NEC 110.14 Conductor sizing an termination temperature

[cmmadrazo]

I will pose my question as an example.

A 225 kVA (480 VAC) transformer wil be installed in the state of AZ outdoors in a 50C ambient. I plan to run one large conduit from a 480 VAC panelboard to the transformer.

I prim = 225 / .48 / 1.73 = 271 amperes
125 % of I prim = 1.25 * 271 = 339 amperes

The minimum feeder size shall be (2) per phase 4/0 THWN-2 CU
310.16 - 90C column (4/0 AWG) = 260 ampere
Derate 50C = 82%
310.15(B)(2)(a) 80% (4-6 current carrying conductors)

Minimum Feeder Size = 2 * 260 * .82 * .8 = 341 amperes > 125% I prim, 339 (Code compliant)

Termination Verification
310.16 75C column (4/0) = 345 amperes
Derated 50C = 75%
Minimum Feeder Size = 2 * 230 * .75 = 341 amperes 1> 25% I prim, 339 (Code compliant)

Question
Do I need to derate by 75% for the termination verification part of this calculation to be code compliant?

 

[david luchini]

I will pose my question as an example.

A 225 kVA (480 VAC) transformer wil be installed in the state of AZ outdoors in a 50C ambient. I plan to run one large conduit from a 480 VAC panelboard to the transformer.

I prim = 225 / .48 / 1.73 = 271 amperes
125 % of I prim = 1.25 * 271 = 339 amperes

The minimum feeder size shall be (2) per phase 4/0 THWN-2 CU
310.16 - 90C column (4/0 AWG) = 260 ampere
Derate 50C = 82%
310.15(B)(2)(a) 80% (4-6 current carrying conductors)

Minimum Feeder Size = 2 * 260 * .82 * .8 = 341 amperes > 125% I prim, 339 (Code compliant)

I don't quite follow what you are asking here. There is no requirement for the ampacity of the feeder conductors to be at least 125% of the transformer rated current. The feeder needs an ampacity at least as high as the load to be served, but you don't tell us what the load current is.

Termination Verification
310.16 75C column (4/0) = 345 amperes
Derated 50C = 75%
Minimum Feeder Size = 2 * 230 * .75 = 341 amperes 1> 25% I prim, 339 (Code compliant)

Question
Do I need to derate by 75% for the termination verification part of this calculation to be code compliant?

Here, I think you are asking about 215.2(A)(1). Assuming your load was the full 225kVA, and the load was all continuous, then per 215.2(A)(1), the minimum feeder size would need to be one set of 500mcm or two sets of 2/0 (an ampacity greater than 339A.)

The ampacity of the conductors, adjusted for temperature (and for number of current carrying conductors) would need to be 271A or greater. A single set of 500mcm THWN-2 would have an ampacity of 353 (430 * 0.82.) The single set of 500mcm THWN-2 would be acceptable.

Two sets of 2/0 THWN-2 would have an ampacity of 256 (195 * 0.82 * 0.8 * 2.) This is not acceptable for the 271A load. Two set of 3/0 THWN-2 would have an ampacity of 295A. This would be acceptable for the load.

 

[Bill318]

Assuming that the OP is loading the transformer fully at 271A and also assume that requirements are that the conductor must be sized to 125% of load i.e. 339A. Then there are two sets of calculations that follow.

1. Determining the proper cable size for 90C conductors within a single conduit at 50C ambient. Must assume 2 sets of cable in same conduit.
2. Determining that the 90C rated cable that was selected in step 1 also meets 75C terminal temperatures at the end device(s).

Another assumption might be made at this point which is the device with 75C rated terminals also lives in the same 50C ambient environment.

Calculation 1 was clearly answered in the OP and we can assume 2 set?s of 4-0 meet the conditions of use.

Calculation 2 is a bit trickier and I have seen it done several different ways, but in this case, what is the correct way?

Method 1: Take the selected conductor, in this case 2-sets 4-0, and look up the maximum ampacity for the cable in the 75C (our target terminal temperature) column. Which in this case is 230A x 2 = 460A which is > 271A full load current.

Method 2: Similar but it now applies the 50C ambient conditions for the 75C rated cable because our terminals also live at 50C ambient.
230A x 2 x .75 = 345A which is still > 271 full load current.

Since calculation 1 already took into account the 50C ambient, do we also need to apply the same 50C ambient derating when viewing the conductor as a 75C conductor for the terminal rating?
Another question also arises, should the terminal temperature in this case be 75C at rated full load (271A) or should it be 75C at 125% full load (339A)? The latter seems to be suggested by the OP.

Bottom line, what is the correct way to compensate terminal temperatures at >30C ambient when we are directed to use table 310.16 {2008 NEC} or 310.15(B)(16) {2011 NEC}?

Thanks for the very interesting topic!