What is a submain in an electrical installation?
A submain is a distribution circuit carrying electricity between distribution boards, rather than the final circuit supplying equipment [S1]. For example, assume 1 cable feeds a car-park board, which supplies separate charger circuits. Your designer must check its load, protection and route; cable length affects voltage drop [S2]. Checked October 2026.
What does a submain mean for your site?
A submain is the distribution circuit connecting an upstream distribution board to another board. A final circuit then supplies equipment from that downstream board. The IET illustrates this distinction by separating a circuit feeding a distribution unit from the final circuits it serves [S1].
Why it matters for your site: a car-park charger board may have spare outgoing ways while the cable feeding it still needs assessment for the proposed loads. Cable selection involves current capacity, voltage drop and protection, rather than merely finding space for another breaker [S3].
An illustrative route is: main board → submain → car-park board → final circuits → chargers. Ask for your actual route on the drawing. Read single-phase and three-phase supplies for the separate question of supply arrangement.
Why do distance and capacity matter?
The IET’s charging guidance calculates voltage drop using cable characteristics, load current and length [S2]. A route to distant parking therefore needs a calculation; do not copy a cable specification from a nearby charger.
For buried cables, ground conditions and grouping also affect current capacity [S3]. Ask your designer to record the route, installation method, intended simultaneous load and protective device. Include planned battery charging or other equipment on the same board in the assessment.
What should you request before accepting the cable design?
Request a drawing showing both boards and the cable between them, the design calculations and the protection schedule. Keep the test records described in the electrical certificate guide. For the wider supply question, use maximum import capacity.
Next step
Bring the cable route and load schedule to the site-power tool, then get matched with an installer that covers your area.
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Common questions
Is every cable feeding a charger a submain?
No. Distinguish a circuit feeding another distribution board from the final circuit serving equipment. Ask the designer to mark both on the installation drawing [S1].
Does a longer route affect the design?
Yes. Cable length is part of the voltage-drop calculation. Request the calculation for the proposed route and load, including the upstream installation [S2].
Can cable size alone prove suitability?
No. The IET identifies installation conditions, current capacity, voltage drop and protective-device energy as design considerations. Have the actual cable and route assessed [S3].
Is a buried cable's rating the same in every trench?
No. Soil thermal conditions, burial depth and grouping affect current capacity. The designer must select ratings and correction factors appropriate to your route [S3].
Can an existing breaker simply be retained?
Have the designer verify protection against the cable's capacity and fault-energy withstand. A spare breaker does not by itself establish that the new cable arrangement is suitable [S3].
Ask about your site
Sources
- MythBusters 6 - distribution circuits and final circuits, Institution of Engineering and Technology,Last checked 19 October 2026
- How voltage drop can affect EV charging point open PEN detection devices, Institution of Engineering and Technology,Last checked 19 October 2026
- Current-carrying capacity of cables buried in the ground, Institution of Engineering and Technology,Last checked 19 October 2026