Conductor Temps in Duct Bank

[Flapjack]

I came across a couple of articles about cable ampacity/temperature for duct banks. One article was from a cable manufacturer, and the other article was from an engineering firm. Both articles said the design would be non-compliant once the conductor temperatures in the duct bank exceeded 75C, even though the conductors are 90C rated. Can someone explain why the cables can't be utilized up to their 90C rating for the underground portion of the circuit?

In the image below, they use 6 sets of 750 kcmil and the total load is 2,000 amps. The conductor ampacity at the 75C rated equipment terminations would be 475 amps * 6 sets = 2,850 amps. So no issue at the terminations.

NEC 110.14(C) says "Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both." Wouldn't this apply due to mutual heating from underground ducts & ground being an insulator?

 

[wsbeih]

How do you guarantee that conductor's temperature outside the duct bank will be less than that at termination rating (75 C). this might be why these articles may read as non compliant. I would suggest to:
1- Validate that 2000A is the demand load.
2- Increase the spacing between conduits

 

[Flapjack]

Couldn't you pose the same question to an above ground installation? Code allows us to use the 90C column for derating due to ambient temperature & # of conductors per conduit. If the six sets of 750's were installed above ground in three 5" conduits with 6 CCC's per conduit and an ambient temperature of 43C, we'd go to 90C column for 750 kcmil Cu and apply the derate factors to the ampacity (535 amps * 0.87 * 0.8 * 6 sets = 2,234 amps). Why would we not use the same temperature limit for below grade?

 

[Julius Right]

• The Table 310.15(B)(3)(a) Adjustment Factors for More Than
  Three Current-Carrying Conductors in a conduit only and in connection with the presented tables 16,18 and 20 only.
• RW90 cable it is not a NEC recognized cable and no cable duct bank direct buried [not embedded in concrete] in NEC.
• 3) according to art.110.14 Electrical Connections.C(C) Temperature Limitations.

(b) Termination provisions of equipment for circuits rated over 100 amperes, or marked for conductors larger than 1 AWG, shall be used only for one of the following:

(1) Conductors rated 75°C (167°F)

conductors does not exceed the 75°C (167°F) ampacity of the conductor size used, or up to their ampacity if the equipment is listed and identified for use with such conductors

That means if the equipment connected with terminal allows higher temperature [90oC for instance] then this may be the final temperature of cable conductor also.

 

[Flapjack]

• The Table 310.15(B)(3)(a) Adjustment Factors for More Than
  Three Current-Carrying Conductors in a conduit only and in connection with the presented tables 16,18 and 20 only.

Not sure what you're saying here. The duct bank example in the article isn't applying Table 310.15(B)(3)(a). I used that adjustment factor for an alternative above-ground example in response to the question posed by @wsbeih.

• RW90 cable it is not a NEC recognized cable and no cable duct bank direct buried [not embedded in concrete] in NEC

Yes, the article references RW90, which is recognized in CEC. It's still a 90C rated conductor that the article claims is non-compliant once the conductor temperatures in the duct bank exceed 75C. You can call it RHW-2 if you want. My question still applies.

• 3) according to art.110.14 Electrical Connections.C(C) Temperature Limitations.

(b) Termination provisions of equipment for circuits rated over 100 amperes, or marked for conductors larger than 1 AWG, shall be used only for one of the following:

(1) Conductors rated 75°C (167°F)

conductors does not exceed the 75°C (167°F) ampacity of the conductor size used, or up to their ampacity if the equipment is listed and identified for use with such conductors

That means if the equipment connected with terminal allows higher temperature [90oC for instance] then this may be the final temperature of cable conductor also.

I showed in my first post that the conductor ampacity at 75C was sufficient for the load in order to comply with terminal temperature requirements of 110.14(C).

 

[Julius Right]

If instead of direct buried in soil were embedded conduits and an increased distance of 250 mm instead of 190 the ampacity will be 334 A.

 

[Julius Right]

Couldn't you pose the same question to an above ground installation? Code allows us to use the 90C column for derating due to ambient temperature & # of conductors per conduit. If the six sets of 750's were installed above ground in three 5" conduits with 6 CCC's per conduit and an ambient temperature of 43C, we'd go to 90C column for 750 kcmil Cu and apply the derate factors to the ampacity (535 amps * 0.87 * 0.8 * 6 sets = 2,234 amps). Why would we not use the same temperature limit for below grade?

It is very interesting, indeed. How six conduits with 3*750 kcmil copper conductor cables- each- will heat the ambient air up to 43oC [or the inside air of conduit reaches this temperature?]

 

[Flapjack]

If instead of direct buried in soil were embedded conduits and an increased distance of 250 mm instead of 190 the ampacity will be 334 A.

Yes, I understand there are different ways to get the conductors under 75C in the duct bank. I'm asking why does the example claim the installation is non-compliant if the conductors in the duct bank exceed 75C? The conductors with the highest temperature are 79.1C. The conductor insulation is 90C. Seems compliant to me.

For an above ground installation, we derate the conductors ampacity due to ambient temperature, # of conductors per conduit, conduits not being properly spaced, etc. For an underground installation, a cable's ampacity is also impacted by temperature, # of conductors per conduit, conduit spacing, depth, soil rho, etc. Whether above or below ground, it's all about heat.

The conductors must separately meet requirements for (1) terminations and (2) conditions of use throughout the run. In the example being discussed, the termination requirements at 75C have been met for the load. Now we look at the conditions of use throughout the circuit. Code allows us to use the 90C ampacity to apply any deratings. Shouldn't that apply whether above or below ground? Why does the article limit it to 75C in a duct bank? It doesn't make any sense to me.

 

[david luchini]

Seems compliant to me.

Me too.

 

[alehman]

I think it is acceptable to operate 90C rated conductors at up to 90C for underground portions, as long as you don't exceed the 310.16 ampacity for the temperature rating of the terminations. This is explained in 110.14(C)(1).

 

[Julius Right]

It is correct. I think it is another limit possible in the connection space between last manhole and the switchgear-usually easier- as cable tray or conduits but eventually the Table 310-16 will state the actual limit [if any].