single conductor cable ampacity

[hw123]

I been asked to find out how many 240mm² 90°C single conductor cable should be used for a 2150A generator at 690V, according to NEC standard. I normally use IEC standard.

the problem is, that have difficulty to find the ampacity for single conductor cables. The values i find is much lower than the values I find in IEC.

can anyone help me

 

[dpc]

In order to determine allowed current-carrying capacity per NEC, you have to specify if the conductor is in free air, underground, conduit, tray, etc. (and if it is copper or aluminum ) Table 310.16 will be the basic table, but you will have to convert your 240 mm2 size to a US equivalent. I believe 240 mm2 is just slightly smaller than 500 kcmil, so to be safe, I would use the NEC data for 400 kcmil. Assuming copper at 75 deg C, this has a rating of 335 A, so you would need a minimum of 7 conductors per phase.

Also, if this project must meet NEC requirements, you should limit yourself to the 75 deg C temperature rating even though you plan on using 90 deg C insulation. Most equipment and especially terminations are only rated for use up to 75 deg C ratings.

Hope that helps.

 

[hw123]

its copper cables, they are placed in free air and its a 3-phase installation

must tables recarding mm² size to US size says that 240mm² is equivalent to kcmil 500.

If I place the cables with space in between, would that give me a higher rating.

According to the IEC standard 4 conductors pr phase its enough.
Thats why the costumer has asked if the amount of cables could be lower than 6 or 7 conductors according to NEC

 

[wareagle]

Look at table 310.17 for conductors in air at 30C.
500 kcm = 620 amps

 

[dpc]

If the conductors are in free air, then Table 310.17 may be used, with resulting reduction in number of conductors, as wareagle mentioned.

If the conductors are in tray, you'll need to refer to Article 380 to determine the ampacity.

240 mm2 is slightly smaller in cross-section than 500 kcmil, according to the tables I have. From an engineering perspective, they probably equivalent, but you'll run the risk of a local inspector having a different opinion. I'd use the 400 kcmil ampacity and have one less thing to worry about. But that's just me, YMMV.