Power Distribution Block Symbol
Definition: The Power Distribution Block symbol represents a bus-bar or terminal assembly that accepts a single high-current input conductor and distributes power to multiple branch output circuits, depicted in industrial wiring and panel diagrams as a labelled block with one IN pin and multiple output pins (Out1–Out4), indicating a current-dividing junction point used in control panels, motor control centres, and power-distribution enclosures.
Also known as: power distribution block, bus block, distribution terminal, tap block, branch terminal block, bus bar tap.
What the Power Distribution Block symbol means
The Power Distribution Block symbol represents a mechanical current-distribution component that connects one large-gauge incoming conductor to several smaller-gauge branch-circuit conductors at a common electrically bonded point. In a panel wiring diagram the symbol identifies where a single supply cable fans out to multiple loads, sub-panels, or branch circuits without the need for splicing or wire nuts.
Power distribution blocks are rated for specific maximum input and output currents, and the symbol communicates that all output pins share the same electrical potential as the input. In industrial control panels (IEC 61439) and UL-listed enclosures (UL 508A), the distribution block is a common component for 24 V DC control buses, 120 V AC branch circuits, and three-phase 480 V distribution. The symbol shows IN on the left and Out1 through Out4 on the output side.
How to identify the Power Distribution Block symbol
The Power Distribution Block symbol is drawn as a rectangle labelled 'DIST BLOCK' or 'POWER DIST' with a single thick line (IN) entering from the left or top and multiple thinner output lines (Out1, Out2, Out3, Out4) exiting from the bottom or right. The multiple-output fan-out geometry distinguishes it from a simple terminal block (one-in, one-out per pole) and from a busbar (a continuous bar serving many points). The block may be annotated with its current rating (e.g. '100 A in / 30 A per output') and voltage class.
Function in a circuit
A power distribution block electrically bonds an incoming power conductor to multiple outgoing branch conductors through a conducting copper or brass bus element, typically with separate input and output clamping mechanisms rated for different wire gauges. The input side accepts heavy-gauge wire (e.g. 4 AWG / 25 mm² for a 100 A block); the output side accepts smaller wires (e.g. 10–14 AWG / 6–2.5 mm²) for individual branch loads. All outputs are at the same potential as the input. Blocks are rated by maximum continuous current, voltage class, and number of poles; they must be selected so the sum of output currents does not exceed the input rating.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60947-7-1 (Low-voltage switchgear — Terminal devices for copper conductors) covers terminal blocks including distribution types. IEC 61439-1 (Low-voltage switchgear and controlgear assemblies) governs their use in panel assemblies. IEC 60617 does not define a dedicated distribution block glyph; it is drawn as a junction or terminal block symbol. |
|---|---|
| ANSI/IEEE 315 | UL 1059 (Terminal Blocks) covers distribution blocks for US applications; NEMA ICS 4 addresses industrial control terminal devices. ANSI Y32.2 / IEEE 315 represents distribution blocks as junction symbols or labelled rectangles; no dedicated standard glyph is defined. |
| Key difference | IEC and ANSI/UL use the same rectangular block convention in schematic diagrams. The physical product ratings differ (IEC uses mm², ANSI/UL uses AWG wire gauges) but the schematic symbol representation is identical. |
Terminals / pins
| Pin | Name |
|---|---|
| in | IN |
| out1 | Out1 |
| out2 | Out2 |
| out3 | Out3 |
| out4 | Out4 |
Typical values
Typical input ratings: 60–400 A; output ratings: 15–100 A per position; number of output positions: 2–16; voltage class: 300 V, 600 V, 1000 V AC/DC; wire sizes: input 1/0 AWG to 4 AWG (6–70 mm²); output 22–6 AWG (0.5–16 mm²); materials: copper bus with UL-rated plastic housing; temperature rating: 75°C or 90°C (per NEC 110.14 conductor temperature rating).
