Applying 310.15(B)(3)(a)

[cdcengineer]

I have a question, let's hope I can verbally describe this adequately for the sake of this discussion.

The distribution is 208Y/120V, 3-phase, 4-wire and the conduit contains conductors to serve (4) 20A, 120 receptacles and (4) 50A, 208V receptacles. Because the 50A receptacles are 3-wire served from a 3-phase system, I must consider the grounded or neutral conductors as current carrying when applying the demand from table 310.15(B)(3)(a) per NEC 310.15(B)(5).

Therefore, I have underground PVC-sch 40 conduit with (16) current carrying conductors derated 50%. I am specifying THWN-2 copper conductors. Do I apply the demand of 50% to the ampacity of the 90 deg C column of table 310.15(B)(16)? And if so, (without considering voltage drop) would I need a minimum of #10 AWG CU for the 20A and #3 AWG CU for the 50A receptacles respectively?

Thanks for your input.

 

[charlie b]

Well, actually you have 20 current carrying conductors in the conduit. But the derating factor is still 50%. Yes to your other questions.

 

[cdcengineer]

Charlie B.

Wouldn't the 20A neutrals not be counted per 310.15(B)(5)(a)?

Thanks

 

[charlie b]

No, because the neutral of a single phase 120 volt circuit does not carry "only the unbalanced" current from the other conductors. In fact, they carry 100% of the current from the ungrounded conductor.

 

[GoldDigger]

But the neutral of the 208 three phase circuits will not be counted as long as the outlets serve only linear loads.
In other words the OP had the sense of the current carrying neutral rule exactly backwards.
And if the 120V circuits were wired as MWBCs, the shared neutral would possibly not be counted either.
It is slightly paradoxical at first, but in that specific case adding individual neutrals increases the power loss.


Tapatalk!

 

[cdcengineer]

Without knowing the loads, I must assume the worst and count them.

Thx

 

[GoldDigger]

Without knowing the loads, I must assume the worst and count them.

Thx

One thing that is not completely clear to me is the exact wiring of the 50A receptacles.
You say "3 wire". Is that two phases and a neutral without an EGC (not allowed in new work), or two phase wires and an EGC (without a neutral), which would have only two CCC but would be very limited in the type of loads it could serve? Or would it be two phases, a neutral and an EGC ( four total)?
Or even three phases, a neutral, and an EGC?

Tapatalk!

 

[cdcengineer]

(2) ungrounded phase conductors, a neutral and EGC. A 4-wire, 50A, 208V receptacle for a temp spider.

 

[david luchini]

C column of table 310.15(B)(16)? And if so, (without considering voltage drop) would I need a minimum of #10 AWG CU for the 20A and #3 AWG CU for the 50A receptacles respectively?

If the 50A receptacles are single receptacles on individual branch circuits, you could use #4 as long as the load current wasn't greater than 47.5A.

 

[GoldDigger]

(2) ungrounded phase conductors, a neutral and EGC. A 4-wire, 50A, 208V receptacle for a temp spider.

Definitely 3 CCC then.
(since 2.x is not an option)

Tapatalk!

 

[cdcengineer]

If the 50A receptacles are single receptacles on individual branch circuits, you could use #4 as long as the load current wasn't greater than 47.5A.

No way to know the load, and therefore because the OCP is rated at 50A, I believe it necessary to go with the larger #3 AWG.

 

[david luchini]

No way to know the load, and therefore because the OCP is rated at 50A, I believe it necessary to go with the larger #3 AWG.

If you don't know the load, how do you know that a 50A circuit is adequate?

 

[cdcengineer]

The circuit is sized for the max rating of the 50A spider. Who knows what they'll try to plug into it, but it'll have capacity for the max rating of 50A based on the wire and OCP I provide.

 

[Smart $]

Regarding the first OP question, you apply the adjustment factor to the allowable ampacity?in NEC speak, that means the value listed in the respective table for the conductor size, type, and condition of use.

For your application using THWN-2, that would be the value in the 90?C column of Table 310.15(B)(16).

 

[cdcengineer]

Regarding the first OP question, you apply the adjustment factor to the allowable ampacity?in NEC speak, that means the value listed in the respective table for the conductor size, type, and condition of use.

For your application using THWN-2, that would be the value in the 90?C column of Table 310.15(B)(16).

Thanks smart $ and all who responded