Fluke i400 vs i400s (2026): Which AC Current Clamp Wins?

Why you should trust this comparison

I built this comparison entirely from Fluke’s published documentation and the widely circulated specifications for the i400 and i400s current probes, not from any bench test of my own. I have not clamped these probes around a live conductor or measured their output on a calibrator, so I will not pretend to report numbers I generated. Instead, every figure below is attributed to Fluke’s spec sheet or to documentation that has been consistent across the manufacturer’s listings for years. Where the public record is fuzzy on an exact value, I say so plainly rather than inventing a precise figure to fill the table.

The reason this kind of comparison still helps is that the i400 and i400s are easy to confuse. They share a name, a 400 A class, and a similar clamp body, but they belong to two different output families. One is a current-output probe and the other is a voltage-output probe, and that single distinction decides whether the accessory will even work with your instrument. My goal here is to make that difference unmistakable so you order the right part the first time, using only what Fluke documents.

How we compared them

I focused on the criteria that actually change a buying decision for a clamp-on current accessory: the output type and what instrument it connects to, the rated current range, the safety category of the jaw, the conductor-size limit the jaw can physically accept, and the intended use case. Output type is first because it is the make-or-break factor. A probe that outputs a small proportional current is meant to extend a digital multimeter’s range, while a probe that outputs a voltage is meant for a scope or recorder, and the two are not interchangeable even when their current ranges match.

After output type, I weighed the practical handling and safety details Fluke publishes, since these accessories are clamped around live conductors. I treated accuracy and bandwidth as secondary, partly because exact published figures vary by document revision and partly because both probes are designed for general 50/60 Hz power work rather than high-frequency analysis. Where I am confident of a spec I list the number; where I am not, I describe the behavior qualitatively instead of guessing.

How they compare at a glance

Fluke i400

The Fluke i400 is a clamp-on AC current probe whose job is to let a standard digital multimeter read currents it could not otherwise reach. Rather than breaking the circuit and running it through the meter, you clamp the jaw around a single conductor and the probe outputs a small current that is proportional to the current flowing in that conductor. Fluke designs the scaling so the output plugs into a meter’s current jacks and reads back in a convenient ratio to the actual load, which is what makes this such a common shop accessory for electricians and field technicians.

It suits anyone who already owns a capable multimeter and simply wants to add safe, clamp-style AC current measurement up to the 400 A class without buying a whole new clamp meter. The handling is straightforward, the jaw is rated for the live work it is meant to do, and there is nothing to configure beyond clamping the right conductor.

The honest limitation is scope: the i400 measures AC current only and produces a current output, so it is the wrong tool if you need to feed an oscilloscope or a data logger that expects a voltage input. It also reads a single conductor at a time, so multi-phase work means moving the clamp rather than reading channels in parallel.

Fluke i400s

The Fluke i400s covers the same 400 A AC class as the i400 but answers a different question. Instead of outputting a proportional current, it outputs a proportional voltage, typically expressed as so many millivolts per amp, which is exactly what an oscilloscope or a voltage-input data logger wants to see. That makes the i400s the natural choice when you need to visualize the shape of an AC current waveform, capture inrush, or log current over time on equipment that records voltage rather than current.

It suits technicians and engineers working with scopes and recorders who need current information rendered as a voltage they can trace, trigger on, and store. The clamp body and safety-rated jaw work the same way as the i400’s: clamp around one conductor and read, with no circuit interruption.

The honest limitation is that the voltage output makes the i400s a poor match for a plain multimeter set to measure current, so buying it for general DMM use would be a mistake. Like its sibling it is AC-only and single-conductor, so it will not help with DC measurement or simultaneous multi-phase capture.

Which should you buy?

Match the probe to the instrument you already own. If your goal is to extend a digital multimeter so it can read AC current up to the 400 A class, buy the Fluke i400, because its proportional current output is built to plug into a meter’s current input. If your goal is to see or record the current waveform on an oscilloscope, a scope meter, or a voltage-logging recorder, buy the Fluke i400s, because its proportional voltage output is what those instruments expect.

If you are buying for a mixed bench and can only pick one, let the destination instrument decide: the i400s is the more flexible signal source for scopes and loggers, while the i400 is the cleaner fit for everyday multimeter work. Neither replaces the other, and trying to use one in the other’s role is the single most common mistake with this pair.

Frequently asked questions

What is the real difference between the i400 and i400s? The i400 outputs a proportional AC current meant for a multimeter, while the i400s outputs a proportional AC voltage meant for an oscilloscope or data logger. Same current class, different output.

Can I use the i400s with a regular multimeter? It is not the intended pairing, because the i400s produces a voltage signal rather than the current signal a DMM’s current jacks expect. For straightforward meter use, the i400 is the right part.

Do these measure DC current? Based on Fluke’s documentation, both are AC current probes, so neither is the tool for measuring DC current.

The verdict

These two probes are siblings separated by one decisive trait: output type. Buy the Fluke i400 to extend a multimeter’s AC current reach, and buy the Fluke i400s to push a current waveform into an oscilloscope or logger. I have leaned only on Fluke’s published specs and well-documented features here, and where the public record is imprecise I have kept the table qualitative rather than inventing numbers. Pick by the instrument you plan to plug into and you will not go wrong.