Electrical

Distribution board (panelboard) sizing: nominal current, breakers, demand and busbar

Sizing a distribution board means computing the nominal current of each circuit, picking a standard breaker and pole count, applying demand and diversity factors to size the main breaker and busbar, and laying out the DIN modules plus the spare reserve.

When to use

Use it whenever you specify or check a low-voltage distribution board (panelboard) for a building, machine skid or process panel: residential and commercial feeders, motor groups, HVAC, lighting and outlets. It is the step that turns a load list into hardware — it sets the nominal current of each circuit, the standard breaker and pole count, the diversified demand that the main breaker and busbar must carry, the short-circuit rating, and how many DIN modules (and the spare reserve required by code) the enclosure must hold. It is also the tool for auditing an existing board that trips, overheats or has no room for expansion.

What distribution panel sizing is

A distribution board (panelboard) is the node where one feeder fans out into many protected circuits. Sizing it is not about choosing a pretty enclosure: it is the disciplined chain that turns a load list into hardware — the nominal current of each circuit, the standard breaker and pole count that protect it, the diversified demand the main breaker and busbar must carry, the short-circuit rating, and the number of DIN modules (plus the code-required spare) the enclosure must hold.

The most common field mistake is to add up every installed watt, ignore diversity, and oversize everything — a main breaker, a busbar and an enclosure built for a peak that never occurs. The opposite mistake is just as costly: a board with no spare modules, no margin on the breaking capacity, and a breaker rounded down below the nominal current. The method below avoids both.

Step 1 — Nominal current per circuit

Everything starts at the circuit. The nominal current depends on the number of phases:

Single- or two-phase: In = P / (V · fp)

Three-phase: In = P / (√3 · V · fp)

P is the active power [W], V the circuit voltage [V] (line-to-line for three-phase), and fp the power factor. The √3 on three-phase circuits is the single most forgotten term — drop it and you overstate the line current by about 73 %. If you already know the current of a load, you can enter it directly in current mode and skip the conversion.

Once In is known, the breaker is the smallest standard rating not below it, drawn from the commercial series 6, 10, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125, 160, 200, 250 A. The breaker always rounds up: it protects the conductor, so its rating must be at least the nominal current. The pole count equals the number of phases — one module per pole on the DIN rail.

Step 2 — Demand factor per circuit

A circuit rarely draws its full installed power continuously. The demand factor fd captures that:

I_dem = In · fd

For a continuous resistive load (a water heater) fd ≈ 1.0; for a lighting or outlet circuit it is lower. The demand current, not the nominal current, is what flows toward the board total.

Step 3 — Board demand and the main breaker

The board total is not the sum of every demand current — different circuits do not peak at the same instant. The diversity (simultaneity) factor fs corrects for this at the board level:

I_dem_total = (Σ I_dem) · fs

The main breaker is then the smallest standard rating not below I_dem_total, and its pole count follows the “highest” circuit present: three poles if any three-phase circuit exists, two if any two-phase, otherwise single-pole. Confusing fd (per circuit) with fs (whole board) is a classic error — they act at different levels and are multiplied in sequence, never swapped.

Step 4 — Busbar by current density

The busbar carries the full diversified demand to every breaker. Its minimum cross-section comes from the admissible current density J:

A_bus = I_dem_total / J

For bare copper bars in air, J is typically 1.5–2.0 A/mm²; a lower J gives a larger, cooler bar with less voltage drop along its length. The value is a minimum — the bar finally adopted must also withstand the short-circuit thermal and mechanical stress, which IEC 61439 covers through the assembly’s rated short-time withstand current.

Step 5 — Modules, spare reserve and the enclosure

Each pole occupies one DIN module (18 mm pitch). The modules used are:

modules_used = Σ poles(circuits) + poles(main)

On top of that, the board needs a spare reserve so the installation can grow without a full rework. A practical reading of NBR 5410 gives a minimum by circuit count — about 2 modules up to 6 circuits, 3 up to 12, 4 up to 30, and roughly 15 % of the modules in use for larger boards — and the calculator takes the larger of this minimum and any percentage you specify. The enclosure is then the smallest market panel whose capacity covers used + reserve, with its rows and modules-per-row driving the physical layout (and the 2D/3D diagram).

Step 6 — Short-circuit rating

Finally, every breaker must survive the worst-case fault. Its breaking capacity (Icu/Icn, in kA) must equal or exceed the prospective short-circuit current Icc at the board’s location. The calculator rounds Icc up to the next standard rating in the series 3, 4.5, 6, 10, 15, 25, 36, 50, 65 kA. A breaker chosen below Icc can fail to clear the fault and weld closed — a safety failure, not just a nuisance.

How to read the result

The per-circuit table gives In, the demand current, the adopted breaker and the poles — check that no breaker rounded down and that three-phase circuits used the √3 form.
The summary gives the diversified demand, the main breaker and poles, the busbar area, the modules used / reserve / total, and the recommended kA. If “modules used” exceeds the enclosure capacity, the board does not fit — split it or move to a larger frame.
The layout places the main breaker first, then the circuits, then the reserve, row by row across the DIN rails — the single source of truth for the panel diagram.

