{"id":3636,"date":"2026-10-26T12:25:20","date_gmt":"2026-10-26T04:25:20","guid":{"rendered":"http:\/\/www.grupoinvercap.com\/blog\/?p=3636"},"modified":"2026-10-26T12:25:20","modified_gmt":"2026-10-26T04:25:20","slug":"how-to-select-the-rated-current-of-a-soft-starter-45a0-0254d9","status":"publish","type":"post","link":"http:\/\/www.grupoinvercap.com\/blog\/2026\/10\/26\/how-to-select-the-rated-current-of-a-soft-starter-45a0-0254d9\/","title":{"rendered":"How to select the rated current of a soft starter?"},"content":{"rendered":"

Hey there, if you\u2019re here, odds are you\u2019re staring at a soft starter spec sheet and that \u201crated current\u201d line is making you go, \u201cWait, do I size this for my motor\u2019s full load, peak startup, or some random number in between?\u201d Don\u2019t worry\u2014we get it, this is the kind of thing that can turn a simple motor upgrade into a headache, especially when you\u2019re 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\u2019s talked to a hundred maintenance managers, electricians, and plant engineers, I\u2019m gonna break this down like we\u2019re geeking out over coffee, no stuffy jargon (okay, a little jargon, but I\u2019ll explain it). Soft Starter<\/a><\/p>\n

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First off, let\u2019s clear up the basics: rated current for a soft starter is NOT the same as a motor\u2019s nameplate full load current (FLC). That\u2019s the most common mistake we see, and it\u2019s why folks end up with a starter that either dies fast or doesn\u2019t work right. Wait, let\u2019s define FLC quick\u2014your motor\u2019s nameplate has that number, usually labeled as \u201cFLA\u201d (full load amps) sometimes, yeah, that\u2019s FLC. It\u2019s how much current the motor pulls when it\u2019s running at normal operating speed, no extra load.<\/p>\n

Now, soft starters have two key current ratings you\u2019ll see: the continuous rated current (that\u2019s the one we\u2019re talking about, printed big on the front or spec sheet) and the short-time overload current. The continuous one is non-negotiable\u2014this is the max current the starter can handle 24\/7 without overheating, even if the motor\u2019s running at full load all day. The overload is for startup spikes, which we\u2019ll get to in a sec.<\/p>\n

So step one: start with your motor\u2019s FLC. But here\u2019s the catch\u2014you can\u2019t just match them 1:1. Why? Because every startup is a little different. If you\u2019re driving a conveyor belt or a fan that doesn\u2019t 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\u2019ve got a heavy load\u2014think a crusher, a large pump, a mixer with a full vat of material\u2014startup spikes can hit 7-10x FLC, and that\u2019s where sizing matters.<\/p>\n

Let\u2019s 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\u2019d 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\u2019t 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\u20141.3x his FLC\u2014and that fixed it. He texted us two weeks later saying no more blown starters, no downtime on his shift.<\/p>\n

Wait, why 1.3x? That\u2019s not a magic number, it\u2019s a rule of thumb for light loads. Heavy loads? We usually recommend 1.5x FLC. What about super heavy loads\u2014like 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.<\/p>\n

But hold on, there\u2019s more than just the motor. Let\u2019s talk about your system\u2019s voltage. Wait, did you know that if you\u2019re running a lower voltage than the motor\u2019s rated, the FLC goes up? For example, a motor rated 480V FLC 140A\u2014if you\u2019re running it at 400V, that FLC jumps to 168A. Oh, that\u2019s another mistake! People use nameplate voltage FLC even if their plant runs a lower voltage (sometimes called \u201csite voltage,\u201d which is common in industrial setups). So you\u2019ve gotta adjust for that. Let me make that easy: if your site voltage is 10% lower than nameplate, FLC goes up 10%. If it\u2019s 10% higher? FLC drops 10%. That\u2019s a quick math trick that saves you guessing.<\/p>\n

Next, what about ambient temperature? That\u2019s a big one we don\u2019t see enough people checking. If your starter\u2019s installed in a hot area\u2014like a plant room that hits 40\u00b0C (104\u00b0F) instead of the standard 30\u00b0C (86\u00b0F) that most starters are rated for\u2014you can\u2019t use the same continuous current. Every 10\u00b0C above standard, you have to derate the starter by about 5%. So if you\u2019re at 40\u00b0C, that\u2019s 1.05x your needed current, right? Let\u2019s say you needed 140A at 30\u00b0C, at 40\u00b0C you need 147A, so bump up to 150A. Hotter areas? Like if it\u2019s 50\u00b0C (122\u00b0F)? That\u2019s two 10\u00b0C jumps, so 10% derate, so 154A, go for the 160A starter. If you put a 140A starter in a 50\u00b0C room, it\u2019ll overheat in a month, no matter what your motor\u2019s FLC is.<\/p>\n

Then there\u2019s the load type, which we touched on, but let\u2019s make a quick cheat sheet for that:<\/p>\n