---
title: "How to size a cable correctly: the six-step sequence"
description: "Design current, derating, tabulated rating, breaker coordination, volt drop and short-circuit withstand — the full sequence, worked through for a 45 kW motor."
date: "2026-01-15"
author: "Divakar B"
source: "https://energycalchq.com/blog/how-to-size-a-cable-correctly"
license: "© 2026 Divakar B. Quote with attribution to https://energycalchq.com/blog/how-to-size-a-cable-correctly"
---

Ask ten site engineers how to size a cable and most will describe the same
two steps: work out the current, then look up the table. That gets you a cable
that is right about half the time. The half that fails does so quietly — the
cable does not trip anything, it just runs hot for six years and then the
insulation gives up on a Tuesday afternoon.

The full sequence has six steps. Here they are, with a real example.

Each step produces a minimum size, and the cable you specify is the **largest**
of them — not the first one that passes. That is the single idea most sizing
mistakes come down to: someone finds a size that clears the current table,
stops there, and never discovers that volt drop or fault withstand wanted more.

![Six step cable sizing sequence for a 45 kW motor on an 85 m run: design current 79.2 A, derating 0.677, required rating 117 A giving 50 mm², breaker coordination, volt drop 1.35 percent, and a short circuit withstand check needing 68.7 mm², ending at 70 mm² copper](/blog/cable-sizing-six-steps.webp "The current table asked for 50 mm². The fault check asked for 70 mm². The cable you install is the largest answer, not the first one.")

## The worked example

A 45 kW induction motor, 415 V three-phase, power factor 0.86, efficiency 0.92.
The run is 85 metres from the MCC to the motor, on a perforated tray, bunched
with three other cables, in a plant room that reaches 45 °C in May.

## Step 1 — Design current

```
Ib = P / (√3 × V × cos φ × η)

Ib = 45000 / (1.732 × 415 × 0.86 × 0.92)
Ib = 45000 / 568.4
Ib = 79.2 A
```

Note the efficiency term. A lot of people leave it out, which understates the
current by 8–10 % on a typical motor. The motor draws what it draws on the
input side, not the shaft side.

The [three-phase current calculator](/tools/three-phase-current) does this step
on its own if that is all you need, and it shows the apparent and reactive
power alongside.

## Step 2 — Derating

This is where most designs go wrong. The table value assumes one cable, in
free air, at a reference ambient. Your cable is none of those things.

| Condition | This job | Factor |
|---|---|---|
| Ambient 45 °C | plant room in May | 0.95 |
| 4 circuits bunched | on the tray | 0.75 |
| On perforated tray | not free air | 0.95 |

```
Total derating = 0.95 × 0.75 × 0.95 = 0.677
```

The cable has lost a third of its capacity before you have chosen it.

## Step 3 — Required tabulated rating

```
Iz ≥ Ib / (Ca × Cg × Ci)

Iz ≥ 79.2 / 0.677
Iz ≥ 117 A
```

From the copper table, 35 mm² carries 113 A — not enough. **50 mm² at 138 A**
is the first size that clears it.

If you had skipped derating you would have picked 25 mm² (92 A) and been two
sizes light. That cable would run at roughly 127 % of its real capacity.

## Step 4 — The breaker has to fit between the two

A cable and its protective device are sized together, and there is a rule that
ties them:

```
Ib ≤ In ≤ Iz
```

The device rating `In` must be at least the design current, or it trips on
normal load. It must also not exceed the cable's **derated** capacity, or the
cable can sit above its rating indefinitely without the breaker ever noticing.
A cable protected by a breaker that is too large is not protected.

Note which `Iz` this is. Not the table value — the table value after your
derating factors. For the 50 mm² we were about to choose:

```
Iz(actual) = 138 × 0.677 = 93.4 A
```

So `In` has to land between 79.2 A and 93.4 A, and the only standard rating
that fits is 80 A. That is under 1 % above the design current on a motor
circuit, which is uncomfortably tight — you would be relying on the starting
dip never quite reaching it.

