1200A Feeder Size 1

[birddogger]

Per NEC 240.4(B), a 277/480V, 800A feeder is allowed to be the industry standard of 2 sets of 3-1/2”C, (4)#500, (1)#2G even though the ampacity of the feeders are only 380A. However, it seems that I’ve seen this standard applied to 1200A and 1600A feeders as well, even though 240.4(C) requires any OC device over 800A to have conducters rated equal to or greater than the device rating.

So am I correct in assuming that the proper size for a 1200A feeder would be 4 sets of 3”C, (4)#350, (1)#3/0G? Or is there some loophole I’m not aware of that would allow three sets of #500's to be applied to this circuit as well?

 

[davidbeach]

No loophole. If installed per code a 1200A circuit requires more conductor than just 3 sets of 500. Your 4 sets of 350 works if you do not need to count the neutral as a current carrying conductor. If something, such as triplin harmonics, causes you to consider the neutral as a current carrying conductor, you will need 4 sets of 500 for that 1200A circuit to account for the derating.

 

[WoodrowJWeen]

For 1200A feeders we often use 3 sets each: 4#600, 1#3/0 - 4". Have not had any complaints about pulling 600MCM copper.

 

[birddogger]

In the consulting engineering field, we've always been told not to go over #500 due to skin effect. Any comments on that? I'd be interested in hearing what other's opinions are.

 

[jbartos]

Comment: The 500MCM conductor ampacity of 380A is related to RHW, THHW, THW, THWN, XHHW, USE, and ZW Type 75°C(167°F).
insulated conductors.
75°C temperature rating is maximum required for landing the conductors in circuit breaker terminals.
If there are switches and fuses, 90°C conductor temperature rating may be considered, if the Utility permits.
90°C insulated conductor types are RHH, THHN, THHW, XHHW, XHHW-2, USE-2, THWN-2, etc. as they appear in NFPA 70-2002 NEC Table 310.16 on page 70-144. The allowable ampacity for 500MCM for these conductors is 430A.

 

[alehman]

The rule of thumb about not exceeding 500kcmil is based on cost effectiveness of the larger conductors, ability to pull in conduit and stress on terminals. I've used 600 and rarely 750 in special situations. Some utilities will use up to 1000. Some utilities dont permit >500 (CU) or >750 (AL) for termination on transformer bushings due excessive stress and tendency for the bushings to fail.

You can take advantage of the higher temperature insulations even if your terminations don't allow, if deratings are necessary for >3 conductors in a conduit or in underground duct banks.

 

[dpc]

Up to 800A, the NEC allows use of "next larger standard size" overcurrent device for feeder conductor protection. Above 800A, this exception does NOT apply.

So a feeder protected by a 1200A breaker, for example, must have an ampacity of at least 1200A - 1199A is not acceptable. I'd suggest 4-350 kcmil copper per phase or 3-600 kcmil copper per phase for a 1200A feeder.

As others have mentioned, the upper limit of 500 kcmil is really an economic one. Beyond 500 kcmil, the added conductor cross-section does not provide a corresponding increase in cable ampacity, for a variety of reasons. Installation of larger copper conductors is also more difficult due to weight and stiffness of the cable.

 

[jbartos]

Suggestion: Current limiting fuses may be applied upstream of the breaker to reduce short circuit current levels and increase the conductor temperature from 75°C to 90°C. This type of application will allow to apply conductors with higher ampacities, e.g. 500MCM Cu ampacity will increase from 380A to 430 amps, which is not a bad deal.

 

[davidbeach]

What do short circuit current levels have to do with which conductor temperature can be used for conductor ampacity? Lugs are rated 75 or 90 degrees C and there is nothing that can be done about that through fault current limitation.

 

[DanDel]

jbartos, lug ratings don't change by simply installing fuses. What is your basis for this statement?

 

[jbartos]

Comment on the previous posting: 75°C temperature rated conductor limit is normally aligned with molded case circuit breakers. To circumvent this restriction, the 90°C temperature rated conductors may be terminated on fuse blocks that accept that temperature. Alternately, a terminal box will do to transition 90°C rated insulated conductors to 75°C insulated rated conductors.
If fuses are designed installed, why not to design and install the current limiting ones? They will reduce the short circuit level downstream within the power distribution system, which is always considered a safety measure as well as it prolongs contact life-cycles.

 

[DanDel]

Isn't there still some type of cable lug needed to make the connection? I haven't seen too many 90°C rated lugs in production, though I understand that some are available somewhere.

 

[REDDOG]

Check UL White Book for ratings

 

[jbartos]

Comment: How would all those 90°C conductors be terminated at equipment that permits 90°C, e.g. fuse blocks, terminal blocks, disconnect switches, etc.????

 

[REDDOG]

If Overcurrent devise is 100 amp or greater use 75 degree column for the ampacity you apply derating factors ( fill and temperature ) against the 90 degree column.

 

[DanDel]

This is the basis of NEC Art 110.3(B) 'Listed and Labeled'. Most lugs are listed at no more than 75°C, which means that the conductor must be derated to the 75°C column in Art 310.

 

[jbartos]

Comment: What if lugs are not used and compression terminal blocks are used instead?
Visit

http://www.faa.gov/avr/afs/300/pdf/4d-ch11_14.pdf

for terminal lugs application and terminal blocks

http://www.squared.com/us/products/...534c035585256984005f0ebb/$FILE/9080CT9603.pdf

for temperature ratings of terminal blocks
etc. for more info

 

[DanDel]

Any lug, whether mechanical or compression, will have a temperature rating.

 

[alehman]

Just checked Cutler-Hammer and SquareD safety switches. Lugs on fused and non-fused switches are mechanical type, rated 60C for sizes up to 100A and 75C above 100A.

 

[jbartos]

Comment on the previous posting: The higher temperature rated conductor is better for fused disconnect switches since fuses can become very hot thus damaging the conductor insulation. I noticed that the Square D current Digest Catalog 173 list 75°C conductor temperature rating for circuit breakers in enclosures. They resemble switches. I do not see similar temperature limitation for fusible disconnect switches and lugs in Digest 173.