NEC ?'s - 110.14(C), tables 310.16 & 310.17, 392, 400

[antigfk]

We are using #4/0 AWG dual rated "type-W/RHH" flexible power cable on a job. The manufacturer's data sheet lists the cable as rated 405A @ 90C based on NEC table 400.5. We will be terminating the cable to a power terminal block rated 75C. The cable is going to be installed in an uncovered cable tray in a single row, no spacing between cables. No temperature deration is required - it's a temporary 3-week job and it is cold outside.

Based on the RHH rating:
The conductor is rated 405A per table 310.17 90C column. Must be multiplied by 0.65 per article 392.11. New rating is 263.25A @ 90C. Per article 110.14(C), the cable ampacity must not exceed 230A per table 310.16 based on 75C column. The lower of the two values is 230A, so that must be my cable ampacity. Correct?

Based on the type-W rating:
If I had the cables in "free air" run on the ground and spaced appropriately, the ampacity of the cables @ 90C would be 405A. If I were using camlock connectors on both ends (rated 405A @ 90C), the entire cable could be rated 405A, right? If I take that same cable and terminate to that terminal block or to a breaker, would I then have to use 230A ampacity per article 110.14(C)?

Another question:
What is the purpose of article 392.11(B)(3) if article 110.14(C) forces you to limit the ampacity of the 75C column of 310.16?

 

[rcw retired EE]

"What is the purpose of article 392.11(B)(3) if article 110.14(C) forces you to limit the ampacity of the 75C column of 310.16?"

If the cable tray ran along a boiler where ambient temperature was 55C, we could start the derating from the 90C columsn. The final ampacity of the complete cable, breaker, & termination system would be the least rated element. In many cases that is the 75 C termination.

 

[dpc]

There are virtually no connectors or terminations rated for more than 75 deg C (for low voltage). Even if 90 deg C insulation is used, the conductors will have to terminate somewhere eventually and that termination will likely be rated for 75 deg C wire. Especially if it is a circuit breaker or a fuse. Without getting in a lot of specifics that apply in limited situations, you can save a lot of hassles by just using the 75 deg ampacity.

Article 392.11(B)(3) allows you to use the ampacity given in 310.16 without further de-rating. Normally, cable in tray must be de-rated below ampacity in 310.16 unless conditions in 392.11(B) are met.







David Castor

http://www.cvoes.com

 

[racobb]

If you have 75 deg connectors, then as already stated, 75 deg is the rating you must accept as max, unless you have additional derating factors as also stated, then it will be even less than the 75 deg rating.

Best to start with the 75 deg number and work down from there if you need to.

Alan

 

[antigfk]

So do you guys agree with my interpretations of the code as stated above? Yes or No

Thank you for your help!

 

[rcw retired EE]

"Best to start with the 75 deg number and work down from there if you need to."

This is true only if the conditions are the same along the whole length of a circuit. The 75C limitation of the connectors at the breaker, fuse or MCC is the maximum circuit ampacity, but the rest of the circuit may have a higher rating.

For example: a 250 kcmil circuit starts at an MCC in an air-conditioned room, ambient 30C, runs underground in a duct bank then overhead with conductors spaced one diameter apart in a tray on a pipe rack with process piping at 60C, then down to the equipment in a conduit with 3 other circuit carrying conductors in a 40C ambient.

At the MCC, the 75C rating column is applicable with no derating for temperature. (255A).
For the underground, we can use the 90C rating as the starting point for the underground duct bank ampacity calculations. (290A x derating factor from Ampcalc calculations= ??)
Conductors in the tray have to be derated for the high ambient temperature conditions, but again the starting point can be the 90C ampacity for conductors in free air. (455 A x .71= 323A)
The wires in the final run of conduit carrying 6 current carrying conductors has to be derated 80% plus derated for 40C ambient (91%). (290 x 91% x 80% = 211 amps).

The wires are good for 211 amps in this circuit, (assuming the UG duct bank calc's work out). If we had started with the 75C rating for all of the derating calc's ampacity would be 186 amps.

 

[racobb]

You are correct, I was only speaking about the connectors. As long the 75 deg is used as a limiting factor, unless the thermal rating of the cable (derated as req'd) ends up being the limit.

Alan

 

[antigfk]

Thank you guys.

Is there some sort of official document by NFPA or other group that explains these sizing requirements (especially the termination requirements of 110.14(C)) in detail and gives examples like you have given?

Our business group is going to have a hard time with this, so it would be nice to have something from an official electrical safety group. The mentality is "we've been using this cable for 405A service for 20 years, there's no way it can be rated 202.4A." (this is where a panelboard or breaker or similar device has terminations rated 75C installed in an area of 40C ambient temperature)

 

[dpc]

Other than the NEC itself, you might want to get a copy of the NEC Handbook published by the NFPA. It contains a lot of details regarding the basis for a lot (but not all) of the requirements in the NEC.

You can also subscribe to NECPlus - on-line service of the NFPA and get access to similar, but more extensive background data, in addition to a searchable NEC.

David Castor

http://www.cvoes.com