Practical design considerations

Never forget the √3 on three-phase circuits, and never round a breaker below the nominal current.
Apply diversity once, at the board (fs), after the per-circuit demand (fd) — multiplying both correctly avoids a busbar and enclosure built for an impossible peak.
Match the kA rating to the site: the prospective fault current near a transformer or a strong utility supply can be high; size the breaking capacity to it, not to a default.
Leave a real spare reserve: a board with no free modules is obsolete the day it is energized.
Align standard and method: NBR 5410 governs the low-voltage installation, IEC 61439-3 the distribution board as an assembly, and IEC 60898-1 / 60947-2 the breakers themselves.

Following this chain — nominal current per circuit, per-circuit demand, board diversity, busbar by density, modules with the code spare, and the kA check — yields a board that fits, protects, clears its faults and still has room to grow.

Formulas and fundamentals

Single/two-phase nominal current In = P / (V · fp)

Nominal current of a single- or two-phase circuit. P is the active power [W], V the circuit voltage [V] and fp the power factor [dimensionless]. If the circuit is entered directly in amperes (current mode), In is taken as given.

Three-phase nominal current In = P / (√3 · V · fp)

Nominal current of a three-phase circuit. V is the line-to-line voltage [V]. The √3 factor converts three-phase active power into the line current that the breaker and conductors carry.

Circuit demand current and breaker I_dem = In · fd ; breaker = next standard ≥ In

I_dem is the demand current after the per-circuit demand factor fd [dimensionless]. The breaker is the smallest standard rating (6, 10, 16, 20, 25, 32, 40, 50, 63, 80, 100, 125, 160, 200, 250 A) not below In; the pole count equals the number of phases.

Board demand and main breaker I_dem_total = (Σ I_dem) · fs ; main = next standard ≥ I_dem_total

The total demand is the sum of each circuit's demand current times the board diversity (simultaneity) factor fs [dimensionless], which accounts for loads that do not peak together. The main breaker is the smallest standard rating not below I_dem_total.

Busbar cross-section by current density A_bus = I_dem_total / J

Minimum copper busbar area [mm²]. I_dem_total is the total demand current [A] and J the admissible current density [A/mm²] (typically 1.5–2.0 A/mm² for bare copper bars in air). A larger bar lowers the temperature rise and the voltage drop along the bus.

Modules and code-required spare modules = Σ poles + poles_main ; reserve = max( NBR min, ⌈%·modules⌉ )

Total DIN modules used equals the sum of all circuit poles plus the main breaker poles (1 module = 18 mm DIN pitch). The spare reserve is the larger of the code minimum by circuit count and a chosen percentage of the modules in use; the enclosure is the smallest market panel whose capacity covers used + reserve.

Standards & methods

ABNT NBR 5410 — Low-voltage electrical installations
IEC 60364-5-52 — Selection and erection of wiring systems (current-carrying capacity)
IEC 61439-1 and IEC 61439-3 — Low-voltage switchgear and controlgear assemblies / distribution boards (DBO)
IEC 60898-1 — Circuit breakers for overcurrent protection (household and similar)
IEC 60947-2 — Low-voltage switchgear: circuit breakers
IEC 60269 — Low-voltage fuses (coordination and back-up protection)

Typical reference values

Quantity	Typical range	Note
Busbar current density (bare copper)	1.5 to 2.0 A/mm²	Lower values for poorly ventilated enclosures or high ambient temperature.
Board diversity (simultaneity) factor fs	0.6 to 1.0	Lighting/outlets allow lower fs; continuous process loads approach 1.0.
DIN module pitch	18 mm per module	A single-pole breaker = 1 module; a three-pole breaker = 3 modules.
Code-required spare modules	≥ 2 (up to 6 circuits) … 15 % of used (large boards)	NBR 5410 practical reading: 2 / 3 / 4 modules for ≤6 / ≤12 / ≤30 circuits.
Standard breaker ratings	6, 10, 16, 20, 25, 32, 40, 50, 63, 80, 100 A …	Always round the nominal current UP to the next commercial rating.
Standard short-circuit ratings (Icu/Icn)	3, 4.5, 6, 10, 15, 25, 36, 50, 65 kA	Breaking capacity must equal or exceed the prospective fault current Icc at the board.

