I was looking at an installation the other day, rather then change the main service panel, the electrician ran a 2" sealtight from the main service to a larger load center. He removed the feeder going into the house from the main breaker and ran it to the new panel. From the new load center he ran a feeder back (through the same raceway) to the main panel and spliced it to the original feeder. There are now six 2/0 cu. in the 2" sealtight, 4 current carrying conductors. this is a 200 amp service and they used table 310.15(B)(6) to use 2/0cu instead of 3/0cu. So my question is, since there are now 4 CCC's in the same conduit and it requires derating to 80% based on 310.15(B)(2)(a), would you start to derate from 200 amps or 175 amps? I say 175 amp, This means a jump to 250MCM. Now the 2" sealtight is too small. :blink:
Derating question
- Thread starter acrwc10
- Start date
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- Location
- Placerville, CA, USA
- Occupation
- Retired PV System Designer
I am having a hard time understanding what was removed and what was spliced, as well as what happened to the branch circuits.
One major concern is that service wires (before the first OCPD) and branch or feeder wires (after the first OCPD) are not allowed to run in the same conduit or raceway.
One major concern is that service wires (before the first OCPD) and branch or feeder wires (after the first OCPD) are not allowed to run in the same conduit or raceway.
Carultch
Senior Member
- Location
- Massachusetts
The way that I would approach this problem:
A. Start with the service rating, take 83% of it when 310.15(B)(7) applies to the service conductors. Ignore this if it does not apply. Call this value A.
B. Select a conductor from 310.15(B)(16) in the 75C column by default (or the column applicable to the terminations), that meets or exceeds the value of A.
C. Calculate total derate, the product of both temperature correction and ampacity bundling adjustment. Calculate the value of A/total derate. Select a conductor from 310.15(B)(16) in the column applicable to the wire's ampacity, usually 90C in most of my applications.
D. Choose the larger of parts B and C.
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
How long is the 2" spacer? If it is less than 24", the adjustment factors of table 310.15(B)(3)(a) do not apply.
- Location
- Lockport, IL
- Occupation
- Retired Electrical Engineer
My reading of the situation is that the branch circuits that had been served from the original panel were not altered. The new panel will have its own breakers to serve additional branch circuits. So no service conductor is sharing a conduit with a branch circuit conductor. Is that right?
sorry for not being clear enough, there is a main service panel with a 200Amp main OCPD . The feeder was taken off the main OCPD and a new feeder run from the main OCPD to feed a new load center. The new feeder is run in the 2" sealtight conduit, about 5' long from the OCPD to the new load center. In the new load center a 200Amp breaker was installed to feed back to the original main service panel and are spliced to the old feeders that were taken off the main OCPD. None of these are service conductors. The new feeder is run in the same 2" sealtight as the feeder returning to the main, to splice to the original feeder that was disconnected.
I would say that with the 5 ft sealtite if would be a violation. 310.15(B)() will allow the 2/0 which at 75° has a 175 ampacity. With 4 cc conductors using the 90° column you still end up below the 175 amp rating.
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