Cable Derating Factor: How to Calculate Current Capacity

A cable's derating factor is a multiplier applied to a conductor's base current rating (its ampacity) to find the current it can carry safely under real conditions. Ampacity tables assume a reference ambient temperature and a conductor in free air or in a small group. When the surroundings are hotter, or many loaded conductors are bundled together, the wire cannot shed heat as quickly, so the allowed current must be reduced to protect the insulation.

In short:

• Safe current is the base ampacity multiplied by a temperature factor and a bundling factor, with an optional extra margin for continuous loads.
• Most published ampacity tables use a 30 °C ambient reference, so any hotter environment needs a correction.
• The cable is rarely the only limit. The connector contact is often the weaker link, and the lower of the two derated ratings sets the system limit.

Why Cable Derating Is Needed

Current heats a conductor through resistive loss (I² × R). The wire settles at the temperature where that heat equals the heat it can lose to the surroundings. Four conditions change that balance:

• Ambient temperature: a hotter environment leaves a smaller temperature gap between the copper and the air, so less heat leaves the wire.
• Bundling: loaded conductors packed into a loom, conduit or tray heat each other, so each one runs hotter than it would alone.
• Continuous loading: a load that runs for hours reaches full steady-state temperature, whereas short bursts may not.
• Altitude: thinner air cools less effectively, so conductors run slightly hotter at the same current. This is usually handled with a separate correction and is not part of the formula below.

The Derating Formula

The safe current is the base ampacity times each correction factor:

I_safe = I_base × f_temp × f_bundle

Where I_base is the rated current from the cable datasheet or the ampacity table, f_temp is the temperature correction, and f_bundle is the bundling correction. A design margin for continuous loads is covered below.

1. Temperature correction factor (f_temp)

Ampacity tables such as those in the US National Electrical Code (NEC) are based on a 30 °C ambient temperature. For a conductor with a given insulation temperature rating, the correction for a hotter ambient follows this relationship:

f_temp = √( (T_rated − T_ambient) / (T_rated − 30) )

As a check, for a 90 °C-rated conductor at 40 °C ambient the formula gives 0.91, which matches the NEC correction table. NEC tables only list 60, 75 and 90 °C ratings, so for wire rated at 105 °C or 125 °C, which is common in automotive and appliance cable, use the formula above or the manufacturer's own table. Calculated values:

• 30 °C or below — 105 °C insulation: 1.00; 125 °C insulation: 1.00
• 40 °C — 105 °C insulation: 0.93; 125 °C insulation: 0.95
• 50 °C — 105 °C insulation: 0.86; 125 °C insulation: 0.89
• 60 °C — 105 °C insulation: 0.77; 125 °C insulation: 0.83
• 70 °C — 105 °C insulation: 0.68; 125 °C insulation: 0.76
• 80 °C — 105 °C insulation: 0.58; 125 °C insulation: 0.69

2. Bundling correction factor (f_bundle)

When more than three current-carrying conductors share a raceway or cable bundle, NEC requires an adjustment. The commonly used values are:

• 1 to 3: 1.00
• 4 to 6: 0.80
• 7 to 9: 0.70
• 10 to 20: 0.50

Count only the conductors that are actually carrying current at the same time. Section numbering and details vary between code editions, and automotive and aerospace harnesses are often designed to their own standards, so check the rules that apply to your product.

3. Design margin for continuous loads

The NEC treats a load that runs for three hours or more as continuous, and requires circuits to be sized at 125% of that load, which is the same as using 80% of the rating. That rule sizes the protective device and the circuit. It is not a thermal property of the wire itself, because ampacity tables already assume the conductor is loaded continuously. Many designers still carry an 80% margin in harness designs as a conservative practice, and it is reasonable to do so, but treat it as a design choice and label it that way.

Worked Example

Take a 16 AWG copper wire with 105 °C insulation in a bundled loom near an engine.

• Base ampacity: 20 A (an assumed figure for illustration; take the real value for your wire and insulation from the datasheet).
• Ambient temperature: 60 °C, so f_temp = 0.77.
• Bundle: 8 current-carrying conductors, so f_bundle = 0.70.

I_safe = 20 A × 0.77 × 0.70 = 10.78 A

If the design also applies an 80% margin for a continuous load, the working limit drops further:

10.78 A × 0.80 = 8.62 A

So a wire rated for 20 A at room temperature is limited to roughly 9 to 11 A in this installation, depending on the margin you choose.

Connector vs. Cable Derating: Which Is the Weaker Link?

A common mistake is to derate the cable and forget the connector, or the reverse. The two fail in different ways:

• Cable limit: set by the insulation's temperature rating and by how well the wire sheds heat along its length.
• Connector limit: set by heating at the contact interface, where a small contact area and contact resistance concentrate heat, and by the plastic housing's maximum rated operating temperature.

Connector current ratings are normally defined at a fixed temperature rise, commonly 30 °C measured to EIA-364-70, so the usable current falls as the surrounding ambient rises. Multi-pin connector datasheets also often show a lower per-pin rating when every pin is loaded at once. In dense harnesses the connector often reaches its limit before the attached wire does. The system rating is the lower of the two derated values.

Selection and Derating Checklist

• Find the worst-case ambient temperature inside the enclosure, not the outside air temperature.
• Count the conductors that carry current at the same time in the tightest section of the bundle.
• Apply the temperature and bundling corrections for your insulation rating, or use the manufacturer's tables.
• Decide whether to add a margin for continuous loads, and state it explicitly.
• Check the connector's derating curve at the same ambient temperature and with all powered pins loaded.
• Use the lower of the cable and connector results as the system current limit.

For the thermal physics and voltage-drop side of this topic, see our companion guide on cable temperature ratings, current derating and voltage drop.

The correction values above come from the NEC approach and a standard temperature formula. Always confirm against the cable manufacturer's tables and the electrical code or product standard that applies to your design before finalizing a harness.