# Wire Harness Voltage Drop and Power Loss Calculator by AWG

> Source: https://netray.co/tools/mil-spec-wire-voltage-drop-and-power-loss-calculator
> Publisher: Netray (netray.co), AI and integration engineering for ERP and manufacturing systems.
> Section: Free Tools

## Short answer

Find voltage drop, percent drop and I squared R loss in a copper wire run by gauge, current, length and conductor temperature.

## Category

engineering-plm

## Industry

Defense Electronics

## Audience

- Harness designer
- Electrical engineer
- Test engineer

## Tool

**Kind:** calculator

**Inputs:**

#### Wire gauge (copper)

**Key:** ohm_kft

**Type:** select

**Default:** 6.385

**Options:**

##### AWG 24 (25.67 ohm/1000 ft)

**Value:** 25.67

##### AWG 22 (16.14 ohm/1000 ft)

**Value:** 16.14

##### AWG 20 (10.15 ohm/1000 ft)

**Value:** 10.15

##### AWG 18 (6.385 ohm/1000 ft)

**Value:** 6.385

##### AWG 16 (4.016 ohm/1000 ft)

**Value:** 4.016

##### AWG 14 (2.525 ohm/1000 ft)

**Value:** 2.525

##### AWG 12 (1.588 ohm/1000 ft)

**Value:** 1.588

##### AWG 10 (0.9989 ohm/1000 ft)

**Value:** 0.9989

**Help:** Solid copper DC resistance at 20 C. Stranded wire is slightly higher; use the wire spec value for final design.

#### Load current

**Key:** amps

**Type:** number

**Default:** 5

**Min:** 0

**Step:** 0.1

**Unit:** A

#### One-way run length

**Key:** length

**Type:** number

**Default:** 25

**Min:** 0

**Step:** 1

**Unit:** ft

**Help:** Length from source to load; the return path is added automatically.

#### Supply voltage

**Key:** volts

**Type:** number

**Default:** 28

**Min:** 1

**Step:** 0.5

**Unit:** V

**Help:** For example 28 V DC aircraft or vehicle bus.

#### Conductor temperature

**Key:** temp

**Type:** number

**Default:** 20

**Min:** -55

**Max:** 200

**Step:** 1

**Unit:** C

**Help:** Copper resistance rises about 0.39 percent per degree C above 20 C.

#### Allowed voltage drop

**Key:** limit

**Type:** number

**Default:** 3

**Min:** 0.1

**Max:** 20

**Step:** 0.1

**Unit:** % of supply

**Outputs:**

#### Round trip resistance

**Key:** r_total

**Expr:** ohm_kft * 2 * length / 1000 * (1 + 0.00393 * (temp - 20))

**Format:** decimal

**Description:** Ohms for supply and return conductors at the stated temperature.

#### Voltage drop

**Key:** vdrop

**Expr:** amps * r_total

**Format:** decimal

**Primary:** true

**Description:** Volts lost in the wire.

#### Voltage drop percent

**Key:** vpct

**Expr:** vdrop / volts * 100

**Format:** percent

**Primary:** true

#### Voltage at the load

**Key:** vload

**Expr:** volts - vdrop

**Format:** decimal

#### Power lost as heat in the wire

**Key:** ploss

**Expr:** amps * amps * r_total

**Format:** decimal

**Description:** Watts.

#### Maximum one-way length within allowed drop

**Key:** max_len

**Expr:** if(gt(amps, 0), (limit / 100 * volts) / (amps * ohm_kft * 2 / 1000 * (1 + 0.00393 * (temp - 20))), 0)

**Format:** decimal

**Description:** Feet.

**Chart:**

#### Voltage drop vs one-way length

**Type:** line

**X:** length

**From:** 1

**To:** 100

**Steps:** 40

**Series:**

- vdrop

**Disclaimer:** DC resistance estimate for copper. It does not cover ampacity, bundle derating, altitude, insulation temperature rating or connector and contact resistance. Use the applicable wire specification and your harness design rules for current ratings.

## Content

**Intro:** Long wire runs on vehicles, shelters and aircraft quietly consume supply voltage and turn it into heat. This calculator gives the voltage drop, percentage loss and I squared R heating for a copper run by gauge.

**Sections:**

#### How to use it

**Body:** Select the wire gauge, enter load current and one-way length, then supply voltage and conductor temperature. The tool doubles the length to include the return conductor, applies a temperature correction to copper and returns the drop in volts and percent, the load voltage, the wire heating loss and the longest run that stays within your allowed drop. Use the chart to compare how drop grows with length.

**Bullets:**

- Enter one-way length; do not double it yourself.
- Use hot conductor temperature for worst case.
- Set the allowed drop from your interface requirement.

#### The formulas

**Body:** Resistance equals the wire resistance per 1000 ft times round trip length divided by 1000, corrected for temperature with R(T) = R20 x (1 + 0.00393 x (T - 20)) for copper. Voltage drop is I x R and wire loss is I squared x R. The AWG resistances used are standard values for bare copper at 20 C. Real wire to a military wire specification may be tin or silver plated and stranded, so check the specification resistance.

#### Worked example

**Body:** A 5 A load on 25 ft of AWG 18 at 28 V has round trip resistance 6.385 x 50 / 1000, or 0.319 ohm. Drop is 1.6 V, about 5.7 percent of supply, and the wire dissipates 8 W. If the budget is 3 percent, the longest run is about 13 ft, or you can move to AWG 14 which cuts the drop to roughly 0.63 V. Voltage drop is separate from the current carrying limit, so always check ampacity too.

#### Common mistakes

**Body:** Using only the one-way length is the classic error and halves the answer. Others ignore connector and splice resistance, which can add as much drop as several feet of wire in a harness with many mating pairs. For AC or high frequency, inductance and skin effect change the picture. For safety, size wire for both ampacity and voltage drop and take the worse case.

**Cta:** If harness definitions and wire lists live in PLM and ERP systems, Netray can discuss how to make that data searchable and traceable on your own infrastructure.

**Faq:**

#### Why is the length doubled?

**Answer:** Current flows out on the supply conductor and returns on the ground or return conductor, so the voltage drop includes both. If a chassis return is used, replace the return length with the chassis path resistance.

#### Does temperature matter?

**Answer:** Yes. Copper resistance increases about 0.39 percent for each degree C above 20 C, so a hot harness drops more voltage than the table value. Use worst-case conductor temperature.

#### Is this the same as wire ampacity?

**Answer:** No. Ampacity is limited by heating, insulation rating, bundle size and altitude. A wire can satisfy voltage drop and still be overloaded thermally, so check both limits.

#### What allowed drop should I use?

**Answer:** Use the figure in your interface control document, power budget or customer specification. Many designs allow only a few percent in distribution wiring, but there is no universal value.

## Search terms

wire voltage drop calculator, awg voltage drop, harness voltage drop, wire resistance per foot awg, copper wire power loss, dc voltage drop wire gauge, cable harness design calculator, wire gauge selection

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