Move to 70 mm² and the picture changes:

```
Iz(actual) = 175 × 0.677 = 118.5 A
79.2 ≤ 100 ≤ 118.5  ✓
```

A 100 A device fits with room either side. This is a good example of a step
that does not strictly *fail* at 50 mm² but tells you something is wrong — when
only one breaker rating fits, and barely, the cable is marginal.

There is a second condition for devices whose tripping characteristic is not
adjustable, `I₂ ≤ 1.45 × Iz`, where `I₂` is the current guaranteeing operation
in the conventional time. Modern MCBs and MCCBs to IS/IEC standards satisfy it
automatically when `In ≤ Iz`, so in practice it only needs checking with fuses
and older devices.

**Motors are the exception worth knowing.** Where a starter provides the
overload protection through its own relay, the upstream device is doing
short-circuit protection only and is deliberately sized above `Ib` — the
`In ≤ Iz` limit then applies to the overload relay setting, not to the breaker.
Apply the rule to whichever device is actually protecting the cable against
overload.

## Step 5 — Volt drop

Now check the run length. For 50 mm² copper, volt drop is 0.83 mV per ampere
per metre.

```
Vd = 0.83 × 79.2 × 85 / 1000 = 5.59 V
Vd% = 5.59 / 415 × 100 = 1.35 %
```

Comfortably inside the 5 % limit for power circuits. On a longer run — say
250 m — the same cable would give 4.0 %, still passing but close enough that
you would go up a size for the motor starting dip.

**Volt drop governs more often than people expect.** Any run past about 100 m
at low voltage, check it before you commit.

The [voltage drop calculator](/tools/voltage-drop) will also tell you the
longest run a given size can carry within the limit, and the smallest size that
clears it — useful when you are deciding between upsizing the cable and moving
the board closer.

## Step 6 — Short-circuit withstand

The step almost everyone skips. The cable must survive the fault current for
as long as the protective device takes to clear it:

```
S ≥ √(I² t) / k
```

where `k` is 115 for PVC-insulated copper and 76 for PVC-insulated aluminium.
If the board fault level is 25 kA and the breaker clears in 0.1 s:

```
S ≥ √(25000² × 0.1) / 115
S ≥ 7906 / 115
S ≥ 68.7 mm²
```

Which means 50 mm² **fails** on short-circuit withstand even though it passed
current and volt drop. You need 70 mm².

This is the step that catches good engineers out. A cable sized correctly for
load can still be destroyed by a fault it has to hold for a tenth of a second.

There is more to it — the right `k` for a phase conductor versus a protective
one, what current-limiting devices change, and why the far end of a long cable
needs checking as well as the near end. It has its own post:
[short-circuit withstand](/blog/cable-short-circuit-withstand).

## The answer

Laying the six steps out together shows where the decision actually came from:

| Step | Check | Minimum size |
|---|---|---|
| 1 | Design current, 79.2 A | — |
| 2 | Derating, total 0.677 | — |
| 3 | Tabulated rating ≥ 117 A | 50 mm² |
| 4 | Breaker fits between Ib and Iz | 50 mm², marginal |
| 5 | Volt drop 1.35 %, limit 5 % | not governing |
| 6 | Short-circuit withstand ≥ 68.7 mm² | **70 mm²** |

**70 mm² copper**, governed by short-circuit withstand, not by load current.

Read down the last column and the point of the sequence is obvious. Volt drop
had plenty of margin on an 85 m run and never came into it, the current table
asked for 50 mm², and the fault check — the one most often skipped — is what
actually set the answer. Any one step taken alone gives the wrong cable.

If you had used only the current table you would have specified 25 mm² — three
sizes light and unsafe under fault.

## Try it

The [cable sizing calculator](/tools/cable-size) runs the current and volt drop
steps with the derating factors built in. Short-circuit withstand depends on
your board fault level and breaker curve, so check that one against your
protection study.

## A caution worth repeating

Cable current ratings vary between manufacturers by 5–10 % for the same
nominal size and construction. The tables here are representative. Before you
issue a design, pull the actual datasheet for the cable you are buying.