it crapped out. Turns out his startup current was 8x FLC, so 140A starter couldn’t handle that 1120A spike for the 10 seconds it took the crusher to get up to speed. We suggested he go with a 180A starter—1.3x his FLC—and that fixed it. He texted us two weeks later saying no more blown starters, no downtime on his shift.

Wait, why 1.3x? That’s not a magic number, it’s a rule of thumb for light loads. Heavy loads? We usually recommend 1.5x FLC. What about super heavy loads—like a mine conveyor that has to start with a full load? Sometimes 2x FLC, because the startup ramp is longer, so the overload current is active for more time.

But hold on, there’s more than just the motor. Let’s talk about your system’s voltage. Wait, did you know that if you’re running a lower voltage than the motor’s rated, the FLC goes up? For example, a motor rated 480V FLC 140A—if you’re running it at 400V, that FLC jumps to 168A. Oh, that’s another mistake! People use nameplate voltage FLC even if their plant runs a lower voltage (sometimes called “site voltage,” which is common in industrial setups). So you’ve gotta adjust for that. Let me make that easy: if your site voltage is 10% lower than nameplate, FLC goes up 10%. If it’s 10% higher? FLC drops 10%. That’s a quick math trick that saves you guessing.

Next, what about ambient temperature? That’s a big one we don’t see enough people checking. If your starter’s installed in a hot area—like a plant room that hits 40°C (104°F) instead of the standard 30°C (86°F) that most starters are rated for—you can’t use the same continuous current. Every 10°C above standard, you have to derate the starter by about 5%. So if you’re at 40°C, that’s 1.05x your needed current, right? Let’s say you needed 140A at 30°C, at 40°C you need 147A, so bump up to 150A. Hotter areas? Like if it’s 50°C (122°F)? That’s two 10°C jumps, so 10% derate, so 154A, go for the 160A starter. If you put a 140A starter in a 50°C room, it’ll overheat in a month, no matter what your motor’s FLC is.

Then there’s the load type, which we touched on, but let’s make a quick cheat sheet for that:

  • Light loads (fans, pumps with empty lines, conveyors with no load at start): 1.0–1.2x FLC
  • Medium loads (conveyors with full load, general purpose mixers): 1.2–1.5x FLC
  • Heavy loads (crushers, mining conveyors, large pumps with full suction): 1.5–2.0x FLC
  • Ultra-heavy loads (rotary kilns, ball mills): 2.0x FLC or higher, sometimes you gotta go off the starter’s overload curve instead of just FLC ratio.

Wait, what about if you’re using the soft starter for other stuff? Like, do you have a cooling fan for the starter itself? Some people forget that the starter’s own fan adds a tiny bit of load, but honestly, that’s negligible—we’re talking less than 1% of the starter’s rated current, so don’t stress about that. The big stuff is motor FLC, voltage, ambient temp, load type.

Now, let’s talk about what NOT to do. Don’t size it by the motor’s rated power (kW/HP) instead of current. A 100kW motor at 480V has a different FLC than a 100kW motor at 240V, so power alone doesn’t tell you squat. We had a guy come in with a 100kW motor, tried to buy a 100kW rated starter, and it died because the motor was 240V, so FLC was way higher than the starter’s current rating. That’s a $500 mistake he won’t make again.

Another don’t: don’t ignore the starter’s overload capability. Some cheap starters have a 2x overload for 10 seconds, but good ones (like the ones we supply) have 3x overload for 15 seconds. If your startup time is 12 seconds, a starter with only 2x overload might not cut it, even if the continuous current is right. That’s why we always ask customers for their startup time when they call—we don’t just go by numbers, we ask the specifics of their setup.

Wait, what about retrofit jobs? A lot of people are replacing DOL starters with soft starters. If your old DOL starter was rated for 140A, does that mean the soft starter should be 140A? Not necessarily. DOL starters handle peak current differently—they’re just contactors that close full power, so they’re rated for short peaks. Soft starters have built-in current limiting, so their continuous rating is for steady state, but their peak is a bit lower than DOL. So if your old DOL was 140A, you might need a 150A soft starter to match the startup peak, because soft starters can’t handle the same short-term spike as a contactor. That’s a common retrofit pitfall.

Let’s wrap this up with a quick step-by-step, no fluff:

  1. Grab your motor’s nameplate FLC (full load current) — don’t use HP/kW.
  2. Adjust FLC for your site voltage: +10% if voltage is 10% lower than nameplate, -10% if higher.
  3. Check your installation’s ambient temp: if over 30°C, derate starter current 5% per 10°C above 30.
  4. Pick a multiplier based on your load type: light (1.0-1.2), medium (1.2-1.5), heavy (1.5-2.0).
  5. Double-check your startup time (if it’s over 10 seconds, go for the higher multiplier).

And hey, if you’re still confused? That’s why we’re here. We see all sorts of weird setups—plants in desert heat, mines with super heavy loads, retrofits where old starters didn’t work. We don’t just sell you a starter; we ask questions about your motor, your load, your environment, and make sure you get the right size so you don’t have downtime or wasted money.

Medium Voltage Cabinet If you’re shopping for a soft starter and wanna run your specs by us, hit us up to talk through your needs. We can even help you cross-reference your motor’s info to make sure the current rating is spot on. No sales pitch, just straight answers based on what works for your setup.

References

  1. "IEC 60947-4-2: Low-voltage switchgear and controlgear – Part 4-2: Contactors and motor-starters – AC motor-starters for starting protection"
  2. NEMA Standard ICS 2: "Control and Protection of Motor Circuits"
  3. US Department of Energy: "Motor System Optimization Guide: Soft Starter Sizing and Selection"
  4. "Application Guide for Medium Voltage Soft Starters", IEEE Industry Applications Society, 2021

Nanjing Aubo Electric Co., Ltd.
Nanjing Aubo Electric Co., Ltd. is one of the most reliable soft starter manufacturers and suppliers in China. Welcome to wholesale high quality soft starter made in China here from our factory. For more cheap products, contact us now.
Address: No.15, Zhufeng Road , Zhuzhen Town Luhe District, Nanjing, Jiangsu
E-mail: aubochina@aubochina.com
WebSite: https://www.vfd-softstarter.com/