Question About Derating Cable Ampacity

[Sam.Ying]

Hello All,

My question is fairly simple:
Is it the rated current or the breaker’s rating that determines whether the current value of a 194°F cable should be correctly derated to a 167°F cable?

Example:
A separate container with equipment requires a branch feeder and is specified as follows:

• 35 kW (PF = 1)
• 208 V, 3-phase
• Current = 32 / 0.36 = 88.9 A
• Branch feeder = Load + 20% = 106.7 A
• Breaker size = 125 A
• Wire size = AWG 2, 130 A (we only use 194°F cable on this site)
  (No other ampacity adjustment factors are considered in this example)

And here comes the question:

Postulate A:
Due to the terminals on the breaker (max 167°F), the current rating of the AWG 2 cable is reduced to 110 A (from the 167°F table), which is still more than the actual load of 88.9 A.
Therefore, sizing with AWG 2 cable is acceptable.

Or

Postulate B:
Since the maximum possible current in the cable is 125 A, a cable should be selected that, according to the 167°F table, will not exceed 167°F at 125 A.
Therefore, sizing with AWG 1 cable is acceptable.

So, please assist me here:
Is Postulate A, using AWG 2 cable, correct?
Or is Postulate B, using AWG 1 cable, correct?

Thanks and best regards,
Sam

 

[infinity]

Does this seem correct?
35000/208/1.73=97.25 amps
If continuous load 97.25*125%=121.58 amps
125 amp conductors protected at 125 amps equals #1 AWG minimum size.

 

[ramsy]

Is it the rated current or the breaker’s rating that determines whether the current value of a 194°F cable should be correctly derated to a 167°F cable?

Since wire is sized to the load, rated current governs cable size.

Example:
A separate container with equipment requires a branch feeder and is specified as follows:

• 35 kW (PF = 1)
• 208 V, 3-phase
• Current = 32 / 0.36 = 88.9 A

* 208 V, 3-phase: presumes 3 legs, not 2
* Current = 35000/208/1.732/Pf = 97A * 1.25 = 121.3A

*208, 1-phase: presumes 2 legs
* Current = 35000/208/Pf = 168.3A * 1.25 = 210.3A

And here comes the question:

If all terminations, equipment, wire, & terminals are listed for 75°C, or higher:
T.310.16 shows #1cu for 3 legs @ 121.3A, using 125A breaker, or
4/0cu for 2 legs @ 210.3A, using 225A breaker

 

[jim dungar]

• 35 kW (PF = 1)
• 208 V, 3-phase
• Current = 32 / 0.36 = 88.9 A
• Branch feeder = Load + 20% = 106.7 A
• Breaker size = 125 A
• Wire size = AWG 2, 130 A (we only use 194°F cable on this site)
  (No other ampacity adjustment factors are considered in this example)

The NEC uses degrees C for cable and termination ratings.

The math of 32/.36 does not look right. See infinity's post #2.
He has assumed a continuous load (one lasting more than 3 hours) so he included the required 1.25 factor.
Is your 20% for future loading?

But I think the answer you are looking for is: the conductors need to be sized based on the current at the temperature rating of the conductor. However they cannot be smaller than the sizing required to meet the temperature rating of the lugs.
The simple methodology is to make two different selections, one based on the insulation rating and a second based on the terminal temperature. Then you choose the larger of the two results.