Ch. 9 Table 9

[tbro723]

This table is meant for 75 deg C, is this the ambient temp of the conductors or the insulation of the cable? I thought it was ambient temp of the conductor but this is weird because the ampacity tables in Chapter 3 are based on 30 deg C ambient temp, then you can derate if you need to, so if this is the case why not put Table 9 on a 30 deg C base and then recalculate for different temps from there with the equation they give you.

Thanks for the help and insight

 

[jghrist]

Its the temperature of the conductor with current flowing in it. This is the same as the insulation temperature (the insulation touches the conductor). The ambient temperature is the temperature of the surrounding air (or soil if the cable is buried) at a distance from the hot cable.

 

[tbro723]

Thanks jghrist

Now if I am using 90 C rated conductors (THWN-2) and the corresponding ampacities from Table 310.16. Does that mean I need to correct the values in Chapter 9 Table 9 for 90 C. Do conductors get this hot? I know on rooftops temperatures inside conduits can reach 75 C and even this could be on a rare basis.

Any thoughts from anyone?

 

[tbro723]

To give a little more detail this is a PV application. These will be the wires run from the combiner boxes to the inverter and will be in conduit.

 

[jghrist]

If you are loading the conductors up to the limit so that the conductor temperature is 90?C, then you should adjust the resistance. Conductors get that hot if you load them to the limit of the ampacity table when the ambient temperature is at the assumed value of the ampacity table. If the cables are on a hot roof, then they should be derated for the high ambient temperature.

 

[tbro723]

Things are really starting to make sense now...thanks!