MV Cable Ampacity

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ElectricalPancake

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I need to determine ampacity for 2 sets of 500kcmil CU cable using a layout corresponding to NFPA 70 Table 310.60(C)(77), however the table only provides ampacities for 1, 3, or 6 circuits. Since I have 2 circuits, is it typical to use the lower ampacity of a 3 circuit configuration?

Thanks!
 
According to Table 310.60(C)(77) Det.1[one circuit] 500 MCM 465 A and Det.2[3 circuits] 310 A. Calculated as per Neher&McGrath minimum for 2 ducts [2 circuits] 500 MCM is 380 A [(465+310)/2=387.5 A it is not so far].

However, in my opinion, you have to take only 310 A per each conductor using the table instead of calculation [310.60 Conductors Rated 2001 to 35,000 Volts.(B) Engineering Supervision]
 
Do remember that you don't have to protect it at its ampacity per 240.100. You just need to make sure that the connected load doesn't exceed the ampacity.
 
Part IX. Overcurrent Protection over 1000 Volts, Nominal

240.100 Feeders and Branch Circuits

(C) Conductor Protection. The operating time of the protective

device, the available short-circuit current, and the conductor

used shall be coordinated to prevent damaging or

dangerous temperatures in conductors or conductor insulation

under short-circuit conditions

So in my opinion here it is about short-circuit current. The ampacity is connected with overload. See : 240.101 Additional Requirements for Feeders.
 
Very good replies to take to heart already provided.

Of course in most MV applications, I've never really had concrete load calculations that couldn't be fudged a little.

Protect below its withstand and all is good.
 
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