Each standard takes a fundamentally different approach to sizing residential electrical infrastructure based on local installation codes and statistics:

• US NEC 220: Relies on floor area sizing (3 VA/sq ft for general lighting/receptacles) plus dedicated appliance branch circuits (small appliance kitchen circuits, laundry circuits) and prescriptive demand factors (Table 220.42). Sizing is conservative to support high load density.
• International IEC 60364-8-1: Uses general housing demand estimations based on floor space in square meters (usually 20-30 VA/m² for lighting and general sockets) combined with statistical diversity factors applied to individual circuits or total consumer loads.
• Indian Standard (IS 732 & IS 4648): Point-based sizing is used. Circuits are divided into dedicated lighting sub-circuits (max 800W or 10 points) and power sub-circuits (max 3000W or 2 points). Sizing is based on actual point ratings (40W lights, 60W fans, 100W/5A sockets, 1000W/15A sockets) and applies flat domestic diversity factors.

For a balanced wye-connected system, the total active power $P$ is the sum of the power in all three phases:

$$ P = 3 \times P_{phase} = 3 \times V_{L-N} \times I_{phase} \times \cos\phi $$

In a wye-connected system, the phase current equals the line current ($I_{phase} = I_{line}$), while the line-to-line voltage $V_{L-L}$ is mathematically related to line-to-neutral voltage $V_{L-N}$ by the phasor relationship:

$$ V_{L-L} = \sqrt{3} \times V_{L-N} \implies V_{L-N} = \frac{V_{L-L}}{\sqrt{3}} $$

Substituting this back into the total power equation yields the line-based three-phase formula:

$$ P = 3 \times \left(\frac{V_{L-L}}{\sqrt{3}}\right) \times I_{line} \times \cos\phi = \sqrt{3} \times V_{L-L} \times I_{line} \times \cos\phi $$

Electric Vehicle (EV) charging draws maximum current for hours at a time, classifying it as a continuous load (defined by NEC Article 100 as running continuously for 3 hours or more). Overcurrent Protection Devices (breakers) and conductors accumulate heat during this sustained operation.

Standard circuit breakers are calibrated to carry 100% of their rated load for short periods, but are limited to 80% for continuous loading. To accommodate this limit, the circuit rating must be upsized by 125% ($1 / 0.80 = 1.25$):

$$\text{Circuit Size} = \text{Continuous load rating} \times 1.25$$

For example, a standard 32A EV charger requires a $32\text{A} \times 1.25 = 40\text{A}$ rated breaker and wiring to prevent thermal fatigue tripping.

In a 3-Phase wye (star) system, the neutral conductor returns the unbalanced current. Using phasor addition, the neutral current $\vec{I}_N$ is the vector sum of phase currents $\vec{I}_A$, $\vec{I}_B$, and $\vec{I}_C$:

$$ \vec{I}_N = \vec{I}_A + \vec{I}_B + \vec{I}_C $$

Assuming pure resistive loads (currents in phase with voltages and offset by 120°), we apply vector mathematics using the law of cosines to derive the scalar neutral current formula:

$$ I_N = \sqrt{I_A^2 + I_B^2 + I_C^2 - I_A I_B - I_B I_C - I_C I_A} $$

For instance, if $I_A = 80\text{A}$, $I_B = 50\text{A}$, and $I_C = 30\text{A}$:

$$ I_N = \sqrt{80^2 + 50^2 + 30^2 - (80\times50) - (50\times30) - (30\times80)} = \sqrt{9800 - 4000 - 1500 - 2400} = \sqrt{1900} \approx 43.6\text{ A} $$

To ensure safety and prevent total power loss during faults, Indian Standard IS 732 sets strict point and power limitations for branch wiring. A domestic distribution board divides load into two distinct sub-circuit types:

1. Lighting and Fan Sub-Circuits: Limited to a maximum of 10 points (lights, fans, exhaust fans, and 5A/6A sockets) or a connected load of 800 Watts per sub-circuit, whichever is reached first. Standard wiring is $1.5\text{ mm}^2$ copper.
2. Power Sub-Circuits: Sized for heavier appliances (AC, geyser, refrigerator, microwave). Limited to a maximum of 2 points or a connected load of 3000 Watts per sub-circuit, whichever is reached first. Standard wiring is $2.5\text{ mm}^2$ or $4.0\text{ mm}^2$ copper.

