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Lectromec Harness Derating Tool (HDT) tutorial for EWIS


As mentioned on the WireFacts blog, the Lectromec Harness Derating Tool (HDT) is now up and running. The HDT is designed to follow the guidance provided in AS50881 on how much electrical current should be in the wires for a given harness. If the guidance is not followed, the electrical current in a harness will generate heat, and, if there is sufficient heat built up, the wires may overheat, damage the insulation and result in smoke, fires, or electrical arcing.

Obviously the formula provided is not exact and cannot predict all scenarios or the impact of all environmental conditions, but it is a conservative estimate. If it is necessary to push the limit beyond what is recommended by the HDT, it is recommended that a simple test be performed to represent the target harness.

We will now run through a quick tutorial on using of the Lectromec HDT.


Suppose we need to know the ampacity for the wires in a harness consisting of 10 – #20 wires, 8 – #22 wires, and 6 – #24 wires. There are 6, 2, and 0 power wires per gauge respectively. All of the wires use AS22759/34 style wire construction.

The first three values that we enter for this configuration are the altitude, temperature, and rating of the wires.



The altitude is the operational altitude to be considered for the assessment. If there are multiple stages of flight or operation to be assessed, it is recommended to select the highest altitude (the higher the altitude the greater the derating of the harness). Enter this value in feet.

For our example, we will set the operational limit at 40,000ft.

Ambient Temperature

This is the operational ambient temperature in the area being considered. As this has a large impact on the harness derating values, it is important to use the best data available.

For our example, we will set the operation temperature to 50C.

Wire Temp Rating

This is the max rated temperature for the wires in the harness. If the harness considered contains multiple wire types with different max temperatures, use the minimum of these.

For our example, the 22759/34 wire is rated to 150C.

Harness Information

Since we have three wire gauges, we will have to enter nine data points into the Harness Information section.

Number of Wires

This refers to the number of wires that are in the harness for the given gauge.

For our example, we will enter ‘10’ into the first cell as we are entering only the information for the 20AWG wires.

# of Power Carrying Wires

The power carrying wires refers to the number of wires that will carry electrical current to power devices (not signal wires).

For our example, we will enter ‘6’ into the second cell, still only entering the information for the 20AWG wires.


This is straightforward; this is the gauge of the wires considered in this line of data entry. Each line must be for a different gauge.

For our example, we will enter ‘20’.


Given the information provided at a given moment, the form will update the ampacity for a particular wire gauge. The ampacity is the recommended limit for the amount of current down a particular gauge wire.

Now repeat the last couple of steps for the 22 and 24AWG wires. When you are done, the form should look like the following:


The results for this bundle indicate that the maximum combined current (the Harness Ampacity) is 50.47Amps with a limit of 7.8A, 6.0A, and 4.5A for the #20, #22, and #24 gauge wires respectively.

Ensuring that a harness is adequately designed to handle power is one more step in understanding the Electrical Wire Interconnection System (EWIS) and assessing the overall risk for the aircraft. To find out more about the HDT or services Lectromec has to offer, contact Lectromec.

Michael Traskos

Michael Traskos

President, Lectromec

Michael has been involved in wire degradation and failure assessments for more than a decade. He has worked on dozens of projects assessing the reliability and qualification of EWIS components.