The standard ammeter schematic symbol is a circle with a capital 'A' in the center, recognized globally by both IEC 60617 and ANSI/IEEE 315 standards. When dealing with high-current AC panels, this symbol is almost always paired with a Current Transformer (CT) symbol, while DC applications typically show it in series with a millivolt shunt. Below is the complete reference for meter symbols you will encounter on electrical schematics and panel diagrams.
The Complete Meter Symbol Reference Table
Use this table to quickly identify meter symbols on single-line diagrams, control schematics, and PCB silkscreens. The designations below map directly to the IEEE 315 and IEC 60617 standards.
| Instrument | Schematic Symbol | IEC 60617 / ANSI Designation | Typical Jobsite / Bench Use Case |
|---|---|---|---|
| Ammeter | Circle with 'A' | S01462 / 315-1975 | Measuring load current; wired in series or via CT secondary. |
| Voltmeter | Circle with 'V' | S01463 / 315-1975 | Measuring potential difference; wired in parallel across phases. |
| Ohmmeter | Circle with 'Ω' | S01465 / 315-1975 | Insulation resistance testing (Megger) or continuity checks. |
| Wattmeter | Circle with 'W' | S01466 / 315-1975 | Measuring real power (kW); requires both current and voltage coils. |
| Galvanometer | Circle with 'G' | S01461 / 315-1975 | Null-detection in Wheatstone bridges; highly sensitive micro-amps. |
| CT + Ammeter | Two overlapping circles (CT) feeding a circle with 'A' | Composite Symbol | Panel metering for feeders >50A; steps down primary current to a 5A secondary. |
Rows People Get Wrong: Ammeter vs. Voltmeter & Shunts
The most dangerous mistake on the bench or jobsite is confusing the physical wiring requirements of an ammeter versus a voltmeter, despite their nearly identical schematic symbols (just a different letter inside the circle).
The Dead-Short Hazard
An ammeter must be wired in series with the load. Its internal resistance is designed to be as close to zero as possible to avoid altering the circuit. For example, a standard Fluke 87V multimeter has an internal resistance of roughly 0.2 ohms on the mA range, and essentially 0 ohms on the 10A range. If you misread the schematic and wire an ammeter in parallel across a voltage source (like a voltmeter), you create a dead short. This will instantly blow the meter's internal fuse, destroy the PCB traces, or cause an arc flash on mains circuits.
Ignoring the Shunt or CT Symbol
On DC schematics, you will rarely see a standalone ammeter symbol for currents above 10A. Instead, you will see an ammeter symbol wired across a shunt resistor. Standard DC panel shunts are rated for a specific voltage drop at full scale—typically 50mV or 60mV. If your schematic shows a 500A/50mV shunt, the ammeter symbol connected to it must be calibrated for a 50mV full-scale deflection. Connecting a standard 60mV meter to a 50mV shunt will result in a 20% reading error at full load.
Regional Standards: IEC, ANSI, and Legacy UK
While the 'circle with an A' is nearly universal today, the surrounding context and older drawings vary by region. Understanding these variants is crucial when troubleshooting legacy equipment or working with imported machinery.
- IEC 60617 (Global / EU / Modern UK): Uses clean, minimalist circles for all indicating instruments. The ammeter is strictly a circle with an 'A'. Current transformers are depicted as two overlapping circles or a circle with a line through it, clearly separated from the metering circle.
- ANSI/IEEE 315 (North America): Also uses the circle with an 'A', but older North American schematics (pre-1990s) sometimes used a square or a rectangle with an 'A' for panel-mounted switchboard meters to distinguish them from portable test instruments. You will still see this on legacy Motor Control Center (MCC) drawings in the US and Canada.
- BS 3939 (Legacy UK): Before the UK fully harmonized with IEC standards, BS 3939 was the norm. It frequently used different line weights and occasionally rectangular boxes for instruments. If you are retrofitting a control panel in an older British facility, do not assume a rectangle with an 'A' is a relay; check the wiring to confirm it is a legacy ammeter symbol.
For a deeper dive into how these symbols integrate into broader circuit diagrams, the All About Circuits reference library provides an excellent breakdown of standard component designations.
Safe Interpretation of Faded or Missing Panel Markings
In industrial environments, panel meter glass gets cracked, faceplates fade from UV exposure, and schematic labels peel off. If you are faced with a panel meter where the 'A' or 'V' is completely illegible, you must safely interpret the meter's function before applying power or taking measurements. Never guess based on the needle position or the physical size of the meter.
Step 1: Trace the Wire Gauge and Routing
De-energize the panel, lock out/tag out (LOTO), and verify dead with a calibrated CAT III or CAT IV meter. Once safe, trace the wires connected to the back of the meter.
- Ammeter (CT-driven): Will typically have 12 AWG or 14 AWG THHN wires. These wires will route back to a Current Transformer wrapped around a main busbar or feeder cable. The wires will often terminate on a shorting block (a terminal strip with sliding links designed to short the CT secondary).
- Voltmeter: Will typically have much smaller sensing wires, usually 18 AWG or 22 AWG. These wires will route to a voltage terminal block, often with high-impedance fuses (e.g., 0.5A or 1A glass fuses) or tap directly into a busbar via a high-resistance dropping resistor.
- Ammeter (Direct-wire DC): Will have massive cables (e.g., 2/0 AWG or 4/0 AWG) passing through a heavy brass or manganin block (the shunt) mounted directly behind the panel. The meter itself will only have two small twisted-pair wires connecting it to the shunt's Kelvin (sense) terminals.
Step 2: Check for Burden Resistors
If the meter is an analog ammeter driven by a CT, look for a burden resistor wired in parallel across the meter terminals. CTs require a specific burden (load) to maintain accuracy. If you find a high-wattage wirewound resistor (e.g., 5W, 10Ω) across the terminals, it is almost certainly an ammeter. Voltmeters do not use burden resistors.
When working with clamp-on ammeters for field verification of these panel meters, ensure you are using a true-RMS meter capable of handling the harmonic distortion common in modern VFD-driven panels. The Fluke guide on clamp meter usage details how to properly zero and orient the clamp jaw to avoid phantom readings from adjacent magnetic fields in crowded panels.
Pro-Tip for Bench Work: If you are designing your own PCB or control panel and need to silk-screen an ammeter symbol, always add the maximum expected current next to the symbol (e.g., 'A [0-50A]'). A standalone 'A' symbol tells the technician what the meter does, but not what scale to expect, which can lead to dangerous misinterpretations during a fault condition.