The neutral conductor carries the unbalanced current in split-phase or wye installations. NEC 220.61 permits the neutral feeder conductor to be derated relative to the phase conductors under specific conditions:

• A demand factor of 70% is allowed for the neutral feeder for electric household cooking ranges and clothes dryers, recognizing that the 240V components (heating coils) do not pull current through the neutral.
• The neutral feeder can be reduced for the portion of the unbalanced load above 200 Amps (70% demand factor for anything above 200A).
• Crucial Exclusion: No reduction is allowed for circuits supplying non-linear loads (fluorescent lighting, computers, VFDs) because triplen harmonics sum in the neutral rather than canceling.

The Grounding Electrode Conductor (GEC) connects the electrical system's neutral grounding bus to earth grounding electrodes (such as rods, plates, or metal water pipes). Sizing depends on standard criteria:

• US NEC Table 250.66: Sized based on the cross-sectional area of the largest service-entrance conductor. For example:
  • Phase size 2 AWG Copper or smaller $\implies$ Requires 8 AWG Copper GEC.
  • Phase size 1/0 AWG or 2/0 AWG Copper $\implies$ Requires 6 AWG Copper GEC.
  • Phase size 3/0 AWG to 350 kcmil Copper $\implies$ Requires 4 AWG Copper GEC.
• Indian Standard IS 3043: Sized based on the prospective short circuit current. For residential installations, the earth lead is typically sized at 50% of the phase wire size, with a minimum limit of $6.0\text{ mm}^2$ copper ($10.0\text{ mm}^2$ aluminum or $16\text{ mm}^2$ GI).

Electric cooking ranges cycle heating elements on and off to maintain temperature. The NEC recognizes this massive diversity and allows derating ranges via Table 220.55 instead of calculating at full connected wattage:

• Single Range (9 kW to 12 kW): Calculated at a flat demand value of 8 kW (Column C), resulting in a major reduction ($12\text{ kW} \to 8\text{ kW}$).
• Single Range under 8.75 kW: Calculated using Column B (45% demand factor for ratings 3.5 kW to 8.75 kW) or Column A (80% demand factor for ratings under 3.5 kW).
• Ranges over 12 kW: Column C demand (8 kW) is increased by 5% for each additional kW (or fraction thereof) that the range rating exceeds 12 kW.
  Example: A 15 kW range exceeds 12 kW by 3 kW. Sizing is: $$ \text{Demand} = 8\text{ kW} \times [1.00 + (3 \times 0.05)] = 8\text{ kW} \times 1.15 = 9.2\text{ kW} $$

Commercial kitchen equipment (fryers, ovens, griddles) draws large loads but is thermostatically controlled. NEC 220.56 allows developers to apply aggressive demand factors to the aggregate rating of kitchen loads if there are 3 or more kitchen units:

• 3 Units: 90% demand factor.
• 4 Units: 80% demand factor.
• 5 Units: 70% demand factor.
• 6 or more Units: 65% demand factor.

These values apply to all thermostatic kitchen appliances, heaters, ovens, and preparation devices, except for HVAC and space heating systems.

Electric motors draw a high surge current during starting (known as Locked Rotor Amps or LRA), which can be 5 to 8 times the Full Load Amps (FLA). To prevent nuisance tripping while providing short-circuit protection, OCPDs must be sized using coordinate trip curves:

• Inverse Time Circuit Breakers (Standard Breaker): Sized up to 250% of the motor FLA per NEC Table 430.52 to let starting inrush pass.
• Dual-Element Time-Delay Fuses: Sized up to 175% of the motor FLA, providing reliable thermal protection with built-in time delay.
• The thermal overload relay (overload heater) is sized near the motor FLA (typically 115% to 125%) to protect the windings from running overloads.