Derating Cable Ampacity for Short Segments in Burried Conduit

[majesus]

I have an application where I'm running 1000ft of parallel single conductors on tray across a site. I could use free air rating, however, there is one instance where I have to bury the cables to traverse a road and the buried segment is no more than 45ft. I will determine the conductor size assuming that the entire system is burried (worst case). However, as a hypothetical question: at what maximum buried length is the free air rating still valid? Say you have to bury the cable for a 3 ft length, does the cable require derating? (Think about when we connect a free air cable to switchgear. We can still have 3ft of cable running inside the enclosure to terminate at a breaker lug and that is similiar to an enclosed conduit.)

 

[Reg87]

I would reccommend you look up your wiring rules/standards.

If your in Australia, the cable would need to be derated for the 3ft underground length. The rest of the free air cable would have its own derating factor.

 

[musaUKPN]

In my view it also depends on how are you burying the 3ft length?

If it is in proper size duct where air can circulate; and there is less probability of the duct air passage blockage; there is no harm to use the free air ratings. However, being conservative I would personally like to de-rate it to some extent; but not to the de-rated value used for fully buried cable. It is also important to see the codes/ regs in your local area before deciding the ratings.

musa UKPN

 

[majesus]

In my view it also depends on how are you burying the 3ft length?

Cable would be buried in earth, if free air circulates the 3ft cable, then there is no derating problem.

I would reccommend you look up your wiring rules/standards.

Canadian Electric Code falls silent on partial burried segments. Does anyone know if the NEC addresses this?

The cable would need to be derated for the 3ft underground length. The rest of the free air cable would have its own derating factor.

You can't have two ratings for one cable feed. It is either derated or not.

 

[davidbeach]

You can certainly have multiple deratings for one run of cable. You then go with the derating that produces the lowest rating.

 

[dpc]

Generally the NEC requires the lowest ampacity be used for the entire circuit, but see 310.15(A)(2).

David Castor

http://www.cvoes.com

 

[rcw retired EE]

NEC 2011 Article 310.15(A)(2) "Selection of Ampacity - Where more than one ampacity applies for a given length, the lowest value shall be used. Exception: Where two different ampacities apply to adjacent portions of a circuit, the higher ampacity shall be permitted to be used beyond the point of transition, a distance equal to 3.0 m (10 ft) or 10 percent of the circuit length figured at the higher ampacity, whichever is less."

The philosophy is that the short section of cable will be cooled by heat transfer along the cable.

I know a couple of forensic engineers who disagree with this philosophy based on their work investigating electrical accidents and fires.

 

[Reg87]

Sorry, majesus. I misinterpreted what you wrote as having a cable underground to switchgear then another cable from there going free air elsewhere. Regardless, ASNZS 3000 doesnt apply here

 

[majesus]

No worries, I appreciate the input. Thanks


All, for bedtime reading, if you take a look at
IEEE Transactions "Ampacity Derating Factors for Cables Buried in Short Segments of Conduit" it discusses the following:
The results show that once the length of conduit exceeds about 20 times its outer diameter, then the ampacity of the circuit must be reduced to the value that it would have if the entire length were buried in the conduit. Factors that result in lower cable ampacities, such as high soil thermal resistivity and deeper burial depths lead to larger derating factors.

The transaction is really for buried cables that go through segments of different thermal resistivity, but it does give you an idea.