Electrical
LV distribution design
Low voltage distribution design including maximum demand, cable sizing, volt drop, earth fault loop impedance and discrimination.
- Discipline
- Electrical
WHAT IT IS
LV distribution design determines how power gets from the incoming supply to every point of use: the boards, the submains, the cable sizes, the protective devices and the way they discriminate.
The calculations are the design. Cable sizing to BS 7671 takes account of installation method, grouping, ambient temperature and thermal insulation, then is checked against volt drop and against earth fault loop impedance for disconnection time. A cable that satisfies current-carrying capacity and fails volt drop is not a design.
WHEN YOU NEED IT
On any new installation, any significant alteration to an existing one, and on any project where the incoming supply capacity or the existing switchgear is being asked to serve a larger load than it was designed for.
DELIVERABLES
What you receive
- Maximum demand and diversity calculation
- Distribution schematics
- Cable sizing calculations
- Volt drop and earth fault loop impedance calculations
- Discrimination study
- Distribution board schedules
STANDARDS
Standards applied
The editions in force at the time of design. Where a standard is amended mid-project we confirm which edition the appointment is based on.
QUESTIONS
Common questions
How is cable size calculated to BS 7671?
The design current is established with diversity, then correction factors are applied for installation method, grouping, ambient temperature and any thermal insulation. The resulting cable is then checked for volt drop and for earth fault loop impedance to confirm disconnection within the required time. All four checks have to pass.
What is discrimination in an electrical installation?
Discrimination, or selectivity, means that a fault causes only the protective device nearest the fault to operate, leaving the rest of the installation energised. It is achieved by co-ordinating the time and current characteristics of devices in series and is demonstrated by calculation at design stage.