Connector current derating: from specifications to equipment evaluation conditions

Current reference values can differ within the same series

Page 8 of Molex Nano-Fit wire-to-wire specification 2014471000-PS, revision D3 (May 15, 2026), lists two reference values for 20 AWG configurations using tinned copper wire, terminals with 15 microinches of gold plating and all circuits energized simultaneously: 7.50 A for a single-row, 2-circuit configuration and 5.75 A for a single-row, 8-circuit configuration. The rating table is based on a maximum temperature rise of 30°C. Asterisks in other cells indicate interpolated values. Molex specification, page 8

These two values illustrate how to select the right table entry. They cannot be extrapolated into one rating for the entire series. Suppose a project calls for an 8-circuit configuration with a continuous 6 A per circuit. Using only the 7.50 A entry for 2 circuits would select the wrong configuration during document screening. The matching 5.75 A reference entry also does not support that 6 A requirement. The next step is to select a different configuration or obtain documentation covering the project conditions, rather than add a rule-of-thumb allowance to the tabulated value.

Read the circuit count according to the manufacturer’s table definition. Do not add currents across several contact paths and treat the sum as the capability of a single terminal. The same page separately lists data for tin-plated terminals; do not omit the plating condition and mix the two sets. Actual part numbers, wires and assembly methods still need to match their respective documents. Citing this public revision does not establish it as the final applicable document for a particular purchased part number.

Check total temperature even when temperature rise is acceptable

Phoenix Contact’s explanation of connector current-carrying capacity relates the thermal properties of contacts, connections and housing insulation to ambient temperature and self-heating. Weidmüller’s derating explanation adds that ambient temperature plus the rise caused by the load must not exceed the component’s upper temperature limit. The number of poles and the connected conductors also affect current-carrying capacity. Phoenix Contact: electrical tests; Weidmüller: derating

Record the two checks separately during a review:

Temperature at the contact location = ambient temperature at that location + temperature rise caused by the current

One check determines whether the temperature rise meets the conditions in the applicable documentation. The other determines whether the total temperature exceeds the applicable limit of any relevant component. Room temperature outside the equipment cannot directly replace ambient temperature near the connector. Nearby heat sources, enclosed spaces and wiring arrangements need to be included in the project conditions.

The following are purely hypothetical arithmetic examples. Assume that the project review uses a total-temperature limit of 90°C and a temperature-rise limit of 30°C. These are not recommended limits for any product or measurements made by MTTJ.

The 5°C in case A is only the result of this subtraction. It is not a safety margin confirmed through design evaluation. The examples contain no measured relationship between current and temperature rise, so they cannot be used to work backward to an allowable current. Nor does halving the temperature rise mean that the current can be doubled.

Identify the curve before reading it

Use a graph that matches the actual terminals, wires, number of poles and energization pattern. Find the current boundary at the target ambient temperature, and check whether the wire or another component imposes a lower limit. If only a room-temperature reference table is available, with no data covering the target high-temperature environment, record that additional conditions need to be documented. Do not draw an extension to the curve yourself.

Weidmüller describes one method of deriving a derating curve from a base curve: reducing the base-curve current to 80% to account for variations in contact resistance and uncertainty in temperature measurement. The page also notes that current-carrying capacity at low temperatures may still be limited by the connected conductor. Base curves and derating curves

When you encounter 80%, identify its role in the documentation. If the derating curve has already been corrected, do not apply that same correction again as a universal rule. If the project requires an additional design margin, record its basis and where it is applied separately. A rule consisting only of “rated current multiplied by 0.8” still cannot establish whether the result applies after changing wire size, changing the number of poles or placing the assembly in an enclosed cabinet.

Different evidence calls for different next steps

This table sets out the article’s review recommendations. It separates what can be compared from what remains unproven. This avoids classifying a documentation gap as a failure, or presenting a component curve as an equipment-level validation result.

Narrow the bench question to a difference that can be checked

If document screening leaves an unresolved difference in internal equipment temperature, define the part numbers, wires, assembly and energized-circuit combination, then design the comparison around that environmental condition. If the difference concerns simultaneously energized circuits, first map the load distribution for each operating state. Distinguish normal operation, startup and intermittent loads. Do not use a continuous-energization curve to approve pulses or overloads that it does not cover.

Bench preparation should at least allow another engineer to verify these relationships: the combination used matches the document revision; measurement locations and the ambient temperature at each location are defined; current records correspond to the contact paths operating simultaneously; and stabilization criteria, stopping conditions and the acceptance basis are set before energization. Appropriately qualified personnel must arrange power-supply limits, protection and test operation according to the equipment conditions. Until these conditions are defined, a list of questions to verify is more useful than an estimated current entry.

The review can deliver one complete statement: “This combination has a documented selection basis within a specified document, configuration and temperature range; this particular difference inside the cabinet still needs supporting evidence.” Procurement, design and testing then receive the same question with the same boundaries.

Example file and further reading

The accompanying CSV contains the hypothetical inputs, temperature sums, margins and two independent checks for cases A, B and C above, ready to copy into your own review sheet. It is calculation example data, not a calculator with automatic formulas. Recalculate the results yourself after changing any input. All results appear in the article, so the document-screening process above can be completed without downloading the file.

For assembly retention questions, continue with Connector locking review: what to check at the housing, terminals and contact points. Check current-related thermal conditions and mechanical retention separately, then bring them into the same review of the actual assembly. More published articles are available in the resource library.

Sources accessed: October 4, 2026. Manufacturer documentation is used to explain its test conditions and how to read it; this article does not reproduce the tests in hardware. Recheck affected judgments when a manufacturer revises its documentation, the project changes components or installation conditions change.

Summary: Match the connector configuration before reading its current rating. Check temperature rise and total temperature separately. The three thermal-budget cases are hypothetical arithmetic examples, not measured ratings or a current calculator.

This content was prepared with AI assistance.