Maximizing 12 Existing #2 Conductors (Can't pull new)

[brycenesbitt]

We have twelve legacy #12 conductors in underground 3" pipe, and are looking to maximize the ampacity.
We had hoped to replace this conduit, but the local sewer district denied a request to cross their 9 foot deep clay sewer, without buying them a new HDPE sewer.
Is there a better strategy for utilizing these conductors?

The service is 3 phase 120/208Y.
The 3” conduit has twelve AWG #2 THWN-2 copper, and two AWG #6 THWN-2 copper.
No parallel conductor option (as parallel conductors under 360Hz are allowed at 1/0 AWG and larger only [ NEC 310.10(G)(1) ] )
Terminals are 75C.
All loads are continuous (EV charging).
Voltage drop based on distance is not a concern (EV charging: the cars can adapt).
No local shunt trip requirement for EV.
No neutral required.
Local soil temperature at 18" depth is 15-19°C max all year (10% adder).

Options?
Use
3 conductors 90C rating one 130A three phase run, no capacity derating, limit to 115A based on terminals (115A total).
6 conductors 90C rating two 130A runs, 80% derating, 104A each (208A total)(Two 100A breakers).
9 conductors 90C rating three 130A runs, 70% derating, 91A each (273A total)(270A net w/90A breakers).
12 conductors 90C rating four 130A runs, 50% derating, 65A each (256A total)(Um next size up rule 70A x 4 = 280A per 240.6(A)).

 

[Master Nater]

maximum ampacity - use them as pull wires to pull in bigger conductors.

 

[ptonsparky]

maximum ampacity - use them as pull wires to pull in bigger conductors.

Sounds right to me. 6-250cu will get over 400 amp.

 

[suemarkp]

In your 12 conductor case you should really use the ampacity, not the breaker size. So you have 4x65A which is 260 amps. The 9 wire case seems better at 91x3 which is 273 amps. Those could be rounded up to 100A breakers too, but keeping it at 90 is probably better.

What color are the #6 wires? Green? White?

 

[wwhitney]

A few comments:

Per Table 310.15(C)(1) footnote, "spares" get counted as CCCs. So if you don't want to count some of the conductors, those will need to be terminated as unused neutrals (up to one per set of 3 CCCs) or redundant EGCs or pulled out of the conduit.

If you utilize 215.2(A)(1) Exception 2 / 210.19(A) Exception and provide junction boxes at both ends of the conduit with splices to possibly larger conductors, then for the case of conductors of 104A ampacity, you can protect them at 110A (per 240.4(B)) and support an 88A continuous load. Likewise for the case of conductors of 91A ampacity, you can protect them at 100A and support an 80A continuous load.

If you use 100% rated OCPD (unusual but available at these ratings, no clue on cost), then you can use the full ampacity for continuous load.

Cheers, Wayne

 

[brycenesbitt]

What color are the #6 wires? Green? White?

The #6 are green
(The old setup used 3 phase 4 wire. New is EV charging so no neutral required).

Picture shows the 1" 2" 3" conduit all in the same trench, 230 feet current run length):


The upstream meter, at least one of the choices, has 3/0 wire 225A 3 phase continuous rated AFIK. The vendor is out of business
30 years, all I have to go on is the connecting 3/0 wire.

 

[brycenesbitt]

maximum ampacity - use them as pull wires to pull in bigger conductors.

It's 20 year schedule 40 PVC conduit, with gentle bends, then a 100 degree turn. Total distance
about 180 feet. The guy doing the pulling is concerned about conduit integrity.

The original plan was to use the existing conductors as pull wire, then 2 sets of 3 new wires to maximize an available 400A upstream meter (split it into two 200's).

 

[brycenesbitt]

A few comments:
Per Table 310.15(C)(1) footnote, "spares" get counted as CCCs. So if you don't want to count some of the conductors, those will need to be terminated as unused neutrals (up to one per set of 3 CCCs) or redundant EGCs or pulled out of the conduit.

Excellent, thank you for that. Useless neutrals is a good code solution.

That said if heat ever was an actual issue, the 12 conductor (4 times 3 phase) solution while it calculates out worse,
would have to be superior, right? Can we get engineering supervision to say that, and be done with the code dancing? Is that allowed here?

 

[ptonsparky]

It's 20 year schedule 40 PVC conduit, with gentle bends, then a 100 degree turn. Total distance
about 180 feet. The guy doing the pulling is concerned about conduit integrity.

The original plan was to use the existing conductors as pull wire, then 2 sets of 3 new wires to maximize an available 400A upstream meter (split it into two 200's).

Why would he be worried about conduit integrity? Buried and backfilled properly, the SCH40 will outlast all of us.
That enclosure looks like it needs to be increase in size for pulling bigger conductors, along with bushings etc.