# Circuit and load inputs

Source: https://meptools.com/tools/voltage-drop-cable-checker/docs/circuit-and-load/

Stage **01 Circuit & load** is the first of the two input stages. Filling these in does not
calculate anything by itself — press **Calculate** (or **Load a worked example**) once both
stages are set. See
[Getting started](/tools/voltage-drop-cable-checker/docs/getting-started) for the full
calculate/recalculate flow.

## Circuit type

A segmented control: **DC** / **1-ph AC** / **3-ph AC**. Changing it also syncs the circuit
arrangement used in stage 02 (cable selection).

## Nominal voltage

**Nominal voltage** (V) — the system voltage used to compute design current from a kW/kVA
load and to express the voltage-drop percentage. Must be greater than zero, and cannot exceed
the BS 7671 low-voltage scope this tool's Appendix 4 tables are drawn from: **1000 V** for AC
circuits, **1500 V** for DC. A value above that ceiling is blocked, not just flagged — there is
no applicable table data past it.

## Voltage-drop limit

A select (**Voltage-drop limit**) that sets the allowable voltage-drop percentage:

| Option                                   | Allowable VD       |
| ---------------------------------------- | ------------------ |
| **BS 7671 Table 4Ab — lighting (3 %)**   | 3%                 |
| **BS 7671 Table 4Ab — other uses (5 %)** | 5%                 |
| **Custom / project specification**       | you type the value |

Choosing either BS-referenced option writes the matching percentage into the **Custom limit**
field described below (it stays read-only until you switch to Custom). Selecting **Custom /
project specification** reveals **Custom limit** as an editable field.

The panel displays this scope note next to the field:

> Private-supply whole-installation 6%/8% limits and optional >100 m uplift are not applied.

That is, the two BS-referenced options only implement the BS 7671 Table 4Ab **public LV supply /
final circuit** row (3% lighting, 5% other) — the same row the tool's formula-audit trail cites
by name ("Public/final-circuit lighting" / "Public/final-circuit other"). The private-supply
whole-installation limits and the optional uplift for runs over 100 m are not available — if
your circuit falls under those provisions, select **Custom / project specification** and enter
the applicable percentage yourself.

## Custom limit

Only shown when **Voltage-drop limit** is set to **Custom / project specification**. The
percentage voltage-drop limit used in the pass/fail check. Must be between **0.1% and 30%**.

## Protective device type

A select with five BS-referenced device types:

- MCB - BS EN 60898
- RCBO - BS EN 61009-1
- MCCB - BS EN 60947-2
- Fuse - BS 88
- Semi-enclosed fuse - BS 3036

The adjacent current field's label changes with the selection:

| Device type          | Field label                              |
| -------------------- | ---------------------------------------- |
| MCCB - BS EN 60947-2 | **Long-time trip setting** (symbol Ir)   |
| all others           | **Protective device rating** (symbol In) |

The field offers a list of standard ratings — 6, 10, 13, 16, 20, 25, 32, 40, 45, 50, 63, 80,
100, 125, 160, 200, 250, 315, 400, 500, 630, 800, 1000, 1250, 1600 A — as suggestions, but you
can type any other positive value.

For **MCCB**, the value you enter is a user-supplied long-time trip setting (Ir) that the
tool treats directly as the device's In for the coordination and capacity checks. The tool
does not check trip curves, I2, frame/setting range, breaking capacity, discrimination/
selectivity, earth-fault loop impedance (Zs), or disconnection time — verify the entered
value against the installed trip unit yourself.

For **Semi-enclosed fuse - BS 3036**, the tool applies a protective-device factor **Cf =
0.725** to the required cable capacity (Cf = 1.00 for all other device types). See
[Correction factors](/tools/voltage-drop-cable-checker/docs/correction-factors) for how Cf
combines with the other correction factors.

## Load

A single **Load** field with an inline unit selector — **kW**, **kVA**, or **A** — next to it.
The number you enter is read in whichever unit the selector is set to. Must resolve to a
design current greater than zero.

For an AC circuit with **kW** or **kVA** selected, the design current is derived using the
**Power factor** field below. For a **DC** circuit with **kW** selected, design current is
`P / Vn` and power factor is not part of the formula (see
[Method and formulas](/tools/voltage-drop-cable-checker/docs/method)).

## Power factor (lagging)

Always shown and required, for **every** circuit type including DC — the field does not hide
when you pick DC. The tool supports **lagging power factor only**. Valid range: greater than 0
and no more than 1. DC's own design-current and voltage-drop formulas do not use the value, but
the field must still hold a valid entry before the result panel commits.

## Parallel runs

A positive integer — the number of cable runs in parallel carrying the load between them.
Parallel runs assume **identical cables, routes, lengths, and terminations, with equal
current sharing** between runs. Must be a positive integer (≥ 1).

## Run length and Length unit

**Run length** — the single-direction cable run length, must be greater than zero.
**Length unit** — `m` or `ft`, selects the unit the length value is entered in.

## Validation summary

| Rule                                 | Applies to                                                                                          |
| ------------------------------------ | --------------------------------------------------------------------------------------------------- |
| Must be greater than zero            | Nominal voltage, design current (derived from load/kW+PF/kVA), run length, protective-device rating |
| Must not exceed the BS 7671 LV scope | Nominal voltage — up to 1000 V for AC circuits, up to 1500 V for DC                                 |
| Must be a positive integer           | Parallel runs                                                                                       |
| Range 0.1%-30%                       | Custom limit (only enforced when Voltage-drop limit is set to Custom)                               |
| Range 0 < pf ≤ 1                     | Power factor (required for every circuit type)                                                      |

Any failed rule shows an inline validation message on the offending field instead of a
result. **Not** a blocking rule: a protective-device rating below the design current does not
stop the commit — it is reported as a failed **Device coordination** check inside the result
instead, alongside the other two checks. See
[Reading the result](/tools/voltage-drop-cable-checker/docs/results).

## Next

- [Cable and installation method](/tools/voltage-drop-cable-checker/docs/cable-and-installation) —
  stage 02, cable construction, installation method, and size.