Domestic
When calculating the length of a circuit,it is important to include...
The cable drops to switches/sockets
Diversity formula for cookers in commercial
Not applied
Iz formula
In divided by correction factors
What is the maximum permitted voltage drop for a lighting circuit?
3%
How can poor floor plan design negatively affect cable selection and protective device choice?
Longer cable runs increase voltage drop and earth fault loop impedance, possibly requiring larger cables or lower-rated protective devices.
Minimum height of a socket
450mm from ffl to centre of accessory
It is recommended to have ... instead of switches in places where there is constant movement of objects/people
sensors (PIR as an example)
Voltage drop formula
V.d= mV/a/m times Ib times L divided by 1000
What is the maximum permitted voltage drop for other circuits such as sockets?
5%
Why must electrical symbols on a floor plan match those in the drawing legend?
To avoid misinterpretation, installation errors, and non-compliance with design specifications.
Maximum height of a switch
1200mm from ffl to centre of accessory
Lighting diversity formula
90% of Ib
where is R1+R2/m value found ?
OSG Table I1
Cc value
1 if above ground
0.9 if bellow ground
What information should be included on an electrical floor plan to allow another electrician to install the system without further explanation?
Circuit references, accessory symbols, switch lines, cable routes, scale, legends, and distribution board location.
Where should the CCU ( Customers consumer unit) be placed?
Accessible yet hidden space ( utility room/cupboard etc)
Diversity formula for power circuits (e.g FCUs for hand dryers/ hair dryers)
100% of Ib of the first appliance and 75% of Ib for the rest of the appliances
R1+R2 formula
R1+R2/m times L times 1.2 divided by 1000
How many metres squared can a radial circuit wired in 2.5mm cover?
50m squared
Why must all applicable correction factors be applied before finalising cable size selection?
Because ignoring any factor may result in a cable that overheats or fails to meet design current requirements under real conditions.
The diversity formula for cooker circuits
Ib - 10A, 30% of the remainder, +10A (+5A if socket is incorporated in the cooker switch)
What scale is most commonly used for electrical floor plans?
1:50
Name first 7 steps of cable calcs
ib, in, correction factors, Iz, install. method, It, Actual voltage drop.
Whats the maximum area of a room that a ring circuit wired in 2.5mm can cover ?
100 metres squared
A circuit has design current Ib of 24 A. After applying temperature and grouping factors, a 2.5 mm² cable can carry only 21 A. What design decision must be made and why?
Increase cable size (e.g. 4 mm²) because Iz must be ≥ In ≥ Ib to comply with BS 7671.