Electrical — cables, voltage drop and fault current

Electrical design asks four questions of every circuit: is the cable big enough to carry the load without overheating, will enough voltage survive the journey, what happens when something shorts out, and how dangerous is the equipment to work on live. This guide covers each, then how STING calculates them.

Why cables are not sized by current

The intuitive approach is to look up the load current, find a cable rated for it, and stop. On short runs that works. On long runs it produces circuits that are technically compliant and functionally useless.

The reason is voltage drop. Every metre of cable has resistance, and current flowing through resistance loses voltage. Deliver 230 V to a distribution board and the far end of a long circuit may see appreciably less. Motors run hot and start badly on low voltage, lighting dims and shifts colour, and electronics behave unpredictably.

So on any appreciable run, voltage drop — not current-carrying capacity — is what decides the size. BS 7671 limits it to 3% for lighting circuits and 5% for power circuits. Lighting is stricter because the effect is directly visible.

STING reflects this: rather than reading a size off an ampacity table, it works up through standard sizes and picks the first one whose voltage drop is within limits.

Derating — the capacity you don't get

A cable's tabulated rating assumes reference conditions. Real installations rarely match them, and the tabulated figure has to be corrected downwards for:

• Installation method — a cable clipped to a surface in free air sheds heat well. The same cable buried in insulation cannot, and loses roughly a quarter of its capacity.
• Insulation type — cross-linked polyethylene tolerates a higher operating temperature than PVC, so the same conductor carries appreciably more.
• Ambient temperature — a cable in a hot plant room, or in a tropical climate, starts closer to its limit and can carry less.

These multiply together. A cable in a poor installation method, in PVC, in a hot room, can end up with well under half its headline rating. This is the step most often skipped by hand, and it is the one that causes cables to run hot in service.

Fault current

Voltage drop is about normal operation. Fault current is about the worst moment.

Short line and neutral together and the only thing limiting current is the impedance of the supply and the cable — so current surges to a value far beyond any normal load. Two things must be true: the protective device must interrupt it fast enough to prevent a fire, and the device must be capable of interrupting it at all.

That second point is the one that gets missed. Every breaker has a breaking capacity. Present it with more fault current than it can clear and it may fail to open — welding shut or failing violently. The available fault current at each board must be checked against the rating of the device installed there.

Fault current is highest near the supply and falls as you move away, because each length of cable adds impedance. So the main board needs the highest-rated devices, and a small final board far from the supply may need much less.

Arc flash

Arc flash is the safety case for the people who will maintain the installation, not the equipment.

When a fault arcs across an air gap inside a panel, the arc releases intense heat and light in milliseconds. The measure that matters is incident energy — how much energy would land on someone standing at working distance — expressed in calories per square centimetre. That number sets the protective clothing required to work on the equipment live, and the boundary beyond which it is safe to stand.

The counterintuitive part: incident energy depends on how long the arc burns as much as how large it is. Since clearing time comes from the upstream protective device, a slower upstream breaker can make a downstream panel far more dangerous without changing anything about the panel itself.

STING implements the IEEE 1584-2018 model and maps the result to NFPA 70E protective equipment categories, producing labels for the equipment.

How STING approaches it

Electrical work has its own panel with seven tabs: PNLS, CIRCTS, CALCS, CABLE, SLD, LITE and RPRT.

Boards and circuits

PNLS handles distribution boards and their schedules. + Spare and + Space fill empty ways — a distinction worth keeping straight, since a spare is a fitted device with nothing connected while a space is an empty way with no device, and contractors price them differently. ▶ Run Slot Fill does this across the project.

CIRCTS covers circuit assignment: ▶ Preview All before ▶ Apply to Selected, ▶ Propose Circuits (in panel) for suggestions, and ▶ Apply Balance to even the load across phases. Phase balance matters more than it appears — an unbalanced three-phase supply puts current in the neutral and wastes capacity you have already paid for.

Calculations

On CALCS, the natural order is ▶ Recalculate Load Summary, then ▶ Calculate Fault Levels and ▶ Stamp to Panels, then ▶ Run BS 7671 Audit. ▶ Flag Exceedances and ▶ Auto-Upsize Failing deal with circuits outside limits.

Arc flash sits here too — ⚡ Arc Flash Calc, then 🏷 Arc Flash Label Sheet and 📋 Arc Flash Schedule. 📈 Selective Coordination Viewer addresses a related question: whether a fault at a final circuit trips only its local device, or takes out the main board and the whole building with it. Coordinated protection means the nearest device operates first.

Cables

CABLE carries sizing and containment: ▶ Calculate then ▶ Apply to Circuit, plus Cable size, Voltage drop, CPC size and Route validate. ▶ Calculate Fill and ▶ Validate containment fill in model check that conduits and trays are not overfilled — overfilling traps heat, which derates every cable in the bundle.

Diagrams and reports

SLD generates the single line diagram — the one drawing that shows the whole distribution system as a tree, from incoming supply through main board to sub-boards and final circuits. ▶ Generate SLD Drafting View traces the circuits already in your model and draws them, so the diagram reflects the model rather than being maintained alongside it. ▶ Generate Riser Diagram does the vertical equivalent.

If it reports finding no distribution roots, it means no electrical equipment is modelled as a supply origin — everything is wired as a downstream load, so there is no top of the tree to draw from.

RPRT holds ▶ Full Compliance Audit, register and schedule exports, and handoffs to specialist analysis tools.

A sensible order of work

1. CIRCTS — get circuits assigned and phases balanced first. Everything downstream depends on it.
2. CALCS › ▶ Recalculate Load Summary.
3. CABLE › ▶ Calculate, check the results, then ▶ Apply to Circuit.
4. CALCS › ▶ Calculate Fault Levels and ▶ Stamp to Panels — with real feeder lengths recorded.
5. CALCS › ▶ Run BS 7671 Audit and clear what it raises.
6. SLD › ▶ Generate SLD Drafting View.
7. RPRT › ▶ Full Compliance Audit before issue.

Circuits first is the rule worth internalising. Load summaries, cable sizes, fault levels and the single line diagram are all derived from the circuit tree — build it wrong and every calculation after it is confidently incorrect.