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Joined: Aug 2005
Posts: 64
K
Member
I need to know what is the allowed reduction of parallel conductors to still meet the requirements of the load? If you use parallel conductors does each conductor have to be rated to 80% of full load current? On a service entrace that I am workig on, the existing SEC's are 4/0 aluminum in parallel (RWH 4/0 compact to be exact). It's a 3 phase system and max amps is 208 from the transformer. I know I can use 4/0 copper and just run 3 line conductors and 1 neutral, why didn't they? Cost? It would be easier to pull the 4 coppers instead of the 8 aluminums. What is the deciding factor? Thanks!

Joined: Sep 2004
Posts: 68
H
Member
What is the calculated load and the overcurrent device rating? Sometimes aluminum is used to save money or it is specified by an engineer when the owner has requested that the engineer look for cost savings. You are right that a single set of copper cables will carry the output of the transformer and should be easier to install than double runs of aluminum.
To answer your question about individual conductors needing to meet 80% of the load, I do not believe that has ever been a requirement. You could certainly install parallel aluminum conductors smaller than the 4/0 as long as they combine to meet the requirement for the calculated load and the sizing of the overcurrent device to which they are connected.

Joined: Oct 2000
Posts: 2,721
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Doesn't this fall in the Scope for the Utility Company's Design Engineers to specify?

If YES, then submit the standard items to the Utility Design Engineer:
  • Load Calcs,
  • A site drawing (building footprint),
  • A summary of conditions / scope,
  • Cutsheets / specs for the new Service Equipment to be installed,
  • Filled out Service Planning application form(s),
  • Application(s) for Permit(s),
  • Necessary Fees.


If NO, then you will be the responsible party for compiling the Service Feeders' Design; and so begins the questions list:

  1. What is the Calculated Load?
  2. Will all Parallel Conductors share a single Duct, or will there be Parallel Ducts with Four Feeder Conductors in each?
  3. Will you use Copper or Aluminum Conductors?
  4. Do you need to adjust size, due to Voltage Drop issues?
  5. Is there any requirements from the DBS or Utility for a minimum Conductor size, &/or Conductor material + insulation?
  6. Were the "Original" Service Entrance Feeders + Ducts installed by an EC or the Utility Dept.?
  7. Any ideas of the "Original" Load Calcs' figures, which determined the size of the "Original" Feeders?


Out of curiosity, what size Transformer is used (75 KVA - 208Y/120VAC or 175 KVA, 480Y/277VAC), and is the Service Capacity 400 Amps?

Scott


Scott " 35 " Thompson
Just Say NO To Green Eggs And Ham!
Joined: Dec 2003
Posts: 751
E
Member
I do believe the question was about parallel conductors in the same conduit. If so, then yes, you must derate to 80% when you pull eight conductors (six current carrying) in the same conduit. The last sentence of 310.4 requires this.


Earl

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