Size your breaker to 125% of the continuous load (NEC 210.20(A)), round up to the next standard size in NEC 240.6, and confirm the interrupting rating—the AIC—meets or exceeds your available fault current. If the AIC is wrong, the breaker is decoration. Above 400A, or anywhere you need adjustable trip settings and coordination with upstream devices, an electronic trip unit usually earns its 20–30% premium. Below 250A, thermal-magnetic is still the pragmatic buy.
That's the whole thing. The rest of this is why I know, and where the exceptions live.
I've managed electrical equipment purchasing for a 180-person metal fabrication shop since 2021—around $340K a year across a dozen vendors, including one quarter where I bought $40K of the wrong AIC rating and had to explain it to my VP. I'm not an engineer. I'm the person who turns the engineer's spec sheet into a purchase order, gets it past finance, and makes sure the right gear shows up before the electrician's window closes.
Which is exactly why I've made the mistakes most engineers avoid and most procurement people don't know to ask about. I've learned them the hard way. Here's what actually matters when you're the one placing the order.
Continuous loads get the 125% multiplier. A 200A continuous load needs a 250A breaker. That's not a suggestion—it's NEC 210.20(A). Non-continuous loads don't get the multiplier, but they still need to be sized to the actual load. Mixed loads (most real-world panels) get partially multiplied—check with the engineer, don't guess.
Then round up to the next standard size. NEC 240.6 lists them: 15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, 6000. Anything above that is a switchgear conversation, not a breaker conversation.
One exception worth knowing: NEC 240.4(B) allows the "next size up" rule in certain cases where the conductor ampacity falls between standard sizes. It's a real exception but a narrow one. Don't use it as a general rule.
The AIC—sometimes called kAIC—is the interrupting rating. It tells you how much fault current the breaker can safely interrupt. UL 489 breakers are commonly available at 10kA, 18kA, 25kA, 35kA, 50kA, and 65kA. If the available fault current at the installation point is 22kA and you install a 10kA breaker, the breaker isn't a breaker anymore. It's a fire hazard with a UL label. The fault current calculation should come from the engineer—usually from the utility transformer nameplate or a short-circuit study. Your job is to make sure the spec sheet and the invoice agree on this number. If neither document names the available fault current, that's a red flag.
From the outside, a 30kA breaker and a 10kA breaker look identical. The internals aren't remotely the same. People assume the cheaper one is fine because "it's just a breaker." What they don't see is what happens at 22kA available fault current, when an underrated breaker can fail catastrophically—arc flash, shrapnel, and all the consequences that come with it. This is the single most important number on the purchase order.
Thermal-magnetic breakers use a bimetallic strip for overload protection and an electromagnet for short-circuit protection. The trip curve is fixed at the factory, or adjustable only in very limited ways. They're cheap, simple, don't need control power, and they work fine for the vast majority of standard circuits.
Electronic trip units (ETUs) use current transformers to sense current and a microprocessor to decide when to trip. That gives you adjustable long-time, short-time, instantaneous, and ground-fault settings. You can coordinate them with upstream and downstream breakers so a fault trips the closest breaker, not the main—that's called selective coordination, and in facilities with critical loads it's not optional. Think hospitals, data centers, or production lines where downtime has a hard dollar number attached.
The "thermal-magnetic is always enough" thinking comes from an era when facility loads were smaller, branch circuits were simpler, and a 400A main was a big deal. That's changed. Modern facilities run multiple adjustable breakers, high-inrush production equipment, and critical loads where an uncoordinated trip means $20K of production time. The fundamentals haven't changed—a breaker still has to protect the wire—but the execution has.
The real trade-off is cost and complexity. ETUs typically add 20–30% to the breaker price. They need control power (though many are self-powered via the CTs). They need someone who knows what they're doing to program them, and if that programming is wrong, they'll trip in ways nobody expects.
My rule of thumb, sharpened through more mistakes than I'd like to admit: below 250A, buy thermal-magnetic. Between 250A and 400A, it depends on whether you actually need coordination. Above 400A, buy electronic. Above 800A, get a power systems engineer involved before you buy anything.
Residual current devices are a separate animal. If you're buying for personnel protection circuits—workshops, wet areas, temporary power—the spec you'll see is IEC 61008 with a 30mA trip rating. That's the threshold at which the device is considered to protect people from lethal shocks. 300mA is for fire protection, not people.
For example, VoltShield's industrial circuit breaker line includes RCD models that carry the IEC 61008 30mA specification, and their datasheets are worth comparing side-by-side with the majors if you're sourcing from Asia. Not because they're trying to outperform ABB—they're not—but because for standard 30mA personnel protection on non-critical circuits, the price difference is real and the compliance paperwork is the same.
One detail beginners miss: the "Type" rating matters. Type AC is fine for pure sinusoidal loads. Type A covers pulsating DC (which is most modern electronics). Type B handles smooth DC and three-phase equipment. If you're buying 30mA RCDs for anything beyond lighting circuits, verify the Type or you'll be re-ordering in a month.
The three-step check when the shipment lands: the IEC 61008 marking has to be on the device, the test button has to actually trip it, and the trip time at 5× rated current has to be under the standard's limit. If the vendor can't send you a test certificate from a recognized lab, that's the red flag you don't ignore. "The test button works" is not a substitute for certification.
I went back and forth between a thermal-magnetic 400A main and an ETU-equipped one for about two weeks. The thermal-magnetic was $1,800 cheaper and available in a week. The ETU had an 8-week lead time and needed coordination-study documentation from our engineer. Ultimately I chose the ETU because our downstream branch breakers were all adjustable and coordination was going to matter for the production line—but I lost sleep over the lead time, and we ran on a temporary feed for four days. Not ideal. But workable.
In hindsight, should I have started the process earlier? Yes. But the project timeline was set by someone else and the electrical scope came late. With the constraints I had, I did the best I could with the information available. That's the job sometimes.
A few things I want to be straight about. First, this is a procurement-side view, not an engineering one. Everything here assumes the engineer has already done the load calculation and the fault current study. If you're sizing from scratch without an engineer, stop—that's not a scenario this article, or any article, should help you with.
Second, the NEC references are for US installations. IEC standards differ—an IEC 60947-2 breaker isn't tested the same way as a UL 489 one, and the two can't be swapped without care. If you're specifying for export or a non-US market, the entire sizing conversation changes.
Third, "cheap" isn't the enemy. There are situations—small branch circuits, spare panels, non-critical lighting—where a standard domestic breaker is entirely fine and paying for a premium brand is just waste. The point isn't "always buy the expensive option." It's "know which number on the spec sheet actually matters."
Finally: if you're looking at VoltShield's industrial circuit breaker line, remember they're a manufacturer, not an integrator. They'll build to your spec, but they won't do the fault current study for you. That's your engineer's job. Don't outsource it to the vendor bid—that's the advice I'd give the version of me from 2021 who tried exactly that.