Annex B - Cable Ampacity Studies

[MJP]

Dear All

Hoping someone can provide some advice here.

I'm managing an electrical installation for EV chargers and my A&Es have been using the standard NEC 310.15 for the cable sizing. There will be up to 12 cables in a duct bank, as this is a large installation. There have been some challenges from certain parts of the business, that we should be using Annex B to correct for cable grouping and temperature. The general consensus from my A&Es, is this is not applicable in this use case, wondered if anyone had any opinions on when Annex B is relevant?

Apologies I'm not a PE, so probably have left out some key elements here.

 

[ron]

We use Annex B for all low voltage underground ductbanks that will have a significant load factor (approaching 100%). Depending on the business case, EV charging can definitely be used close to full load for 24 hours (100% LF) if it is at a fleet vehicle arrangement.

 

[MJP]

We use Annex B for all low voltage underground ductbanks that will have a significant load factor (approaching 100%). Depending on the business case, EV charging can definitely be used close to full load for 24 hours (100% LF) if it is at a fleet vehicle arrangement.

Thanks Ron.

This is not a fleet application, so the load factor will vary depending on the vehicle. If there is high utilization and for some reason the cables are not sized correctly, would you say this presents a fire risk?


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[don_resqcapt19]

Thanks Ron.

This is not a fleet application, so the load factor will vary depending on the vehicle. If there is high utilization and for some reason the cables are not sized correctly, would you say this presents a fire risk?


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Not a fire risk, just a conductor failure risk in the ducts.

 

[Julius Right]

Calculated according to Neher and McGrath the ampacity is more than shown in Annex B. Compared with the source-IEEE 835-as NEC declared- it is also less.
Let's take 6 ducts 3*4/0 single core copper cables Table 8.2(7) for 90 Rho ampacity 20oC Earth detail 3 is 137 A.
Calculated according to Neher and McGrath 177 A.
In IEEE 835-page 6- 165 A, but for 25oC Earth temperature.
According to IEEE 835 chapter 3.4.1 Adjust for changes in ambient temperature:
I'=I*sqrt((Tc-Ta')/(Tc-Ta))=165*sqrt(75-20)/(75-25))=173 A.
NEC Table Table 315.60(C) ( l l ) 195 A for 90oC for 2000-5000 V-without shield -196 A. Recalculated as per IEEE 835 chapter 3.4.2 Adjustment for change in maximum conductor temperature or temperature due to dielectric loss I=196*0.907=177.8 A.