Electrical Wiring

How to do Cable Sizing – Step by Step
As per IEC 60364 / IS 732

Step 1: Calculate Load Current (I)

Single Phase: I = P / (V x PF)
Three Phase: I = P / (1.732 x V x PF)

Where P = Load in Watts, V = Voltage, PF = Power Factor 0.8 to 0.9

Example: 15 kW motor, 415V, 3-phase, 0.85 PF
I = 15000 / (1.732 x 415 x 0.85) = 24.5 A

Step 2:
Select Base Cable from Current Table
Pick rating ≥ Load Current.
From table: 24.5 A → 4 sqmm Cu = 25 A. Don’t stop here.

Step 3:
Apply Derating Factors – IEC 60364-5-52
Required Cable Capacity = Load Current / Total Derating Factor

Common factors:
1. Ambient Temp >30°C: 0.91 to 0.71.
If the surrounding temperature of equipment and cable is greater than 30°,the correction factor need.
Ambiant temperature is one iff Tempreature is30°.
Ex: 45°C = 0.79
2. Grouping of cables: 0.8 to 0.5.
The same size cable are grouped together,they impart heat to each other.
Ex: 6 cables in tray = 0.69
3. Installation Method:
Conduit, buried, tray – check table

Total Derating = Temp x Grouping x Installation
Example: 0.79 x 0.69 = 0.545
New required capacity = 24.5 / 0.545 = 44.9 A
Now from table: 44.9 A → 10 sqmm Cu = 45 A

Step 4: Check Voltage Drop
Max allowed: 3% lighting, 5% power

Single Phase: VD% = (2 x L x I x mV) / (V x 10)
Three Phase: VD% = (1.732 x L x I x mV) / (V x 10)

L = length meters, mV/A/m from cable datasheet, V = system voltage
If VD > allowed %, increase cable size.

Step 5: Check Short Circuit Withstand*
Min size: S = (Isc x √t) / k
Isc = fault current, t = breaker time sec, k = 115 for Cu-PVC, 143 for Cu-XLPE

Result: Final cable = largest size from Step 3, 4, and 5.

*Common Mistake*
Picking cable only by load current. Do all 5 steps.
Site example:
25 A load, 80m, grouped, 40°C → 4 sqmm fails. Correct = 16 sqmm.

🔌 Placement of Electrical Switches in Design

1. General Principles

- must be easily accessible.
-They should be installed at a convenient height for users.
-Positioned near the entrance of the room. - -should allow safe operation without the risk of shock.

2. Standard Height of Switches
Typical height from floor level:
1.2 m to 1.5 m (120–150 cm)
In homes, they are commonly:
≈ 1.3 m from finished floor level
👉 This ensures both adults and children can operate them easily.

3. Location Guidelines
a) Near Door Entrance
Place the switch on the latch side of the door (handle side).
Distance from the door frame:
150 mm to 300 mm
✔ This allows the user to switch on the light immediately when entering.

b) In Rooms / Bedrooms
Near the main door.
Additional switches:
Near the bed (for convenience) Near seating areas
c) In
Keep switches:
- Away from sinks and water sources
- Near the entrance

Provide separate switches for:
- Lights
- Exhaust fan
- Appliances

d) In / Toilets
should be placed:
- Outside the bathroom (preferred for safety)
- If inside:
- Use a waterproof switch (IP rated)
e)
Use two-way switches:
-One at the bottom
-One at the top

✔ Allows control from both ends.

f) / Hallways
Use:
-Two-way or intermediate switches
Place at:
-Entry and exit points.