Underground Cable Sizing

[harritr1]

I am a new engineer graduate and I have been given a project where I am sizing replacement cable for a feeder to an ATS. I have everything figured out (voltage drop, grounding etc.) except for ampacity and that's because there is about a 16 foot section of a 560 foot run that goes underground in PVC that is encased in concrete (the rest of the pull is all in uncovered cable tray). Section 310.15(B)(3)(3) in NFPA 70 2017 edition states that:

"(3) Adjustment factors shall not apply to undergroundconductors entering or leaving an outdoor trench if thoseconductors have physical protection in the form of rigid metalconduit, intermediate metal conduit, rigid polyvinyl chlorideconduit (PVC), or reinforced thermosetting resin conduit(RTRC) having a length not exceeding 3.05 m (10 ft), and ifthe number of conductors does not exceed four."

My interpretation of that paragraph is that I need to de-rate this cables ampacity but I cannot figure out with what factors to do it. Is this just a passage that is supposed to refer me to another table in the NEC or is it something that will involve just a little math like adjusting for ambient temperature? If you could be specific in your answers on where you reference the code it would be very helpful.

Thank you.

 

[ron]

Depending on voltage and raceway type I often use Table B.310.15(B)(2)(7) if it is one of the standard configurations.

If not, then I calculate using Neher McGrath formulas.

 

[harritr1]

I am a new engineer graduate and I have been given a project where I am sizing replacement cable for a feeder to an ATS. I have everything figured out (voltage drop, grounding etc.) except for ampacity and that's because there is about a 16 foot section of a 560 foot run that goes underground in PVC that is encased in concrete (the rest of the pull is all in uncovered cable tray). Section 310.15(B)(3)(3) in NFPA 70 2017 edition states that:

"(3) Adjustment factors shall not apply to undergroundconductors entering or leaving an outdoor trench if thoseconductors have physical protection in the form of rigid metalconduit, intermediate metal conduit, rigid polyvinyl chlorideconduit (PVC), or reinforced thermosetting resin conduit(RTRC) having a length not exceeding 3.05 m (10 ft), and ifthe number of conductors does not exceed four."

My interpretation of that paragraph is that I need to de-rate this cables ampacity but I cannot figure out with what factors to do it. Is this just a passage that is supposed to refer me to another table in the NEC or is it something that will involve just a little math like adjusting for ambient temperature? If you could be specific in your answers on where you reference the code it would be very helpful.

Thank you.

I should probably specify that the cables in question are 3/c 350kcmil cables.

 

[ramsy]

I should probably specify that the cables in question are 3/c 350kcmil cables.

Per §310.15(A)(2) max ampacity equals any bottle neck > 10ft. You must use Tbl 310.15(B)(16) @ 60/75/90°C for raceway/cable/earth.

§310.15(B)(3)(a)(3) declares the same bottle neck, which is not exempt from any adjustment factors.

Termination Temperature adjustment is always required:
§110.14(C) Max ampacity listed @ 75°C for lugs, or equipment panel boards, not cable rating. 90°C allowed for derating only.

§392.80(A)(1)(b) would derate Table 310.15(B)(16) by 5% if tray entered the vault, but PVC is not tray so disregard this 5%.

You derate Table Ampacity based on standard adjustment factors.
§310.15(B)(2) - If underground Vault Ambient is not 26-30°C.
§310.15(B)(5) - Current Carrying Conductors shall include neutrals carrying unbalanced loads, or harmonics.

Add Motor Loads adjustments
§220.18(A) - Add 125% of motor full-load currents. 125% Non HVAC motor loads, per Sec II Art. 430.

 

[ramsy]

Under Engineering Supervision §310.60(A)(1) Exception, without 75°C equipment terminations near the underground Vault, that bottleneck can be hotter than 75°C.

Vault temperature may rise to §110.40 temperature terminations if any, or even higher cable-temperature limits for ampacity columns of Tables 310.60(C)(77) thru (86) per FIGURE 310.60(C)(3). At 3-cables per pipe, Detail-1 is a 1-pipe only duct bank with the most ampacity.

However the 3-pipe duct bank of Detail 2, @ 90°C & 105°C ampacity columns of Tbl 310.60(C)(77), offer little advantage over Tbl 310.15(B)(2) @ 75°C, much less ampacity for 6-pipe duct banks per Detail 3.