Where the Power Distribution Block symbol is used
- Industrial control panels distributing 24 V DC control power from a single power supply to multiple PLC input modules, relays, and sensors
- Motor control centres (MCCs) branching 480 V AC from the main bus to individual motor starters
- Electrical sub-panels fanning out a single feeder to multiple 120 V or 240 V branch circuits
- Automotive and marine DC distribution from the battery or alternator output to multiple loads
- Renewable energy systems distributing DC bus power from a battery bank to multiple inverter or charge-controller inputs
- Data-centre PDU (power distribution unit) internal bus connections feeding multiple outlet banks
Example
In an industrial machine control panel diagram, a 100 A power distribution block symbol receives the incoming 24 V DC supply from a DIN-rail power supply through its IN terminal (4 AWG). Four output positions (Out1–Out4) each feed a 30 A branch: a PLC CPU, an I/O module rack, a relay panel, and an HMI touchscreen. The distribution block symbol clearly shows that all four 24 V branches originate from the same supply point without splicing.
Key facts
- The Power Distribution Block symbol represents a current-dividing bus junction with one high-current input (IN) and multiple branch outputs (Out1–Out4), all electrically common.
- Input and output current ratings are independent: a 100 A input block may have four 30 A output positions, but the total connected output load must not exceed the 100 A input rating.
- Wire gauges are different for input and output: the input accepts heavy-gauge conductors (e.g. 4 AWG / 25 mm²); outputs accept lighter gauges (e.g. 10–14 AWG / 2.5–6 mm²) to match branch-circuit ampacity.
- Distribution blocks are governed by IEC 60947-7-1 and UL 1059; they are rated by voltage class (300 V, 600 V, 1000 V) and maximum continuous current.
- All output pins share the same electrical potential as the input; the distribution block is not a transformer or converter — it does not change voltage or current direction.
- In IEC 61439-compliant panels and UL 508A-listed enclosures, distribution blocks must be selected with sufficient current and temperature ratings for the installation environment.
- The sum of all branch-circuit currents must not exceed the input terminal rating; overloading the block causes contact heating, insulation damage, and potential fire.
Frequently asked questions
What does the power distribution block symbol look like?
The power distribution block symbol is a labelled rectangle with a single thick IN connection on one side and multiple output connections (Out1, Out2, Out3, Out4) on the other side. The multiple-output fan-out geometry is its defining visual characteristic, distinguishing it from a simple terminal block with one-in, one-out.
What does the power distribution block symbol mean in a wiring diagram?
The symbol means that a single incoming supply conductor fans out to multiple branch circuits at a common bus junction. All outputs are at the same voltage as the input. It is used wherever a single power source must feed several loads without splicing conductors.
What is the difference between a power distribution block and a busbar?
A power distribution block is a discrete component with defined input and output terminals rated for specific wire gauges and currents. A busbar is a continuous conducting bar that multiple conductors can tap at any point along its length. In schematic diagrams a distribution block shows a finite number of labelled output pins; a busbar is shown as a continuous line with multiple tap symbols.
What standards govern power distribution blocks?
IEC 60947-7-1 (Low-voltage terminal devices for copper conductors) and IEC 61439-1 (Low-voltage switchgear assemblies) govern distribution blocks in IEC-based systems. UL 1059 (Terminal Blocks) covers US listed products. NEMA ICS 4 addresses industrial terminal device ratings.
Can the sum of output currents exceed the input rating of a distribution block?
No. The total of all simultaneously flowing branch currents must not exceed the input terminal's rated current. Exceeding the input rating causes excessive heating at the input terminal clamp, which can damage insulation and create a fire hazard.
What wire sizes does a typical power distribution block accept?
A common 100 A distribution block accepts 4 AWG to 1/0 AWG (25–50 mm²) at the input and 22 AWG to 6 AWG (0.5–16 mm²) at each output position. The input and output terminal chambers are sized differently to match the intended conductor gauges.
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