How to Calculate Electrical Load for a House: Formula, Step-by-Step & Reference Tables
Electrical load calculation is the first step in any proper installation. Without it, every decision is a guess — and guesses mean undersized wires, breakers that trip constantly, or a panel overloaded enough to start a fire. The most common mistake we see in Egyptian apartments: a 2-ton AC wired onto the general outlets circuit. A 2.5mm² wire carrying 24A instead of 10A, with a 16A breaker that runs hot but doesn't trip because it's not the only protection in the loop. The right approach is to calculate before you install: a dedicated circuit for each AC, a correctly-rated breaker, and wire sized to the actual load. This page gives you the formula, a fully-worked 3-bedroom apartment example with 12 circuits, and a complete wire-sizing and breaker reference table.
Quick Answer
Calculate your apartment load with I = P ÷ 220V — worked example for a 3-bedroom flat: 12 circuits, correct wire size and breaker for each. Printable reference table included.
The Core Electrical Load Calculation Formula
aseskahraba.comCurrent (Amps) = Power (Watts) ÷ Voltage (Volts)I (A) = P (W) ÷ 220I = 2,200 ÷ 220 = 10A → use 16A breaker + 2.5mm² wireFor motors and compressors (AC, washer): multiply result by 1.15 — actual current draw exceeds the rated wattage labelWorked Example: Load Calculation for a 3-Bedroom Apartment (120m²)
| Circuit / Appliance | Watts | Amps | Breaker | Wire Size |
|---|---|---|---|---|
| Master Bedroom AC (2 ton) | 2,200W | 10A | 16A | 2.5mm² |
| Living Room AC (1.5 ton) | 1,800W | 8.2A | 16A | 2.5mm² |
| Instant Water Heater (6kW) | 6,000W | 27.3A | 32A | 6mm² |
| Electric Oven | 2,500W | 11.4A | 20A | 4mm² |
| Washing Machine | 2,000W | 9.1A | 16A | 2.5mm² |
| Kitchen Outlets | 1,500W | 6.8A | 16A | 2.5mm² |
| All Lighting | 400W | 1.8A | 10A | 1.5mm² |
| General Sockets | 1,000W | 4.5A | 16A | 2.5mm² |
| TOTAL | 17,400W | 79A | 100A Main Breaker | 16mm² Feed Cable |
Safety margin: 79A × 1.20 = 95A → select 100A main breaker. The supply cable from the meter must be at minimum 16mm².
Wire Sizing & Breaker Reference Table
Colors are illustrative — actual El-Sewedy cables follow IEC standard color coding.
aseskahraba.comDemand Factor — Why You Don't Add Up Every Load
Demand factor is the percentage of loads running simultaneously. Nobody switches on every appliance at once — ACs run at full load in summer while the water heater is off. So panels are designed for the expected simultaneous load, not the theoretical sum of all devices. In Egyptian apartments, a typical demand factor is 60–80%. A 100A panel is sufficient for an apartment with a calculated total load of 130–140A. But there's a catch: buildings constructed before 1995 often have no dedicated AC circuits — every outlet is on a shared ring. In those apartments, demand factor climbs to near 100% because you can't control which loads are running simultaneously. If you're adding split ACs to an old flat, calculate as if demand factor = 100% and plan a full rewire, not just a panel upgrade.
| Load | Demand Factor | Note |
|---|---|---|
| Air Conditioners | 100% | Run at full load in summer |
| Instant Water Heater | 40% | Operates in very short bursts |
| Electric Oven | 50% | Not continuously on |
| Washing Machine | 25% | One cycle per day |
| Lighting | 80% | Most lights on at night |
| General Outlets | 30% | Scattered use |
Load Calculation for Villas & Buildings: Three-Phase (380V)
Three-phase power (380V) is required for villas over 200m², commercial premises, and any unit with central HVAC. Connecting to a three-phase supply from EETC costs between 8,000 and 25,000 EGP and typically takes several weeks to commission. The formula changes: instead of dividing by 220, you divide by 658 — which is 380V × √3. But the formula is almost secondary. The critical decision is load balancing: a phase carrying 20A next to one at 60A means the neutral handles the difference — and chronic imbalance causes transformer heating and early panel failure. Keep the imbalance below 20% across all three phases.
