Electrical load calculation under the Saudi code SBC 401

If you have a villa or building and need to know “what main breaker?” and “what cable from the meter?”, this calculation is where it starts. The Saudi Electrical Code SBC 401 is published by the Saudi Building Code National Committee at sbc.gov.sa; the current edition SBC 401-CR-2024 opens Part Three with clause 31-1, “Maximum demand and diversity factor”.

The earlier (2007) text of that clause says determining maximum demand is essential for an economic and reliable design within thermal and voltage-drop limits, and that diversity may be taken into account. In other words the code does not impose a fixed table of demand factors per appliance as some other codes do; it leaves the judgement to the designer, which is where the project engineer and consultant come in.

1. Load schedule: for each circuit list type (lighting, sockets, AC, water heater, pump…), connected load in kVA, and phase (L1 / L2 / L3).
2. Total connected load: the sum before any diversity.
3. Demand factor: ratio of expected maximum demand to connected load for each group. In Saudi Arabia do not discount AC much; in summer nearly all units run together.
4. Maximum demand: sum of (load × factor) per group.
5. Current: three-phase I = S ÷ (√3 × V), single-phase I = S ÷ V.
6. Phase balance: spread single-phase circuits so phase currents are close; the breaker is chosen on the heaviest phase, not the average.
7. Main breaker, then cable: standard breaker ≥ heaviest phase current, then cable Iz after correction ≥ breaker, then check voltage drop and breaking capacity.

The Saudi Electricity Company’s Distribution Planning Standard (DPS, 2004 edition) uses demand factors to estimate a customer’s maximum demand when planning the network. These are utility-side figures, not a replacement for the installation’s own load schedule, but they are a useful cross-check:

Loads and demand factors in this example are illustrative, to show the method; your project’s figures come from the drawings and equipment nameplates, and the designer sets the factors.

Current: 31,800 ÷ (1.732 × 400) = 45.9 A per phase if perfectly balanced. In practice it will not be; assume the heaviest phase after balancing is 48 A.

Main breaker: the smallest standard breaker above 48 A is 50 A; many designers choose 63 A to leave room for growth. That is a design decision for the consultant to approve, and it must suit the meter breaker set by the utility.

Cable: four-core PVC copper cable in conduit (B2, three loaded conductors) at 45 °C: 16 mm² = 62 × 0.79 = 49.0 A, below a 50 A breaker, so it fails; 25 mm² = 80 × 0.79 = 63.2 A takes a 50 or 63 A breaker.

Voltage drop over 40 m: 1.57 × 45.9 × 0.040 = 2.9 V, 0.72 % of 400 V, leaving ample margin for final circuits within the 4 % limit.

For comparison: with SEC’s villa factor (0.5) the demand would be 42.1 × 0.5 = 21.1 kVA, about 30.4 A. The gap shows why the internal installation is not designed on the utility factor alone; our summer AC load pushes real demand up.

Same steps, with I = S ÷ V. Illustrative example: an apartment with 9 kVA maximum demand after diversity at 230 V: 9,000 ÷ 230 = 39.1 A, so a 40 A breaker, and the cable must carry 40 A after correction: 10 mm² wires in conduit at 45 °C carry 45.0 A and suit, while 6 mm² (32.4 A) does not. If the current approaches the breaker limit, discuss a three-phase supply with the designer.

The main breaker is chosen on two currents: the rated current from the load calculation, and the breaking capacity in kA, which must not be below the prospective short-circuit current where it is installed. The code requires the designer to assess the supply characteristics, including prospective short-circuit current at the origin (clause 31-3 in the 2007 edition). That figure depends on the building’s distance from the transformer and the size and length of the utility cable; the designer calculates it or obtains it from the supplier. Miniature breakers to IEC 60898-1 go up to 125 A and 25 kA; beyond that, moulded-case breakers (MCCB) to IEC 60947-2 are used.

At sub-boards the fault current falls with cable length, so a lower breaking capacity may be enough, but that is a calculation, not a guess. State the breaking capacity for every breaker in the BOQ; it is one of the questions that most often delays pricing.

• Sizing the breaker on the average phase instead of the heaviest.
• Forgetting future loads: an EV charger, home lift, pool, extra AC units in an annex.
• Using the 30 °C cable table without temperature correction.
• Not checking the main breaker’s breaking capacity against the prospective short-circuit current at the intake.
• Designing internal boards on the utility factor (0.5).