The standard schematic symbol for an ammeter is a circle with a capital "A" inside, defined by both IEC 60617 and ANSI/IEEE 315. On physical panel meters, terminals are marked with P1/P2 (IEC) or H1/H2 (ANSI) for direct connection, and S1/S2 or X1/X2 when wired to a Current Transformer (CT). When drafting a single-line diagram, selecting the correct symbol ammeter block depends entirely on the regional standard governing the facility and whether the meter is direct-driven or CT-operated.
Ammeter Symbol & Terminal Reference Table
Use this table to identify schematic symbols and physical terminal markings. This covers both the drawing representation and the physical wiring points on the back of the meter.
| Category | Symbol / Marking | Standard | Practical Meaning & Application |
|---|---|---|---|
| Schematic (Amps) | Circle with "A" | IEC 60617 / IEEE 315 | Measures current in Amperes. Placed in series with the load (or CT secondary) in schematics. |
| Schematic (Milli/Micro) | Circle with "mA" or "μA" | IEC 60617 / IEEE 315 | Used for low-current control circuits, op-amp feedback loops, or leakage current measurement. |
| Panel Terminals (Direct IEC) | P1, P2 or L1, L2 | IEC / EN 61010 | Direct-drive meter. Current enters P1/L1 and exits P2/L2. Internal shunt handles the full load. |
| Panel Terminals (Direct ANSI) | H1, H2 | ANSI C12.1 | North American direct-drive equivalent. Current enters H1, exits H2. |
| CT Secondary (IEC) | S1, S2 | IEC 61869 | Wired to the CT secondary. S1 is the polarity mark (current enters here from the CT). |
| CT Secondary (ANSI) | X1, X2 | IEEE C57.13 | North American CT secondary. X1 is the polarity mark. X2 must be grounded per NEC. |
| Polarity Indicator | Asterisk (*) or Dot (•) | Universal | Found on both CTs and ammeters. Indicates the designated current entry point for correct phase alignment. |
Regional Standard Variants: IEC vs. ANSI vs. Legacy
While the basic circle-and-letter schematic representation is nearly universal, the physical terminal designations and CT wiring rules diverge sharply by region.
IEC (International, EU, UK, AU)
Governed by IEC 60617 for symbols and IEC 61869 for instrument transformers. The IEC standard strictly uses S1 and S2 for CT secondaries. In modern IEC panels, the CT secondary S2 terminal is bonded to earth ground at a single point to prevent floating voltages, while S1 carries the signal to the ammeter.
ANSI / IEEE (North America)
Governed by IEEE 315 for schematic symbols. Physical CT terminals use X1 and X2 (or sometimes H1/H2 for the primary line passing through the window). The National Electrical Code (NEC) strictly mandates that the X2 secondary terminal be grounded to prevent lethal voltage spikes if the CT opens.
Legacy UK (BS 1588) & Old Industrial
If you are troubleshooting a panel built before the 1990s in the UK or older Commonwealth industrial sites, you may encounter BS 1588 symbols. These sometimes used a semi-circle or omitted the circle entirely, relying on the letter "A" inline with the wire. Terminal markings were often just numbered "1" and "2" without polarity designations, which is acceptable for simple analog moving-iron ammeters but will cause errors if retrofitted with modern digital power analyzers.
Rows and Markings People Get Wrong
- Confusing Generic Meters with Ammeters: A circle with no letter inside is a generic meter symbol. If the context doesn't explicitly show it in series with a load or CT, verify the bill of materials. Assuming it's an ammeter when it's actually a voltmeter (which should be in parallel) will result in a dead short and a blown fuse or exploded shunt.
- Ignoring the Polarity Dot (*): For a basic analog moving-iron panel ammeter, AC current direction doesn't matter; the needle will deflect correctly either way. However, if that same CT secondary is shared with a digital power meter or a protective relay, ignoring the S1/X1 polarity mark will result in reversed power factor readings and tripped ground-fault relays.
- Misidentifying Shunt Terminals: On direct-drive DC ammeters, you will often see four terminals: two large ones for the load current and two small ones for the meter movement. People frequently wire the load through the small terminals, instantly burning out the internal 50mV or 75mV shunt. The load always goes through the massive, unmarked or P1/P2 lugs; the meter movement wires go to the small screw terminals.
Decision Path: Wiring Panel Ammeters and CTs
Use this decision tree to select the correct ammeter type and wiring method based on your circuit parameters.
| Condition | Action / Selection | Concrete Part / Spec Example |
|---|---|---|
| IF Load Current is < 50A (AC or DC) | Use a Direct-Drive Ammeter. Wire load in series through main lugs. | Simpson 3323 (Analog) or Modutherm 50A Digital Panel Meter. |
| IF Load Current is > 50A and run distance is < 10 meters | Use a 5A Secondary CT and a matched 5A Panel Ammeter. Ground S2/X2. | Hobut 102-55 Current Transformer (200/5A) paired with a 200/5A scale analog meter. |
| IF Load Current is > 50A and run distance is > 10 meters | Use a 1A Secondary CT and a matched 1A Panel Ammeter to reduce voltage drop on the secondary wiring. | Socomec DIRIS A10 power meter configured for 1A input, with 400/1A CTs. |
| IF Measuring DC Current > 10A | Use a DC Shunt and a millivolt meter scaled to the shunt's rating (typically 50mV or 75mV full scale). | 75mV 100A DC Shunt paired with a 0-100A scaled moving-coil meter. |
Safe Interpretation When Markings Are Faded or Missing
On legacy equipment, terminal markings are often obscured by decades of dust, heat discoloration, or poor stamping. Guessing the wiring on a live panel is unacceptable. Follow this de-energized verification protocol:
- De-energize and Lockout: Shut off the main breaker and apply a lockout/tagout device. Verify the primary circuit is dead using a CAT III or CAT IV rated multimeter.
- Identify the Ammeter Terminals (Direct Drive): Set your multimeter to the lowest ohms range. Place probes across the suspected meter terminals. A direct-drive analog ammeter will show a near-short (typically 0.05 to 0.2 ohms) due to the internal shunt or moving-coil resistance. A voltmeter will show high resistance or an open circuit (OL) due to the internal series multiplier resistor.
- Identify CT Polarity (The Kick Test): If the S1/X1 marks on the CT are faded, you can determine polarity using a 9V battery and an analog multimeter (set to low DC voltage or microamps). Connect the analog meter to the CT secondary. Momentarily tap the 9V battery positive to the CT primary P1/H1 side, and negative to P2/H2. If the analog meter needle kicks positive (upscale) upon connection, the terminal connected to the meter's positive lead is S1/X1. If it kicks negative, swap the meter leads and repeat.
- Verify Secondary Grounding: Trace the wire from the S2/X2 terminal. It must terminate on a verified earth ground bar. If it floats, bond it to ground before re-energizing to comply with NEC 250.110 and IEC grounding mandates.
By cross-referencing the schematic symbol with the physical terminal markings and applying the correct regional standard, you ensure both accurate measurement and operator safety. Always default to CT isolation for any AC circuit exceeding 50A, and never compromise on secondary grounding.