I = P ÷ (√3 × 380) = P ÷ 658I = 30,000 ÷ 658 = 45.6A per phase → 63A three-phase main breakerDistribute appliances evenly across the three phases, keeping the imbalance below 20% to avoid overloading a single phaseIn balanced loads: neutral current ≈ 0. When unbalanced: neutral carries the difference between phasesSteps to Calculate Electrical Loads
Sum every appliance's wattage — note each device's wattage from its label: AC 1500–2500W, washer 2000W, instant water heater 3000–6000W, oven 2000W, refrigerator 150–300W, all lighting 200–400W
Calculate current per circuit — formula: I (Amps) = P (Watts) ÷ 220V. Example: 2200W AC ÷ 220 = 10A. That circuit needs a 16A breaker and at minimum a 2.5mm² wire
Choose the correct wire cross-section — 1.5mm² for lighting (up to 10A), 2.5mm² for standard outlets (up to 16A), 4mm² for high-load outlets, 6mm² for ACs and appliances above 20A
Select the right breaker size — the breaker should be 20–25% larger than the circuit load. A 10A AC circuit needs a 16A breaker. Never use a breaker smaller than the load or far oversized — it defeats the protection purpose
Calculate the total panel load — sum all sub-breakers and add a 20% safety margin. This total is what the main breaker and main supply line must be rated to handle
Avoid these common mistakes — connecting multiple ACs on one circuit, feeding the oven or water heater from a shared line, and failing to account for appliances you plan to add later
Common Electrical Load Calculation Mistakes — And How to Avoid Them
Putting all ACs on one phase in a three-phase setup — correct approach: balance loads across all three phases, keeping the imbalance below 20%.
Skipping demand factor and ending up with a panel oversized by 40% — correct approach: apply 60–80% demand factor for modern apartments with dedicated circuits.
Wiring an instant water heater on 2.5mm² cable — correct approach: water heaters (3–6kW) require 6mm² wire and a 32A breaker at minimum.
Omitting an EV charging point from the load plan — correct approach: a home charging point needs a dedicated 12–16A circuit on 2.5–4mm² wire.
Ignoring motor starting current — correct approach: compressor startup (AC, washer) draws 3–5× rated current; multiply your calculated amps by 1.15.
No headroom for future loads — correct approach: add 20% above current total when sizing the main panel to allow for future expansion.
FAQ
What's the difference between total load and simultaneous load?
Total load is the sum of all appliances if everything ran at once. Simultaneous load is what actually runs at the same time. Panels are designed around the expected simultaneous load with a 20% safety margin.
Can I calculate loads myself?
Yes — sum appliance wattage and divide by 220 to get amperage. But for precise wire sizing, breaker selection, and panel design that meets safety standards, consult a qualified electrical engineer.
What's typically the highest electrical load in an apartment?
Instant water heaters (geysers) reach 3000–6000W and are usually the highest, then ovens at 2000–3000W, then ACs at 1500–2500W. Each of these three must be on a dedicated circuit.
Can an old panel handle new air conditioners?
Not necessarily. Many older panels were designed for much lower loads than today's demand. You need to calculate current plus new loads and verify the panel, main line, and meter can all handle it.
Why does an AC need a dedicated circuit?
ACs draw a high surge current at compressor startup. A dedicated circuit isolates other circuits from this sudden impact, prevents voltage drops, and extends the AC's lifespan.
What is the electrical load calculation formula?
The formula is: I (Amps) = P (Watts) ÷ V (Volts). In Egypt with 220V supply: I = Watts ÷ 220. For motors and compressors, multiply by a power factor of 1.15 to account for startup surge current.
How do I calculate electrical load in a house?
List every appliance with its wattage, group by circuit, divide each circuit's total watts by 220 to get amps, choose a breaker 20–25% above that, then sum all circuit amps and add 20% to select the main breaker.
How do I calculate electrical load in kWh?
Energy (kWh) = Power (Watts) × Hours of use ÷ 1,000. Example: a 2,200W AC running 8 hours a day uses 2,200 × 8 ÷ 1,000 = 17.6 kWh per day.