Can you connect 90°C - or 105°C - rated cable to 75°C equipment? Yes. And it can save you money.
In the National Electrical Code (NEC), exceptions to Section 110-14(c) allow the use of conductors with higher temperature ratings than the equipment they are connected to, provided the ampacity of such conductors is determined based on the temperature limit of the connected equipment.
Section 110-14(c) (1) applies to equipment rated 100 amperes or less. This limits conductor sizing to 60°C ratings. Section 110-14(c) (2) is for equipment rated more than 100 amperes. It limits conductor sizing to 75°C ratings. "In both these sections, the exception permits higher temperature rated conductors provided their ampacity is limited to the appropriate equipment rating," says Dave Mercier, Southwire applications engineering manager.
Reduced ampacities drive up costs
Here's an example of the NEC exceptions at work:
You have equipment rated at 75°C drawing 295 amperes at an ambient temperature of 30°C. Table 310-16 in the NEC says that a 75°C 500 kcmil aluminum conductor can carry this load at this ambient temperature.
But what if that cable also passes through a heated area where the ambient temperature is 40°C? Table 310-16 says that at 40°C ambient you have to derate that 75°C cable by a factor of 0.$$. Your ampacity drops to 273. The result? To carr, your 295 amperes through the heated area with 75°C cable, you have to use 600 kcm conductors. The larger cable pushes up yot installed costs.
Higher temperature ratings can save you money
The exceptions in 110-14(c) give you an option: Service the equipment with a 500 kcmil cable rated at 90°C instead of 75°C. Table 310-16 says the 90°C cable can hand] 350 amperes in the 30°C environment and 319 amperes in the 40°C areas. Even after the derating, you can still carry your 295 ampere load. With 105°C cable, you can g, similar benefits and the ambient temperate can run even higher.
Mercier concludes, "To get the full beni fit of higher temperature ratings in both we and dry locations, design engineers and cab specifiers should consider updating their 600V cable specifications to include `-2' suffixes. For example, XHHW 2 and RHH/RHW 2/USE-2 all give you the advantage of 90°C operation in wet or dry locations - with no cost premium."