1200 A in (2) 3-1/2" conduits?

[Sparky5150]

I've been asked to upgrade a service from 800 to 1200 amps. It has (2) 3-1/2" conduits routed underground and under a parking lot from the utility transformer. The contractor suggested we try to pull the new 1200 amp conductors int he existing (2) conduits. I've tried but cant seem to find the magic combination including derating the neutral.

Any suggestions?


Thanks!


Sparky

 

[petersonra]

I've been asked to upgrade a service from 800 to 1200 amps. It has (2) 3-1/2" conduits routed underground and under a parking lot from the utility transformer. The contractor suggested we try to pull the new 1200 amp conductors int he existing (2) conduits. I've tried but cant seem to find the magic combination including derating the neutral.

Any suggestions?


Thanks!


Sparky

can you get 20 (1/0) in a 3.5" conduit?

 

[infinity]

You didn't say what type of conduit but if you can fit 8-400 kcmil THWN-2 copper conductors in each 3.5" you will end up with:

380 * 2 = 760 * 80% = 608 amps/conduit
608 * 2 = 1216 amps

Sch40 PVC allows 8-400's

 

[Jljohnson]

You can do it until you factor in that pesky 310.15(B)(3)(a) and start derating the conductors. I spent a little time trying to figure out the puzzle but failed every time after derating the conductors properly.
Good luck,

 

[petersonra]

You didn't say what type of conduit but if you can fit 8-400 kcmil THWN-2 copper conductors in each 3.5" you will end up with:

380 * 2 = 760 * 80% = 608 amps/conduit
608 * 2 = 1216 amps

Sch40 PVC allows 8-400's

400 mcm = 380A? I thought it was 500 mcm = 380A.

 

[masterinbama]

You didn't say what type of conduit but if you can fit 8-400 kcmil THWN-2 copper conductors in each 3.5" you will end up with:

380 * 2 = 760 * 80% = 608 amps/conduit
608 * 2 = 1216 amps

Sch40 PVC allows 8-400's

I can't get my calculator to let me put 8 400's in a 3.5"

 

[masterinbama]

400 mcm = 380A? I thought it was 500 mcm = 380A.

Since you are derating you can use the 90C ampacity for 400's.

 

[petersonra]

how about this?

the neutrals don't count as CCC

(3) 300 MCM per phase and (2) 1/0 for neutral in each conduit.

only (9) CCC so only derate to 70%.

 

[infinity]

I can't get my calculator to let me put 8 400's in a 3.5"

I think that I read the table incorrectly. :ashamed:

 

[Sparky5150]

You didn't say what type of conduit but if you can fit 8-400 kcmil THWN-2 copper conductors in each 3.5" you will end up with:

380 * 2 = 760 * 80% = 608 amps/conduit
608 * 2 = 1216 amps

Sch40 PVC allows 8-400's

Thanks guys for all your efforts in this tricky situation!

I like what you suggested Infinity, but I'm stuck on finding where 8 #400's are permitted in Sch40 PVC.

I'm using the table that permits 3.895 sq in of fill in a 3-1/2" PVC conduit, but 8 #400 = 5.3.

Is there a better place to look, that allows more in a PVC conduit or smaller diameter #400?

Thanks!

 

[Dennis Alwon]

I tried with compact conductors and you can get close with 6 runs of 250 kcm xhhw but we all know that is only good for horseshoes. And you come within .10 sq. in with a bare neutral.

 

[Dennis Alwon]

If the calculation is 3.9525 we would go to 4.0 based on Note 7 so if the actual dimension of 3.895 is for 3.5 in PVC can we use 4 sq. in also. If so that would work

6 runs of 250kcm xhhw compact conductors with a bare neutral. Someone can check my math

 

[pete m.]

I've been asked to upgrade a service from 800 to 1200 amps. It has (2) 3-1/2" conduits routed underground and under a parking lot from the utility transformer. The contractor suggested we try to pull the new 1200 amp conductors int he existing (2) conduits. I've tried but cant seem to find the magic combination including derating the neutral.

Any suggestions?


Thanks!


Sparky

Not that I am a big proponent of this type of install it is allowed...

230.90 Where Required. Each ungrounded service conductor
shall have overload protection.
(A) Ungrounded Conductor. Such protection shall be provided
by an overcurrent device in series with each ungrounded
service conductor that has a rating or setting not higher than
the allowable ampacity of the conductor. A set of fuses shall
be considered all the fuses required to protect all the ungrounded
conductors of a circuit. Single-pole circuit breakers,
grouped in accordance with 230.71(B), shall be considered as
one protective device.

Exception No. 3: Two to six circuit breakers or sets of fuses
shall be permitted as the overcurrent device to provide the
overload protection. The sum of the ratings of the circuit
breakers or fuses shall be permitted to exceed the ampacity of
the service conductors, provided the calculated load does not
exceed the ampacity of the service conductors.

Using the code section above and also assuming you have multiple service disconnects you could size your service entrance conductors to the calculated load which may give you the wiggle room you need.

Pete

 

[Dennis Alwon]

T. 310 16 states 250KCM is good for 290 amps. 290 * 6 pair= 1740 amps. # parallel runs in each conduit so we are dealing with 12 conductors but only 9 CCC. This means 70% de-rating. 1740*.7=1218amps.

3.5" pVC = 3.895

250 kcm compact xhhw conductor= .3421
.3421*9= 3.0789 in sq.

250 kcm bare neutral = .260 in. sq
3* .260= .780

3.0789+.780= 3.8589 in. sq.-- It should work.

You may also be able to drop the neutral down a size or so and use insulation

 

[A-1Sparky]

Although this may be permissible, I'd hate to be the one who has to execute the pull!

 

[Dennis Alwon]

Although this may be permissible, I'd hate to be the one who has to execute the pull!

That is only 40% fill. They make tuggers for that. I am sure it will be a hard pull but he was looking for options. I guess I found one-- I did not offer to pull it. :happyno:

 

[kwired]

Install another raceway, use directional boring to minimize how much parking lot needs demolished if that is what it takes.

 

[tkb]

6-400kcmil and 1-250kcmil XHHW CU per conduit

Derated for 90c 80% for 6CCC, not counting neutral as CCC.
Derating neutral down to a single 250kcmil XHHW CU.

PVC40 3-1/2" fill = 39.64%

This assumes that you can derate your neutral and not count it as a CCC.

 

[Smart $]

6-400kcmil and 1-250kcmil XHHW CU per conduit

Derated for 90c 80% for 6CCC, not counting neutral as CCC.
Derating neutral down to a single 250kcmil XHHW CU.

PVC40 3-1/2" fill = 39.64%

This assumes that you can derate your neutral and not count it as a CCC.

Six 350's comes in just under 1200 at 1165A with ambient correction of 1.04 for 21-25?C applied and 80% for 6 ccc's. Underground ambient unknown for OP'er... however would still require engineering supervision under 310.15(C) for approval. This would permit two 250's for non-ccc neutral and come in just under 40% fill.

 

[nakulak]

as kwired said, I think your time would be better spent figuring out where you can run an extra conduit (or two)