Worked example

Light commercial board: five circuits, mixed single- and three-phase

Inputs

Lighting — 1φ, 220 V, fp 1.0: 2200 W
Outlets — 1φ, 220 V, fp 1.0: 3300 W
Air conditioning — 1φ, 220 V, fp 0.92: 4000 W
Motor — 3φ, 380 V, fp 0.85: 7500 W
Water heater — 1φ, 220 V, fp 1.0: 5000 W
Diversity factor fs · busbar density J: 0.8 · 1.5 — · A/mm²

Results

Motor nominal current (3φ): In ≈ 13.4 A
Total diversified demand: I_dem_total ≈ 64.7 A
Main breaker: 80 (3-pole) A
Busbar cross-section: A_bus ≈ 43 mm²
Modules used / spare: 10 / 2 modules
Recommended enclosure: QDC 12 DIN (12 mod.) —

Each circuit's nominal current is computed first: lighting 2200/220 = 10 A → 10 A breaker; outlets 3300/220 = 15 A → 16 A; AC 4000/(220·0.92) = 19.8 A → 20 A; the three-phase motor 7500/(√3·380·0.85) = 13.4 A → 16 A (3 poles); water heater 5000/220 = 22.7 A → 25 A. The demand currents (fd = 1.0) add to ≈ 80.9 A; with the board diversity factor fs = 0.8 the diversified demand is ≈ 64.7 A → an 80 A, 3-pole main breaker. The busbar is 64.7/1.5 ≈ 43 mm². Modules used = (1+1+1+3+1) circuits + 3 (main) = 10; with five circuits the code minimum reserve is 2 modules, so the smallest enclosure that holds 12 modules is a QDC 12 DIN, leaving 2 spare. With a prospective fault current of 5 kA, the next standard breaking capacity is 6 kA.

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Common mistakes

Forgetting the √3 on three-phase circuits — using P/(V·fp) overstates the line current and oversizes the breaker and busbar.
Multiplying every load and then sizing the main breaker with no diversity factor (fs = 1), which inflates the busbar and the enclosure for a peak that never happens.
Picking a breaker below the nominal current 'to be safe on selectivity' — the breaker must always round UP to the next standard rating, never down.
Ignoring the short-circuit rating (kA): a breaker with breaking capacity below the prospective fault current can fail to clear and weld closed.
Counting only the circuit modules and forgetting the main breaker poles and the code-required spare, so the chosen enclosure has no room left.
Confusing the demand factor fd (per circuit) with the diversity factor fs (whole board) — they apply at different levels and are not interchangeable.

Frequently asked questions

How is the nominal current of each circuit found?

For single- and two-phase circuits, In = P/(V·fp); for three-phase circuits, In = P/(√3·V·fp), where V is the line-to-line voltage. If you already know the current you can enter it directly. The nominal current is then rounded UP to the next standard breaker rating (10, 16, 20, 25, 32 A …), and the pole count equals the number of phases.

What is the difference between the demand factor and the diversity factor?

The demand factor fd applies per circuit and reflects that a circuit rarely draws its full installed power. The diversity (simultaneity) factor fs applies to the whole board and reflects that different circuits do not peak at the same instant. The board total is (Σ In·fd)·fs, so fd shrinks each circuit and fs shrinks the aggregate — they act at different levels and are not interchangeable.

How is the busbar cross-section determined?

By current density: A_bus = I_dem_total/J, where J is the admissible density for bare copper bars (typically 1.5–2.0 A/mm²). A lower J gives a larger, cooler bar with less voltage drop. The result is a minimum; the chosen standard bar must also satisfy short-circuit thermal and mechanical withstand.

Why does the short-circuit rating (kA) matter?

Every breaker has a breaking capacity (Icu/Icn) — the maximum fault current it can interrupt safely. It must equal or exceed the prospective short-circuit current Icc at the board. If Icc is higher, the breaker may fail to clear the fault and weld closed, so the calculator rounds Icc up to the next standard rating (6, 10, 15, 25 kA …).

How many spare modules should the board have?

Good practice and NBR 5410 ask for a spare reserve so the installation can grow. A practical reading gives at least 2 modules up to 6 circuits, 3 up to 12, 4 up to 30, and about 15 % of the modules in use for larger boards. The calculator takes the larger of this minimum and any percentage you set, then sizes the enclosure to hold used + reserve.

Why round breakers and ratings up, never down?

The breaker protects the conductor, so its rating must be at least the nominal current — a smaller breaker would nuisance-trip or, if it is the conductor that is undersized, fail to protect it. Likewise the busbar, the main breaker and the kA rating are sized to the next standard value at or above the computed need, building in a margin against tolerance and growth.

Glossary

Nominal current (In): The current a circuit draws at its rated power, voltage and power factor; the basis for choosing the breaker and the conductor.
Demand factor (fd): Ratio of the maximum demand of a circuit to its total installed power; accounts for loads that are not all on at full power.
Diversity / simultaneity factor (fs): Board-level factor that accounts for the fact that different circuits do not reach their peak at the same time; applied to the sum of demand currents.
DIN module: Standard 18 mm pitch on a DIN rail; a single-pole breaker occupies one module, a three-pole breaker three.
Busbar: Copper (or aluminum) bar that distributes current to all circuits in the board; sized by current density and short-circuit withstand.
Breaking capacity (Icu / kA): Maximum prospective short-circuit current a breaker can interrupt without damage; must equal or exceed the fault current at the board.
Main breaker: The board's general protection, sized for the total diversified demand and feeding the busbar that supplies every circuit breaker.