Current Derating: Design Rules and Process Limits

A connector datasheet lists a current per contact, and it is tempting to read that figure as the current the contact can carry. It is not. The rating is established under defined test conditions, and a real application differs from those conditions in several ways at once, which is why the working current is usually a fraction of the rated one.

This article explains how the rating is derived, how the derating factors combine, and what contact resistance and plating contribute over the life of the product.

Why A Rated Current Is Not A Working Current

The rated current is the current at which the contact reaches a defined temperature rise over the ambient, usually thirty degrees, when all the contacts in the connector are energised and the connector is in still air. That condition is rarely the one the design meets: the connector may be in an enclosure, it may be carrying current on only some of the contacts, and the wire attached to it may conduct heat away or add to it.

The rating is therefore a reference point rather than a limit. The design question is what temperature the contact will reach in the actual assembly, and the answer depends on the current, the ambient, the neighbouring contacts and the thermal path through the wires and the board.

How The Rating Is Established

The test energises every contact in the connector with the same current and measures the temperature rise of the hottest one, which is usually in the middle of the row. Raising the current until the rise reaches the threshold gives the rating. That definition explains why the rating changes with the number of contacts: a connector with many contacts has a hotter middle than one with few.

The standard also defines the wire size and the mounting, because both affect the thermal path. A contact with a large wire attached loses heat along the wire, while one with a small wire does not. A connector mounted on a board with copper planes underneath loses heat into the board, while one on a thin board with no copper does not. All of these are part of the test condition rather than of the contact itself.

Derating For Multiple Contacts

When only some contacts carry current, the adjacent contacts are cooler and the temperature rise of the loaded ones is lower than in the full load test. Where the loaded contacts are grouped together, the rise in the middle of the group is higher than if they were separated. Distributing the current carrying contacts across the connector, and leaving an unloaded contact between groups where it is possible, is a simple way to reduce the temperature.

An equal share of the current between parallel contacts is not guaranteed. If two contacts are wired in parallel, the current divides according to their resistance, and a contact with a slightly higher resistance carries less. The distribution is therefore never exactly equal, and the design should assume that one contact carries more than its nominal share. The imbalance is worse at low current, where the contact resistance dominates, and it improves as the contacts heat and their resistances converge.

Contact Resistance And Voltage Drop

The contact resistance is the sum of the resistance of the two contact surfaces and the resistance of the interface between them. The interface resistance depends on the number and the size of the areas where the metals actually touch, which is a function of the contact force and the surface roughness rather than of the apparent contact area.

A clean contact with adequate force has a resistance of a few milliohms, and at ten amperes that produces a small voltage drop but a measurable loss. A contact that is contaminated, oxidised or under less force than it was designed for has a resistance that can be ten times higher, and the resulting drop and heating appear as an intermittent fault rather than a clean failure. The voltage drop across a mated pair is the practical measurement, and it should be checked at the rated current rather than with a low current meter.

Plating, Wear And Fretting

The plating on a contact is selected to resist corrosion during storage and to resist wear during mating. Gold over nickel is the usual choice for a connector that is mated repeatedly, because gold does not oxidise and the nickel provides a hard underlayer. The gold layer is thin, and the wear that matters is the wear through to the nickel, which happens at the point where the two contacts rub.

Fretting is a related mechanism that appears in connectors that vibrate without being mated and unmated. Small relative movements expose fresh metal, which oxidises, and the oxide accumulates in the contact area and raises the resistance. A contact with a force that is too low frets more, and the effect is worse for a tin plated contact than for a gold plated one because tin oxidises more readily. Where vibration is present, a contact with a higher normal force and a gold finish is worth the additional cost.

Specifying A Connector For A Design

The specification should state the current per contact, the ambient inside the enclosure, the number of contacts that carry current and their arrangement, and the wire size. Those five items are what a supplier needs to confirm that the connector will work, and a request based only on the total current cannot be answered properly.

The heat that the connector produces is part of the thermal budget of the enclosure, and it should be included when the internal air temperature is estimated, because the connector is often the component that sets that temperature. Where the connector also carries a signal, the current carrying contacts should be separated from the signal contacts to reduce the coupling, and the general practices for pad design and for sizing the copper that feeds the connector apply to the board side of the joint. The acceptance criteria for the assembly are the ones described under quality characteristics.

Additional Considerations for This Build

Practical attention to current derating pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating current derating explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Deliberate attention to contact plating pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating contact plating explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.

Process Control and Verification

On a design of this kind, contact plating is the item that decides how the rest of the board is arranged. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance. Where a value sits close to a process limit, the drawing should say so, since the shop can then open the process window rather than working to a nominal figure that carries no tolerance.

FAQ

Can two contacts in parallel carry twice the rated current? Not reliably. The current does not divide equally, and the derating for grouping applies. The design should assume an uneven share rather than an ideal one.

Why does the rating depend on the number of contacts? Because the test energises all of them and measures the hottest. A connector with many contacts has a hotter middle, so the current that produces the defined rise is lower.

What causes a connector to fail after years of service? Fretting corrosion is a common cause. Small movements break the plating and the resulting oxide raises the contact resistance until the joint overheats.