Practical guide to selecting AC ammeters and matching current transformers for LV panels: ranges, accuracy class, CT ratio, burden, wiring and a procurement
An AC ammeter is the single most useful instrument you can mount in a low-voltage switchboard. It tells the operator and the maintenance team how much load each feeder actually carries, whether a motor is drawing its nameplate current, and whether a phase has dropped out. For panel builders and procurement engineers, specifying the right ammeter together with its current transformer (CT) is not optional—it is part of a safe, auditable installation.
Current is also the backbone of energy accounting and protection coordination. A feeder running at 90 % of its rating every day is a thermal risk and a hidden capacity bottleneck; the ammeter is the cheapest early-warning device you can fit. For procurement teams, standardising on one meter family across a project cuts spares, training and documentation cost.
This guide covers analogue and digital AC ammeters, the difference between direct-reading and CT-operated models, how to size and match a CT, and a procurement checklist you can hand straight to your supplier.
Both types measure the same quantity, but they suit different panels and budgets.
| Criterion | Analogue (moving-iron) | Digital (LED/LCD) |
|---|---|---|
| Readability at a glance | Excellent, no supply needed | Good, needs auxiliary supply |
| Accuracy class | Typically 1.5 to 2.5 | 0.5 to 1.0 |
| Price | Low | Moderate |
| Extra functions | None | Min/max, alarm, RS-485 |
| Best for | Local indication, genset panels | MCCs, energy dashboards |
For most modern motor control centres and distribution boards, a digital panel meter gives you a 0.5-class reading and often a Modbus output at little extra cost. Analogue meters remain popular where you want a reading even when the control supply is dead.
Ammeters below about 63 A are usually direct-reading: the conductor passes straight through the meter terminals. Above that current, running the full load through the instrument is impractical and unsafe, so you use a CT-operated meter with a standard 5 A (or 1 A) secondary.
If your feeder exceeds 63 A, plan for a CT-operated meter and size the CT before you size the meter—the CT is the component that actually sets the accuracy.
CT-operated meters come in 5 A and 1 A secondary variants. The 5 A type is the global default and matches most panel meters and the widest spare-parts pool. The 1 A type is preferred for long lead runs (above roughly 10 m) because it carries one-fifth of the current, so lead resistance wastes far less of the CT's burden budget and accuracy holds up over distance. If your CT is more than a few metres from the meter, ask for a 1 A secondary or upsize the leads.
The accuracy class printed on the meter and the CT is the maximum permitted error at rated conditions: class 1.0 means plus or minus 1 %, class 0.5 means plus or minus 0.5 %. For general indication, class 1.0 is fine; for any measurement that feeds billing, sub-billing between tenants, or a power-quality report, specify class 0.5 on both the meter and the CT. Also confirm the rated frequency—most low-voltage networks run at 50 Hz, but panels destined for North America or aerospace-related sites may need 60 Hz calibration. A 50 Hz meter reads within class on 60 Hz, but verify the maker's statement rather than assuming.
Match these four parameters when you specify the CT:
| Parameter | What to set | Why it matters |
|---|---|---|
| Ratio | Primary : 5 A | Must exceed max load with headroom |
| Accuracy class | 0.5 or 1.0 | Billing and sub-metering need 0.5 |
| Burden (VA) | At least meter plus wiring | An over-burdened CT loses accuracy fast |
| Window or bar type | Split-core or bar | Split-core retrofits without breaking the bus |
A three-phase three-wire system (no neutral) is usually metered with two CTs in an A-B or A-C arrangement. A three-phase four-wire system (with neutral) needs three CTs, one per phase, summed inside the meter. Getting this wrong gives a reading that is off by roughly a third—an easy mistake that wastes a site visit. Always confirm the system type from the single-line diagram before ordering.
Specify the ammeter and the CT together as a matched pair. A precise CT and a 0.5-class digital meter cost far less than the downtime caused by a misleading current reading.
No. The meter expects a 5 A secondary supplied by a CT. Connecting it across a 100 A line will destroy the instrument. Always insert the current transformer between the conductor and the meter.
Use class 0.5 when the reading feeds billing, sub-metering or energy reports. Class 1.0 is acceptable for pure local indication where no settlement depends on the value.
Check the auxiliary supply first, because digital meters typically need 80 to 270 V to power up. Then verify CT polarity (S1/S2) and confirm the CT is clamped on the correct phase.
Yes. VFD and switched-mode loads are non-sinusoidal, so an average-responding meter under-reads. Specify a True-RMS ammeter for any variable-speed or power-electronic load.
Split-core CTs let you retrofit a measurement point without breaking the busbar, which is ideal for upgrades. Solid-bar CTs are cheaper and slightly more accurate, so they suit new builds where the conductor can be routed through the window.
Send the ratings, dimensions and quantities you are working to and we will confirm the closest match from the ELEHPD catalogue, the compliance documents available for it and the lead time. OEM branding, private model numbering and buyer-specific cartons are available on qualified volumes.