To establish a standard for the derating of relay contacts is, at best, a subjective practice. Limitations are governed by the type of relay, contact gap, maximum voltage capabilities of the relay contact system, and the contact material.
The most common method is to derate the contacts by use of the Power Formula, using the known current and voltage.
This method is valid only for Resistive Loads, and is an approximation only; keeping in mind the limitations mentioned above.

Power = IE (Current x Voltage)
I2 E2 = 2/3 I1E1

A designer is working with a 55 volt DC system and has a relay rated at 10 amps resistive at 28 volts DC.
What is the maximum current that can be switched at 55 Vdc.

I1 = 10 Amperes
E1 = 28 VDC
E2 = 55 VDC
I2 = ? (Current ratings at 55 VDC Resistive)
I2 E2 = 2 I1 E1/3
I2 = 2 I1 E1/E23
= 2 (10 x 28)/55 x 3
= 560/165
I2 = 3.4 Amperes at 55VDC

In addition, the user should always be concerned about the following:
1. Derating contacts that are rated for less than 10 Amperes at nominal voltage.
2. Derating contacts for use in system voltages above 130 Volts DC