Selecting main panel board and DB rating

The rating of a panel board (MDB) or distribution board (SDB) means the rating of its incomer (MCCB/MCB), busbar and incoming cable. It is fixed from the design current of the loads it feeds, after applying utilization and simultaneity (diversity) factors and a safety margin.

Procedure

1. List the loads on each DB: kW, pf, efficiency, single or three phase. Balance single-phase loads on R, Y, B.
2. Find the current of each outgoing circuit: I = P/(√3·V·η·cosφ) for 3-phase, I = P/(V·cosφ) for single phase.
3. Select outgoing MCBs: In ≥ 1.25 × Ib (next standard size: 6, 10, 16, 20, 25, 32, 40, 50, 63 A); C curve for general loads, D curve for motors.
4. SDB demand: add circuit currents and multiply by the simultaneity factor Ks (0.9 for 2–3 circuits, 0.8 for 4–5, 0.7 for 6–9, 0.6 for 10 or more).
5. SDB incomer: In ≥ 1.25 × SDB demand; MCB up to 63/100 A, MCCB above.
6. MDB demand: add the SDB demands, apply Ks of the MDB, add spare for future.
7. MDB incomer: MCCB/ACB ≥ 1.25 × MDB demand; check that it does not exceed the transformer LT full-load current, and its breaking capacity ≥ prospective fault current.
8. Busbar: rating ≥ incomer rating (copper about 1.2–1.6 A/mm²). Keep 20 % spare ways.
9. Coordination: Ib ≤ In ≤ Iz (cable rating), and upstream device larger than downstream for discrimination.

Example

A factory, 400 V, 3-phase: SDB-1 feeds 3 motors of 7.5 kW (η = 0.88, pf = 0.85); SDB-2 feeds 15 kW balanced lighting and sockets at pf 0.9.

Motor current = 7500 / (√3 × 400 × 0.88 × 0.85)
              = 14.47 A
Motor MCB     = 1.25 × 14.47 = 18.09 A → 20 A (D curve)

SDB-1 demand  = 3 × 14.47 × Ks 0.9 = 39.08 A
SDB-1 incomer = 1.25 × 39.08 = 48.84 A → 50 A TP MCB

SDB-2 current = 15000 / (√3 × 400 × 0.9) = 24.06 A
SDB-2 incomer = 1.25 × 24.06 = 30.07 A → 32 A TPN MCB

MDB demand = (39.08 + 24.06) × Ks 0.9 = 56.82 A
MDB incomer = 1.25 × 56.82 = 71.02 A → 80 A TPN MCCB

Cables are then chosen so that their rating is above the protective device rating (e.g. MDB–SDB-1 with 50 A MCB → 10 mm² Cu), and checked for voltage drop.