In low-voltage distribution applications, DIN-rail energy meters are widely used in homes, shops, and industrial distribution boxes due to their compact size, easy installation, and neat wiring.

However, many people are confused when selecting between 1P, 2P, and 3P specifications — they don't understand the naming logic or the size differences. This article explains the industry basics clearly and thoroughly.

Simply Put

P stands for "Pole" in English. But in the context of DIN-rail energy meters, it actually represents the "module width." You can think of it as a "standard occupancy unit."

Internationally, it is uniformly defined that: 1P ≈ 18 mm wide. So:

• 1P meter → 18 mm wide

• 2P meter → 36 mm wide

• 3P meter → 54 mm wide
  …and so on.

Why Define It by “P”?

The use of "P" comes from the broader world of modular terminal electrical devices. A distribution box doesn't just contain meters — it also holds miniature circuit breakers, residual current devices (RCDs), isolators, and more.

To ensure that devices from different brands and with different functions can all fit on the same 35 mm DIN rail, a unified base size is essential. The industry therefore standardized the width of one circuit breaker pole at 18 mm and uses "P" to denote this width unit.

So, when you specify a "2P meter," both the manufacturer and installer immediately understand that it takes up exactly two standard positions — sitting shoulder-to-shoulder with a 2-pole circuit breaker, perfectly aligned.

Common Meter “Sizes” and Their Meaning

Now that the definition of "P" is clear, it's easy to understand the common “body types” of energy meters:

• 1P (18 mm wide): Exists but is relatively rare. Usually used in very simple single-phase scenarios where only the live wire is metered, and the neutral is shared directly.

• 2P (36 mm wide): The most common single-phase meter. One live and one neutral in and out, occupying two positions — the same width as a standard single-phase circuit breaker. Widely used in household sub-metering and rental property energy measurement.

• 3P (54 mm wide): Typically a three-phase three-wire meter without neutral connection. Often used for measuring three-phase motor circuits in industrial applications.

• 4P (72 mm wide): A 4P meter may be a three-phase meter with communication or tariff control functions, requiring additional space. This is also the typical format for a three-phase four-wire direct-connected DIN-rail meter.

• 7P (approx. 126 mm wide): Because it needs to accommodate four current circuits (A, B, C, and N phases), plus sufficient internal space for current transformers and terminals, seven module positions fit the requirement exactly.

There are also transformer-connected three-phase meters that may be 5P, 6P, etc., with designs varying flexibly depending on functionality.

Height and Depth — Also Important, but Standardized

In addition to width, the height and depth of DIN-rail meters also matter. However, since the DIN-rail mounting height is standardized, the vast majority of DIN-rail meters are about 90–100 mm in height and 60–75 mm in depth — designed to fit perfectly into standard distribution box enclosures.

The dimension that truly determines the layout of your distribution box is the "P number" — it directly dictates how much space the meter will occupy on the rail.

Final Takeaway

So next time you see a "2P DIN-rail meter" or "3P three-phase meter," don't think of it as horsepower. It simply tells you: at 18 mm per P, leave that much width in the distribution box, and the meter will snap securely onto the rail, standing neatly in line with all the other switches around it.