Connector current derating

What a connector current rating means

A connector catalogue gives a current rating for one contact, at one ambient temperature, with one conductor size and a defined temperature rise. The rating is not a property of the metal alone. It is the current at which the hottest part of the connection reaches a permitted temperature under those test conditions.

Move the connector into a cabinet at 55 °C, load four adjacent contacts and run the cable through a restricted duct, and the same contact will reach its limit at a lower current. That is derating. The physics is simple: the contact has a resistance, the current produces heat, and the heat has to leave through the contact, the housing, the conductor and the surrounding air.

The consequences of ignoring it are slow rather than dramatic. Insulation softens, plating oxidises faster, contact force relaxes and resistance creeps upward. The joint may still pass a continuity check while it is slowly cooking.

Temperature: the first derating axis

The relevant temperature is the air around the contact, not the room where the panel is built. Inside an enclosure the ambient can be 15 to 25 °C above the room, and a contact near a transformer or a drive can see more. Measure or estimate the local ambient before choosing a connector.

Start with the permitted maximum temperature of the weakest part of the system. That may be the housing plastic, the cable insulation, the plating or the contact spring. A tin plated contact on a 105 °C cable in a 90 °C housing is limited by the housing, not by the tin. The contact must then run cool enough to keep every one of those parts inside its own limit.

The temperature rise is roughly proportional to the square of the current for a resistive joint, so a small increase in current has a larger effect on heat than intuition suggests. For illustration, a contact at 80% of its rated current does not produce 80% of the heat; it produces about 64%.

Grouping and mutual heating

A single loaded contact in free air is the easy case. A row of four or eight loaded contacts in the same housing is not. Each contact heats its neighbours, and the housing that was a heat sink becomes a heat trap. The effect is strongest when all positions carry current at the same time, which is exactly what happens in a power distribution block.

The derating for grouping depends on the housing geometry, the spacing, the contact size and whether the adjacent positions are loaded. There is no universal factor. Connector makers who publish a grouping curve usually give it for a defined number of loaded contacts and a defined housing. Use that curve if you have it. If you do not, treat the grouping factor as a design assumption and confirm it by test.

The same logic applies to bundled cables. A cable that is free to dissipate heat in a tray behaves differently from the same cable packed into a conduit with a dozen others. The connector is often the hottest point because it concentrates the current into a small cross-section.

Altitude and air density

At altitude the air is thinner, so convective cooling is weaker and the clearance needed to prevent breakdown is larger. For a contact carrying current, the first effect dominates. The same heat has less air mass to carry it away, and the contact runs hotter for the same current.

The effect is modest at 1,000 m and becomes significant at 3,000 m and above. Equipment mounted on a tower, in a high-altitude substation or in an aircraft cabin pressurisation fault has to be checked. The correction is not a single number for all connectors; it depends on the housing, the airflow and the pollution degree.

Altitude also changes the creepage and clearance requirements in the applicable safety standard. A connector that is electrically fine at sea level may need a larger spacing at altitude even if the thermal derating is acceptable. Check both, and do not use a thermal factor as a substitute for a clearance review.

Conductor, termination and housing effects

The conductor is part of the heat path. A short length of thin wire conducts heat away from the contact, while a large conductor can act as a heat sink. The termination method matters too. A sound crimp has a low resistance and a predictable heat path. A solder joint with a void, or a screw terminal that has relaxed, adds resistance at exactly the point where heat is least welcome.

The housing material sets the permitted temperature and the rate at which heat leaves the contact. A metal shell spreads heat; a closed plastic housing holds it. Potting or encapsulating a connector improves dielectric protection but can reduce convection, and the compound may have a lower temperature limit than the housing.

Plating also enters the calculation. Tin oxidises faster above about 100 °C, and a tin contact that runs hot for years will see its resistance drift. Silver and nickel are more stable at temperature but have their own limits. The connector's temperature rating should be read with the plating in mind, not only the plastic.

The mounting surface is part of the heat path as well. A connector bolted to a metal plate can shed heat into the plate, while the same connector hanging in free air relies on convection. A terminal block on a DIN rail has a different thermal environment from one on a busbar. If the mounting changes, the derating assumption changes with it.

An illustrative derating table

Suppose a connector is rated at 40 A for a single contact at 25 °C in still air. The table below applies a set of illustrative factors to show how the permitted current falls as the service condition becomes harder. The factors are not a manufacturer's curve. They are a worked example of the method.

How to specify and verify derating

Put the service conditions in the RFQ. State the local ambient, the number of simultaneously loaded contacts, the conductor size and class, the altitude and the enclosure type. Ask for the derating curve, not just the headline rating. If the maker does not publish one, ask for the test report behind the rating.

In the design, keep a margin between the calculated contact temperature and the lowest permitted temperature in the chain. A 20 to 30 °C margin is a reasonable starting point for a joint that will run for years, but the right figure depends on the application and the standard. Document the assumption so that a later change of housing or cable does not silently invalidate it.

Verify with a temperature rise test at the worst-case current, with the connector mounted as it will be in service. Thermocouples on the contact, the housing and the conductor are more useful than a single reading on the outside. If the test is done at a lower ambient than the application, correct the result or repeat at the higher ambient.

• Local ambient inside the enclosure, not the room temperature
• Number of loaded contacts and their arrangement
• Conductor size, class and insulation rating
• Altitude and any enclosure sealing or potting
• Permitted temperature of housing, cable, plating and spring
• Temperature rise test method and acceptance limit