Derating Factor - Duct Banks

Er, one uses British Standard 7671 usually.

If in doubt, consult a qualified local Electrician.

The derating all has to do with heat, how much heating is there due to how close the wires are run together, and the ventilation available for cooling, and the length of run and the grade of the wire....

http://www.cablesizer.com/help/principles/

https://www.nfpa.org/codes-and-standards/all-codes-and-standards/list-of-codes-and-standards/detail?code=70

You need to calculate the derating factor for both situations, and rate the installation for the more severe condition. Some of the factors to rate the conductor/insulation temperature rise have not been presented. These factors, such as cable tray loading, are described in the engineering standards required to assess the allowable ampere rating.

Good citation for NEC. You can see where excess heat developed in a 1.5 meter section (or less) would flow to the cooler sections, limiting temperature rise to within the tolerance of the higher derating factor. This was checked out probably after someone was presented with the situation, maybe missed at design, or a change at install time, and had to scramble to make it work(!)

Thanks Julius!

523.8 (523.7) Variation of installation conditions along a route

Does this applicable for MV cables too? as I checked in IEC 60502-2, this is not mentioned

The de rating factors are derived from the temperature rating of the conductor,and the ambient temperature

Example: A 4/0 copper conductor with 75F insulation is passing through the ceiling of a boiler room,where the temperature is 75F.

How many amps are you allowed to pass through the conductor?

Answer:ZERO.ANY amount of current carried by the conductor will increase the temperature of the conductor beyond the insulation temperature rating.

Even if the conductor passes through a freezer before or after the boiler room.

You would have to have a conductor with a higher temperature rated insulation for the boiler room section.

All de rating factors are based on worst case.Each environment has to be considered separately.

True, except in reality, there is no 75F insulation on the market, it is typically 75C..

And in NEC governed installations, as noted by Julius, if the boiler room flanked by 2 50ft long freezers was 10ft long, you could imply you would be allowed some load on your 75F wire.

Good catch on the typo!Thanks!

Just got back.

In the case of the flanking freezers,amperage would be allowed only within the freezer environment.

When the conductors leave the freezers,they must be de-rated,so outside you would need 90C rated conductors,inside the 75C would be allowed

And since a splice at the entry and exit of the freezers would be required,you would have to use 90C rated terminals outside of the freezer.

This would be a net loss$$$ considering labor and materials.

I don't know of any contractor that would do this for 100 ft of lower rated wire.

The NEC is full of many seemingly contradictory rules,and one must examine them carefully to avoid expensive rework.

The exceptions are the usual tripping points of interpretations,and the NEC is revised every two years to address these concerns and to remain effective with current technological changes.

The NEC tells you the rules,but not how to actually perform the work.

This is left to the contractor and the local AHJ.

Most states require a certain amount of hands on experience before being allowed to sit for the electrical code exam,and in my case,5 years for Unlimited,and a minimum insurance bond of $5million.Then you have to pass the test,which is usually proctored at a local university.

Annual CEU requirements must be met to keep the license,and the CEU's cannot be achieved through correspondence type courses,they must be taken by a state certified instructor,usually at a Technical college

And no matter your level of expertise,consulting with the local AHJ for the gray areas is a good practice.

If your cables are on a ladder indoors, ignoring abnormal temperature etc, then their rating is free-air, method E or F, or as high as it gets [unless you have fan or water cooling]. So anything in a duct could require a de-rating.

However, method 117 of BS7671 in which cables are arranged vertically on side of duct & spaced one cable diameter from each other & at least 1/2 diameter from wall + support metalwork not less than 10% of total area, does not specify a derating. In contrast, method 118 with cables laid on bottom of duct requires de-rating.

So it depends on the detail & on things you have not specified, like number of circuits actual overload fusing/circuit current compared to cable rating.

You need the worst case, Ampacity derating factor. Which will be the cable tray or the duct bank. To make the calculation you must give the cable size awg , type or cores and the cable installed in the duct bank. The ambient design temperature and how the cables are to be installed in the tray, how are they separated or touching and 1/c or 3 / c. What is the voltage. You need to give us these details for the answer you request. Is this cable tray indoors or out doors does it have a cover . Is it ladder type with 9 inch rung spacing. Worst case Ampacity derating factors will apply. Most likely duct bank or cable tray depending on tray details.