Electrical Wiring

19/04/2026

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.

10/04/2026

03/04/2026

𝗛𝗼𝘄 𝘁𝗼 𝗥𝗲𝗮𝗱 𝗮 𝗧𝗖𝗖 𝗖𝘂𝗿𝘃𝗲
• A Time Current Characteristic (TCC) curve is a graph that shows how fast a relay will trip for different fault current levels. It helps us clearly understand which relay operates first during a fault, ensuring proper system protection and coordination.

• I am using DIgSILENT PowerFactory to model and study these TCC curves.
• On the graph, the X-axis represents current (in amperes), usually in logarithmic scale, and the Y-axis represents time (in seconds). The basic idea is simple: as the fault current increases, the relay operates faster.

𝗧𝗵𝗲𝗿𝗲 𝗮𝗿𝗲 𝘁𝗵𝗿𝗲𝗲 𝗺𝗮𝗶𝗻 𝘁𝘆𝗽𝗲𝘀 𝗼𝗳 𝗽𝗿𝗼𝘁𝗲𝗰𝘁𝗶𝗼𝗻 𝗰𝗵𝗮𝗿𝗮𝗰𝘁𝗲𝗿𝗶𝘀𝘁𝗶𝗰𝘀:
𝗜𝗗𝗠𝗧 (𝗜𝗻𝘃𝗲𝗿𝘀𝗲 𝗗𝗲𝗳𝗶𝗻𝗶𝘁𝗲 𝗠𝗶𝗻𝗶𝗺𝘂𝗺 𝗧𝗶𝗺𝗲): This is the most commonly used protection. Here, higher fault current results in faster tripping. In the TCC, it appears as a sloping curve (Standard, Very, or Extremely Inverse).

𝗗𝗠𝗧 (𝗗𝗲𝗳𝗶𝗻𝗶𝘁𝗲 𝗧𝗶𝗺𝗲): The relay trips after a fixed time delay, regardless of the fault current magnitude. So, in the TCC, it appears as a horizontal line.

𝗜𝗻𝘀𝘁𝗮𝗻𝘁𝗮𝗻𝗲𝗼𝘂𝘀 𝗣𝗿𝗼𝘁𝗲𝗰𝘁𝗶𝗼𝗻: This operates immediately when the current exceeds a set value, with no intentional delay. In the TCC, it appears as a vertical line.

• Another important parameter is the pickup current, which is the minimum current at which the relay starts operating. Below this value, the relay will not trip.

• The TMS (𝗧𝗶𝗺𝗲 𝗠𝘂𝗹𝘁𝗶𝗽𝗹𝗶𝗲𝗿 𝗦𝗲𝘁𝘁𝗶𝗻𝗴) controls the operating speed of IDMT relays. A lower TMS means faster tripping, while a higher TMS results in slower operation.

• Relay coordination ensures that the relay closest to the fault operates first, while upstream relays act as backup. This helps in isolating only the faulty section without affecting the entire system.

• If two relay curves overlap on the TCC, it means both relays may trip for the same fault. This is called overlapping curves, and it is undesirable.

• This situation can lead to nuisance tripping, which means unwanted or unnecessary tripping due to improper settings or poor coordination.

• We use the Transformer Damage Curve to know the maximum current and time the transformer can safely withstand without damage.

• To avoid this, we maintain a grading margin, which is the time difference between two relay operations. Typically, this is kept around 200 ms to ensure proper coordination and selective tripping.

25/03/2026

🔌 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.

22/03/2026

17/03/2026

⚡ What is Electrical Earthing?
Electrical earthing (grounding) is the process of connecting the non-current-carrying parts of electrical equipment or the neutral point of a system to the earth.

👉 Purpose:
- Protect people from electric shock
- Protect equipment from damage
- Provide a safe path for fault current

🔸 Main Types of Earthing
Electrical earthing is broadly classified into two categories:

1. 🟢 Equipment Earthing
This type of earthing is used to connect the body (metal parts) of electrical devices to the earth.

✔️ Purpose:
- Prevent electric shock if insulation fails
- Ensure the safety of users

✔️ Examples:
- Refrigerator body
- Washing machine
- Electric motor

2. 🔵 System Earthing
This involves earthing parts of the electrical system, like the neutral point.
✔️ Purpose:
- Stabilize voltage
- Protect the power system

✔️ Types of System Earthing:
- Solid earthing
- Resistance earthing
- Reactance earthing

Types of Earthing Based on Method

1. 🪵 Plate Earthing

A metal plate (copper or galvanized iron) is buried in the ground. The plate is surrounded by charcoal and salt to improve conductivity.

✔️ Features:
- Depth: about 3 meters
- Reliable but costly

2. 🪛 Pipe Earthing (Most Common)

A perforated pipe is placed vertically in the ground. Water is sometimes added to maintain moisture.

✔️ Features:
- Cheap and efficient
- Widely used in homes

3. 🪨 Rod Earthing

A metal rod (copper or steel) is driven into the ground.

✔️ Features:
- Cheap and efficient
- Widely used in homes
- Simple installation
- Suitable for rocky areas

4. 🧱 Strip or Wire Earthing

A metal strip or wire is buried horizontally in the ground.

✔️ Features:
- Used in large installations
- Suitable for long-distance grounding

5. ⚡ Earthing Through Water Mains

An earthing connection is made using underground water pipes.
Conclusion

Electrical earthing is essential for safety in any electrical system. Among all methods, pipe earthing is the most widely used because it is economical and effective.
Plate Earthing: A metal plate buried deep in the soil with charcoal and salt.

Pipe Earthing: A vertical pipe placed in moist soil (most common in homes).

Rod Earthing: A metal rod driven deep into the ground.

Strip Earthing: A horizontal conductor buried in a trench.

16/03/2026