How to Select the Right Temperature Class?

Fegor,

You must have codebooks or manuals for shipboard wiring specifications? If not, many of the CR4 members will have much information for you regarding that.

I will try to explain WHY there are differences, and let the other guys who are engineers explain WHAT the differences are.

i presume you are an electrician of some sort because you mention selecting different wire types for different applications. You need to understand more than just the three variables of Temperature, Current Carrying Capacity, and Nominal Cross Section.

You must consider the application and the type of conductor and the environment.

For example, a 1/0 bare aluminum conductor in open air can carry far more amperage safely than a 1/0 copper conductor with 70°C thermoplastic insulation in an underground conduit at 45% fill.

So, the temperature classes of the cables are to aid in selecting the right insulation for the expected temperature the cable will "operate in" while carrying the expected load. The temperature classes are not to indicate "use this cable when the air temperature in the room is 85 or 90ºC. These classes are for the temperature of the conductor itself in the environment. The room can be 20ºC. The wire might be 100ºC.

There are also standards for minimum and maximum ambient temperatures at which the insulation can freeze/crack or melt. Let's leave that for another post for now.

Your question is one reason why the laser-aimed infrared thermometers are so popular and so helpful for finding "possible problems" and "Oh My God For-Sure" problems in electrical systems. Guys are finding loose terminations, loose lugs in panels, kinked cables that have broken strands and 'hot spots' that are causing insulation breakdowns --- all kinds of temperature related issues. And, lots of guys are finding that their entrance cables and (in my business we call them feeder cables) are FAR HOTTER than the insulation is rated to handle. I just finished a show using five runs off of five 400 amp 3 phase 5 wire generators to drive lighting for a concert. All the feeder cable was 4/0 90ºC SOOW. I love my little laser pointer thermometer. We had 25 4/0 cables running about 200 feet from the generators. 22 of the cables were nice and cool at around 50ºC laying out in the sun. One was at 75ºC and that made sense since it had about 90 amps more load than the others. But... one leg was at 110ºC. The insulation was getting "gooey" and we found out why - the cable had a big damaged gash that was poorly repaired.

So, that's long story to explain that in your engine room the reason you select 90ºC is not just because it is environmentally hotter in your engine room. It's also because the ventilation of the cable is likely poorer, it may be tightly bound against a hot surface, it may be bound with other cables, etc. It's also likely to endure intermittent overloadings and 90ºC cabling is specifically designed to endure many cycles of "dead short" overloads of many times more current than the rated capacity.

Yet another reason is termination derating. Many terminations and panels are rated at 75ºC. So in the accommodation space where ampacity is perhaps not as critical, it could be more economical to use 70ºC cable. Why run 90C cable just to derate it to 75 because of all the terminations?

In another example, when higher amperages are needed, sometimes a smaller wire can be used by using a higher insulation temperature rating. For example, in one wire fill on a piping run the load calculations and fill tables may call for 3/0 wire in 70ºC. But by changing to 90ºC insulation, the same current capacity might be done with 2/0 saving a lot of money on wire and maybe even dropping the pipe size.

I hope this helps. I am sure you will hear much more from others on CR4.

Please ask more questions. That's how I got started and I have learned much here.

Sir,

Txmedic has given you a good answer. I will repeat it with somewhat different words.

• The temperature class is the rating of the temperature the insulation can be at without serious damage. Because wires and cables have resistance, they are producing some heat when they are carrying current, so they will always run hotter (or warmer) than the temperature of the space they are in.
• The temperature class is NOT the maximum permissible temperature for the room the cable runs in.
• The warmer the room or space, the less heat that can be removed from the cable by conduction, convection, or radiation. Therefore, the codes will have tables that list the allowable ampacity for the cable with a particular temperature class in a room or space of a given temperature.
• In choosing the cable by cross-section and temperature class, you have to look at these tables for allowable ampacity, while keeping in mind the warmest or hottest area the cable will be exposed to.
• At the points where the cable terminates, you must also ensure that you are not exceeding the allowable temperature of the equipment at the termination. Frequently this is 75 °C, but this is determined by the equipment--some can allow a higher temperature, but others will require even less.
• Anything that will inhibit the removal of heat from the cable needs to be considered when you choose the cable. This includes whether it is in open air or in a conduit, the number of current-carrying conductors in the conduit (each one is contributing heat), the presence of insulation, etc.

OK? --JMM