A generator one-line diagram uses graphic symbols, equipment tags, electrical ratings, and protection device numbers to show how a power system connects. Use this reference to decode common notation, then check the drawing’s legend for the meaning used on that project. A one-line represents a circuit or system with simplified lines; it does not show every conductor or control connection.
Equipment Symbols and Abbreviations
Graphic symbols vary between drawing conventions. Eaton’s NEMA and IEC schematic comparison, MZ081001EN, effective November 2013, provides side-by-side examples for breakers, switches, fuses, transformers, motors, contacts, and earth connections. Use it to compare actual graphics rather than assuming that one symbol shape applies everywhere.
The following labels are common reading aids, not a mandatory equipment-tagging scheme. Eaton’s Basics of Power System Design, DG081001EN, illustrates equipment tags and ratings in its one-line examples; the revision date of the linked copy was not confirmed.
| Notation or graphic cue | Common meaning | What to check on the drawing |
|---|---|---|
| G beside or inside a generator symbol | Generator | Confirm the connected equipment and equipment schedule. G elsewhere can have another meaning. |
| M beside or inside a motor symbol | Motor | Read the motor’s voltage and load information separately. |
| CB | Circuit breaker | Check its current rating, trip information, poles, and interrupting rating. |
| ATS | Automatic transfer switch | Identify the normal source, alternate source, load connection, and neutral arrangement. |
| T or XFMR | Transformer | Read primary and secondary voltages, winding connections, kVA, and impedance. |
| CT | Current transformer | Check the ratio, quantity, and whether it serves metering or protection. |
| PT or VT | Potential transformer or voltage transformer | Check the voltage ratio and the connected measurement or protection circuit. |
| Bus or busbar line | Common electrical connection serving multiple circuits | Read its voltage, current, and short-circuit rating separately. |
| N | Neutral | Trace the neutral arrangement; do not interpret N as equipment ground. |
| GND or an earth/ground graphic | Grounding-related connection | Identify the conductor or connection from the legend and grounding details. |
| N.O. / N.C. | Normally open / normally closed | Check the drawing’s stated normal condition. These labels do not report the equipment’s live position. |
These interpretations follow the equipment and connection examples in Eaton’s publications. A tag such as G, T, or N must still be read in context: its location and associated symbol matter.
Ratings, Units, and Phase Notation
A rating beside a symbol describes a particular property of that equipment—not necessarily the load it currently carries. For example, a bus current rating, breaker trip rating, and generator output rating are different quantities, even when they appear on the same power path.
| Notation | Meaning | Reading caution |
|---|---|---|
| V / kV | Volts / kilovolts | Establish whether the value is line-to-line or line-to-neutral. |
| A | Amperes | Determine whether it describes equipment capacity, a protective-device rating, or load current. |
| Hz | Frequency in hertz | Read the specified system or equipment frequency. |
| 1Ø / 1PH | Single-phase | The phase designation alone does not establish the voltage or conductor arrangement. |
| 3Ø / 3PH | Three-phase | Read it together with the voltage and system connection. |
| 3W / 4W | Three-wire / four-wire system notation | Check the legend and grounding details; do not infer the equipment-grounding conductor count from this label alone. |
| Y / Δ | Wye / delta winding connection | Confirm whether a neutral point is brought out and how the system is grounded. |
| kW | Kilowatts of active power | Not interchangeable with kVA. |
| kVA | Kilovolt-amperes of apparent power | Read together with the applicable kW and power-factor rating. |
| PF | Power factor | The ratio of active power to apparent power, not generator efficiency. |
| AF / AT | Ampere frame / ampere trip | Distinguish the breaker frame designation from its trip rating and any adjustable settings. |
| LSIG | Long-time, short-time, instantaneous, and ground-fault trip functions | The letters identify functions, not their pickup values or time settings. |
Eaton’s DG081001EN provides the system, winding, breaker, and trip-function examples above. Schneider Electric’s Definition of Power Factor, revised August 5, 2026, supports the distinction between kW, kVA, and PF.
For power ratings expressed on the same basis:
[
P = S \times PF
]
Here, (P) is active power in kW, (S) is apparent power in kVA, and (PF) is dimensionless. This relationship does not establish motor-starting capability, transient performance, or a generator’s site-available output.
Common Protection Device Numbers
Numbers such as 27, 50, and 52 identify device functions in ANSI/IEEE-style notation. They are not current ratings, equipment quantities, or terminal numbers. Eaton’s one-line reference illustrates numbered protective devices and breakers; the SEL-351 product literature, PF00198, revision 20170815, provides a manufacturer’s function-number legend.
| Number or label | Function | Reference |
|---|---|---|
| 25 | Synchronism check | SEL PF00198, August 15, 2017 |
| 27 | Undervoltage | SEL PF00198, August 15, 2017 |
| 32 | Directional power | SEL PF00198, August 15, 2017 |
| 50 | Instantaneous overcurrent | SEL PF00198, August 15, 2017 |
| 51 | Time-overcurrent | SEL PF00198, August 15, 2017 |
| 52 | AC circuit breaker | Eaton DG081001EN; revision date not confirmed |
| 59 | Overvoltage | SEL PF00198, August 15, 2017 |
| 81O / 81U / 81R | Overfrequency / underfrequency / rate of change of frequency | SEL PF00198, August 15, 2017 |
A multifunction relay can contain several numbered functions. A label such as 27/59 therefore need not indicate two separate relay boxes. Likewise, 32 identifies directional power; whether it implements reverse-power protection depends on the relay configuration and settings. Suffixes such as N, G, P, and Q must be checked against the specific relay manual and drawing legend.
How to Read a Generator Label
Consider this illustrative label:
GEN-1 — 125 kVA / 100 kW, 480Y/277 V, 3Ø, 4W, 60 Hz, PF 0.8
Read it as follows:
GEN-1identifies the equipment in this example.125 kVA / 100 kWstates apparent-power and active-power ratings. They are consistent because (125 \times 0.8 = 100).480Y/277 Videntifies a wye system with nominal 480 V line-to-line and 277 V line-to-neutral.3Ø, 4Widentifies three-phase, four-wire system notation; consult the grounding details separately.60 Hzstates the rated frequency.PF 0.8states the power factor associated with these example ratings—not a default for every generator or connected load.
The label does not identify whether the output is a standby, prime, or continuous rating, nor whether a particular motor can start successfully. Check the generator nameplate and model-specific technical data for the rating category and operating conditions.
Neutral and Ground Labels Need Separate Checks
Do not use N, GND, an earth symbol, or an ATS pole count alone to decide where a neutral-to-ground bond belongs. The neutral connection, transfer-switch arrangement, and ground-fault sensing must be considered together. Cummins’ Power Topic #6006, Part 2, December 2006, illustrates how different neutral arrangements affect fault-current paths and sensing; its historical code references are not a substitute for the locally adopted requirements.
For installation or modification, a qualified electrical professional should resolve the arrangement using the project drawings, equipment instructions, and applicable local code edition. A notation reference helps interpret the documents; it does not determine conductor sizing, transfer-switch suitability, protection settings, or installation compliance.