Understanding and Example of a One-Line Diagram for an Electrical Substation (PDF Download Available, in Spanish)
This article explains what a one-line diagram is and provides an example of an electrical substation’s one-line diagram in PDF format for download and further analysis.
The substation is a pole-mounted type featuring a 75 KVA transformer and a 7-200 meter socket feeding a general load.
This article focuses on the one-line diagram of the substation, covering components from the medium-voltage entrance to the main disconnect switch.
We recommend the following Excel file, available on Hotmart, which enables calculations for conductors, conduits, amperage, fuses, grounding systems, and more. This is a valuable tool when preparing a one-line diagram.
Components of a One-Line Diagram for an Electrical Substation
To simplify, we’ll divide the substation diagram into sections:
- Medium Voltage Entrance
- Electrical Transformer
- Meter Socket
- Main Switch and Load
Download the One-Line Diagram of an Electrical Substation in PDF here: Diagram-Unifilar-Substation-ElectricalDownload.
Medium Voltage Entrance
The diagram begins at the medium voltage entrance, describing the medium voltage line and the types and ratings of protection devices up to the transformer.
This example includes a three-phase, 33,000V medium voltage line operating at 60 Hz. The line comprises three phases and four wires: ACSE #1/0 conductors for the phases, and an ACSR #1/0 conductor for the neutral.
The line leads to the fuse cutouts—three in total, one per phase, rated appropriately for the line voltage. Each fuse cutout is rated for 2 A.
Electrical Transformer in the Substation
The next component in the substation’s one-line diagram is the electrical transformer.
In this case, a three-phase, 75 KVA transformer rated for 33,000V on the primary (medium voltage) side and 440V on the secondary (low voltage) side.
This pole-mounted transformer is configured with three #1/0 AWG copper phase conductors and a #1/0 AWG neutral, insulated with THW-LS, all routed through a 2-inch galvanized rigid conduit (GRC).
The diagram should indicate voltage, load, distance, and voltage drop, as illustrated above. Calculations for these values can be completed using the Excel file mentioned at the beginning of this article.
Meter Socket
Following the transformer, the next element in the one-line diagram is the meter base. Types include 5-100 (single-phase), 7-100 (three-phase 100A), 7-200 (three-phase 200A), and 13-20 (three-phase for loads over 200A, with current transformers).
Here, a 7-200 meter base is used, suitable for loads that do not exceed 200A.
In the meter base, the neutral-to-ground bond occurs, connecting the transformer’s neutral to the grounding system, which includes the grounding rod or well.
The outgoing conductors from the meter base include the same conductors from the transformer’s secondary plus a grounding conductor: three #1/0 phase conductors, one #1/0 neutral, and one #2 bare grounding conductor within a 2-inch LICAUTITE conduit.
NOTE: If the neutral is not needed, as in a strictly 440V setup, the neutral can be omitted, leaving only three #1/0 phases and a #2 grounding conductor. This setup is common in agricultural wells or when loads are primarily motors.
Main Switch and Load
The final section of the substation is the main disconnect switch, responsible for safeguarding the feeder that supplies power to the load.
There is a main switch at the substation and another at the load location, typically a distribution panel or load center.
The switch protects the feeder supplying the load, which here consists of three #2 AWG copper phase conductors, one #2 AWG neutral, and one #8 grounding conductor.
With the #2 AWG copper conductors rated for 95A, the switch should be the next standard rating up, 100A.
An #8 grounding conductor is permissible with a 100A breaker; however, if a larger breaker is used, the grounding conductor must also be upsized according to Table 250-122 of the NEC.
The feeder specifications include operating voltage, load, amperage, distance, and voltage drop—in this case, 37 meters.
Finally, the load to be supplied is indicated with its main disconnect switch.
This is the one-line diagram for an electrical substation.
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