I have a 1200amp distribution board fed by (2) underground 4" sch40 PVC conduits from the secondary transformer (A huge engineering screw up but here we are). If I could get a testing company to come out and set the parameters of the breaker to 1000A then I could run parallels of (3) 1000mcm and bond the neutral buss at the panel since it is a separately derived system. The issue there is being able to fit 1000mcm mac adapters for 500mcm lugs. Anyone else out there got any bright ideas other than starting over and running the correct size and quantities of conduits? (The panel is indoors, concrete has been poured, and the building is erected). Another crazy idea is to run DLO cable since we can then run a smaller conductor but pulling DLO in PVC isn't my idea of a good time...
Feeders
- Thread starter Rockinrmt
- Start date
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Have you looked into two parallel sets in each raceway? Each set after derating would need to be 300 amps. (400kcmil=380amps*80%=304 amps) Other choice is a smaller main OCPD in the distribution board.
- Location
- Illinois
- Occupation
- retired electrician
No you really can't use a smaller conductor when using DLO. I expect that you are looking at the published ampacities of the DLO leading you to believe that you could use a smaller conductor, but you will find that those ampacities are for conductors in free air operating at the 90°C ampacity...something you can't really do in an NEC application.
Table 310.16 applied to the use of DLO for your application, however since DLO is not a standard size conductor, 310.15(A) permits, under engineering supervision, you to use interpolation of the adjacent conductors based on the conductor’s circular-mil area to determine the ampacity of non-standard conductor sizes.
I absolutely love that idea! Thanks!
I looked at the derating factor at 75C which would put me at 444DLO but I think I like the other response of running parallels in the same conduit. Appreciate the feedback!
Ugh! Since the wire is in an underground raceway it is already reduced to 75C which the ampacity then becomes 270amps and @80% we then can only get 216amps per. Thoughts?
- Location
- New Jersey
- Occupation
- Journeyman Electrician
What code section? THWN-2 or XHHW-2 90° C wet location?
Elect117
Senior Member
- Location
- California
- Occupation
- Engineer E.E. P.E.
As infinity stated, 4 runs of 400kcmil with 2 runs in each 4in conduit with a single 2/0 SSBJ in each conduit, it would still meet the NEC conduit fill requirement and meet the ampacity of the 1200A OCPD.
I don't see how else to do it.
4in SCH40 - 5.022 in^2
(8 * 0.5863) + 0.223 = ~ 4.9 in^2
400kcmil 90°C ampacity for reduction purposes in same conduit with thwn-2 insulation is (as infinity already put), 380A
380 * 80% derating = 304A per run (80% because you have 6 CCCs in each conduit. We are not counting the neutral. If you need to count the neutral as a conductor, this won't work).
304 * 4 = 1216A which is greater than 1200A OCPD.
I don't see how else to do it.
4in SCH40 - 5.022 in^2
(8 * 0.5863) + 0.223 = ~ 4.9 in^2
400kcmil 90°C ampacity for reduction purposes in same conduit with thwn-2 insulation is (as infinity already put), 380A
380 * 80% derating = 304A per run (80% because you have 6 CCCs in each conduit. We are not counting the neutral. If you need to count the neutral as a conductor, this won't work).
304 * 4 = 1216A which is greater than 1200A OCPD.
Sorry, should have done my research before listening to the peanut gallery in my ear lol. When looking at Table 310.15(C)(1) I would have to derate to 70% due to having (8) conductors. If I am able to use 90C as my derating factor I would be good with that since we could still at least get 1000A however the engineers for this job as well as other comments I have seen in other posts question the lugs with are rated at 75C there for the derating factor would be at that temp...
- Location
- New Jersey
- Occupation
- Journeyman Electrician
He didn't say what the voltage is but assuming that it's 4-wire you could also use reduced size neutrals if the fill is close.
No the derating factor for 4-6 CCC's is 80% not 70%. Unless the load is more than 50% non-linear you do not need to count the neutrals as CCC's.
Pretty sure you have to count the neutral as a CCC however I don't necessarily have to run neutrals since I could technically bond the neutral buss at the panel in which case I could derate at 80% rather than 70% but only if they are allowing the 90C table. It's that grey area where they talk about the lugs being rated at 75C that they seem to have an issue with. (They being the engineers that screwed the whole thing up).
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
Not a grey area, just a misunderstanding. 110.26(C) second sentence says "Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both." So you can start derating at the 90C temperature, but the termination limits you to the 75C ampacity from the table (without derating).
If you like, you can think of it as the conductors being allowed to be up to 90C within the conduit, but limited to 75C in the enclosure. Since the derating for CCCs doesn't apply within the enclosure, no problem.
Cheers, Wayne
Elect117
Senior Member
- Location
- California
- Occupation
- Engineer E.E. P.E.
They are confusing derating conductors in conduit vs the rating of the conductors when it is terminated at the switchgear. Tell them they are wrong.
You are still okay with 400kcmil. You don't derate for lugs or terminations since they don't happen in conduit.
400kcmil 75°C ampacity is 335A * 4 = 1340 which is greater than 1200A which means your terminations are okay.
If your 75°C ampacity for 400kcmil was less than 1200A then your terminations would not be okay.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Is the load more than 50% non-linear? Extremely doubtful that it is so the neutral is not counted as a CCC for derating purposes. Fill is a separate issue.
- Location
- Illinois
- Occupation
- retired electrician
If you don't have to install neutrals, then any neutrals installed would not be current carrying. Remember even where neutrals actually carry current, they often are not considered current carrying conductors for the purposes of ampacity adjustment. See 310.15(E).
Even if you don't run a neutral from the transformer secondary, you would have to install a supply side bonding jumper in each raceway sized per Table 250.102(C)(1) as you need a fault clearing path back to XO at the transformer.
I would have at least 6 CCC (since I am running parallels in the same conduit) if I don't run neutrals and would have to derate at 80% which would give me less than 1200amps. If I am able to use 90C as the derating factor which it sounds like I am then I should be fine.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
Yes use the 90° C ampacity with 6 CCC'S for the derating. Neutrals whether used or not won't be CCC's.
Would you mind elaborating on how the neutrals are not considered CCCs in my case specifically if I were to run them? I think I have been trained to always consider neutrals as a CCC maybe because it's easier than arguing with the engineer but in this case I would love to win an argument lol.
- Location
- New Jersey
- Occupation
- Journeyman Electrician
If the neutral is only carrying the unbalance current then it does not count. This may help.
Check 310.15(B)(5)(a)... I think