How to determine the length of the secondary control wires of a low-voltage power distribution cabin
In a low-voltage power distribution cabinet, the determination of the length of the secondary control
wires (used in control, protection, signal and other circuits) needs to be combined with the **cabinet
structure**, **component layout**, **wiring method** and **process specifications**. The following
are the specific methods:
### I. Basic Principles for Determining the Length
1. **Shortest Path**
- Measure according to the **actual distance between components**, and give priority to straight lines
to avoid detours or intersections.
- Refer to the drawings of the power distribution cabinet, mark the installation positions of each component
(such as terminal blocks, relays, instruments), and calculate the sum of the horizontal and vertical distances
between two points.
2. **Reserve Redundancy**
- **Fixed Margin**: Reserve **50-100mm** at both ends of each wire for the crimping and adjustment of
the terminal connectors.
- **Moving Margin**: If the component needs to move (such as an openable cabinet door), reserve a wire
harness of **1.5-2 times the opening length of the door**, and protect it with a corrugated tube.
3. **Unification of Wire Harnesses**
- The lengths of the wires within the same wire harness should be **consistent** to avoid confusion caused
by inconsistent lengths. Accurate measurement can be carried out through a **pre-laying template** (such as
the device in Abstracts 2 and 8).
### II. Calculate the Length Step by Step
#### 1. Measure the Actual Distance between Components
- **Horizontal Distance**: Measure the horizontal length from the starting point to the ending point along
the wire slot or the installation rail.
- **Vertical Distance**: Measure the height difference (such as the vertical distance from the terminal block
to the upper relay).
- **Example**: The path from the terminal block to the relay is "horizontal 150mm + vertical 80mm", and the
total distance is **230mm**.
#### 2. Consider the Wiring Method
- **Wiring in Wire Slots**: It is necessary to additionally increase the **bending length at the turning points
of the wire slots** (the bending radius is ≥ 2 times the outer diameter of the wire).
- **Wire Harness Bundling**: According to the principle of **horizontal and vertical alignment**, fix it once
every 300-400mm (refer to Abstract 3), and avoid looseness or over-tightening.
#### 3. Calculate the Length of a Single Wire
**Formula**:
**Wire Length = Actual Path Distance + Reserve at Both Ends + Bending Compensation**
- Example: The actual distance is 230mm, and 80mm is reserved at each end. The total length is
**230 + 80×2 = 390mm**.
### III. Refer to Specifications and Empirical Values
1. **Requirements of National Standards**
- Cross-sectional Area of Secondary Wires:
- For ordinary circuits ≥ **1.5mm²** (single-strand) or **1.0mm²** (multi-strand);
- For current circuits ≥ **2.5mm²** (refer to Abstracts 3 and 4).
- There should be no joints in the middle, and the exposed copper at the wire ends should be ≤
**1mm** (refer to Abstract 4).
2. **Industry Experience**
- The length of the control wires between the fixed components in the cabinet is generally between
**200-500mm**, specifically depending on the size of the cabinet.
- For the connection wires across the cabinet door, a **moving length** needs to be reserved. For example,
if the height of the cabinet door is 500mm, the reserved length is **500×1.5 = 750mm**.
### IV. Practical Operation Process
1. **Drawing Marking**: Mark the terminal numbers of each component and the corresponding wire
numbers on the electrical schematic diagram.
2. **Pre-laying of Wires**: Use a template or a flexible wire to simulate the wiring and measure the actual
length (refer to Abstracts 2 and 5).
3. **Unified Cutting**: Cut the wires according to the measured length + reserve, and put on the wire number
tubes (with clear markings and consistent directions).
4. **Wiring and Fixing**: Connect the terminals according to the drawings, fix the wire harnesses with cable
ties, and the spacing should be ≤ **100mm** (refer to Abstract 3).
### V. Precautions
- **Avoid Signal Interference**: The weak-current control wires (such as communication wires) need to be
**isolated** from the strong-current lines, with a spacing of ≥ **100mm** or shielded with a metal wire slot.
- **Clear Markings**: The markings on the wire number tubes should be consistent with the drawings for
easy maintenance (refer to Abstract 6).
- **Testing and Verification**: Measure the conductivity after wiring to ensure there is no short circuit or
open circuit.
Through the above methods, the length of the secondary control wires can be accurately determined,
taking into account both **economic efficiency** and **process aesthetics**, while meeting the
requirements of safety specifications.
