The Siemens 3NJ6940-3BJ22 is a pluggable bushing current transformer rated 400 A primary / 5 A secondary with Class 0.5 accuracy. It is the lowest primary current variant in the SENTRON 3NJ6940 accessory range for 3NJ6 in-line fuse-switch-disconnectors. For 400 A feeders — a common size in commercial distribution boards, sub-main panels, and light industrial switchgear — it delivers precision measurement in the same compact, tool-free plug-in format as its 500 A and 600 A siblings.

Why 400 A instead of 500 A or 600 A?

A current transformer performs best when its rated primary current matches the nominal circuit current. A feeder protected at 400 A with a 500 A CT will typically operate below 80% of the CT rating — in the lower end of the accuracy band where measurement errors widen. Conversely, using a 400 A CT on a 500 A feeder risks saturation. The 3BJ22 is purpose-built for 400 A circuits: it stays within its specified Class 0.5 accuracy window for the currents a 400 A feeder actually carries. If your feeder is fused at 400 A, this is the correctly matched CT.

What does Class 0.5 accuracy give me on a 400 A circuit?

Class 0.5 means the CT contributes at most ±0.5% error to the measurement chain at rated current — half the error of a Class 1 CT. For a 400 A feeder supplying a tenant floor, a production area, or a critical HVAC plant, that tighter tolerance translates to more trustworthy energy data for internal cost allocation and efficiency tracking. The 3BJ22 is the precision tier — choose it when the numbers drive decisions, but no invoice depends on them. If billing is involved, consider the existing 3NJ6940 portfolio for a calibrated alternative or consult the wider Siemens 4NC/5NC current transformer range.

400/5 A Variant Context in the 3NJ6940 Series

Frequently Asked Questions

Is the 3BJ22 the only 400 A option in the 3NJ6940 series?

The 3BJ22 is the 400/5 A Class 0.5 variant. The broader 3NJ6940 range also includes a 400/1 A Class 0.5 (3BJ12) for long-distance metering applications. If you need a 400 A CT with legal-for-trade calibration or a Class 1 accuracy option, consult the wider Siemens SENTRON CT portfolio — not all accuracy/calibration combinations are available at every primary current rating within the 3NJ6940 series.

Does a 400 A CT with a 32 mm window fit my cables?

A 400 A feeder typically uses 120–185 mm² conductors. Single-core cables in this range have outer diameters of roughly 18–26 mm — well within the 32 mm window. For multi-core cables or cables with enhanced outer sheaths, always measure the actual diameter before ordering.

Can this CT be used on a 3NJ6 disconnector rated higher than 400 A?

The CT rating and the disconnector frame rating are independent. You can install a 400 A CT on a 630 A 3NJ6 disconnector if the actual load current on that specific feeder does not exceed 400 A. The CT measures the current in the cable passing through it — not the disconnector rating. Ensure the cable diameter does not exceed 32 mm and the primary current stays within the CT rating.