Conductor Calculations to New Switchgear

[ShockedOneinAZsun]

Gentlemen-I have to run conduit and conductor for 300' overhead (above grid ceiling) from the existing SES to the new MDP-03 (switchgear). I would like to know if you would parallel the load (800A, 3 phase, 4-wire, plus grounding)and what size conduit (EMT) you would use.

 

[jryan]

Re: Conductor Calculations to New Switchgear

(3)paralleled sets of 3 - 300 Kcmil (285A) , 1/0 GND, 2" EMT

If I understand correctly....

Edit:

what type wire? THHN assumed...

[ June 17, 2004, 02:18 PM: Message edited by: jryan ]

 

[ShockedOneinAZsun]

Re: Conductor Calculations to New Switchgear

Thanks, Jryan, THHN insulation sounds like the way to go. You have been quite helpful.

 

[don_resqcapt19]

Re: Conductor Calculations to New Switchgear

jryan,
Three 300 kcmil and a 1/0 exceeds the permitted fill for 2" EMT. Also the original post specified a 4 wire cicuit.

shocked,
What is the load? If the major portion is nonlinear, then the grounded conductor is a current carrying conductor and the 80% adjustment factor fir more than 3 current carrying conductors will apply.
Don

 

[jryan]

Re: Conductor Calculations to New Switchgear

He is right, I did not account for the 4-wire system, use 2.5".

2.343 @ 40% Total area EMT 2.5"
3.538 @ 40% Total area EMT 3.0"

4-300's = 0.4608*4 = 1.8432 sq in
1- 1/0 = 0.1855 sq in

Total = 2.0287 sq in Use 2.5" unless you have to pull it then use 3.0"

[ June 17, 2004, 03:12 PM: Message edited by: jryan ]

 

[ShockedOneinAZsun]

Re: Conductor Calculations to New Switchgear

The load on this 800A, 3-phase, 4-wire switchgear is 68,164 watts lighting, and 256,320 watts of power that is to be installed new. In addition, there will be a 200a/3p breaker for an existing A/C, 70a/3p breaker for another 5 ton A/C, and 3-20a/3p breakers for existing air handlers and evaperative coolers.
Also, with an a 300' run, wouldn't I have to put a pull box somewhere in the center?

[ June 17, 2004, 05:37 PM: Message edited by: ShockedOneinAZsun ]

 

[bob]

Re: Conductor Calculations to New Switchgear

What kind of lighting are you installing? How many 90 degree bends between the SES and the MDP?

 

[ShockedOneinAZsun]

Re: Conductor Calculations to New Switchgear

Bob-they are 120volt, 4-lamp, double-switched fluorescents, for the most part. Some metal halides on the perimeters.

There will only be 3-90 degree bends, but I am concerned about the pull box due to the great distance.

[ June 18, 2004, 10:10 AM: Message edited by: ShockedOneinAZsun ]

 

[don_resqcapt19]

Re: Conductor Calculations to New Switchgear

Shocked,
With three 90s, properly installed and supported raceways, and power pulling equipment 300' should not be a problem.
Don

 

[bob]

Re: Conductor Calculations to New Switchgear

I asked about the lighting because if you have harmonics involved your neurtal conductor may need to be counted as a current carrier and would require you to derate the circuits. The problem with the 90's is not the ability to pull the wire but the possibility of damaging the insulation while pulling. You need to make the pulling caculations to determine the requirements.

 

[don_resqcapt19]

Re: Conductor Calculations to New Switchgear

Bob,
You are correct that you should do the pulling calculations to be sure that the cable won't be damaged. It is likely for this installation that the maximum permitted sidewall pressure for the cable would be exceed if any of the 90s are near the pulling end. Thanks for pointing that out.
Don

 

[riley]

Re: Conductor Calculations to New Switchgear

Hi Shocked,
I may have over looked the applied voltage relating to your post. The following is based on 480v, 3ph, 4w, serving a 800 a SWB, wire length of 300ft.
VD allowed = 480v x 3% = 14.4
VD using 300kcmil
1.732 x1 2.9 x 300 x 800/14.4 = 17.87
VD using 350kcmil = 15.3
VD using 400kcmil = 13.4
Select 400 kcmill @ 75c = 335a x 3 runs = 1005a.
If derating is required to major % of load
being nonlinear 335a x 80% = 268a x 3 runs =
804a
3runs 3" EMT, 3- #400kcmil @ 1- 1/0 in ea.

 

[don_resqcapt19]

Re: Conductor Calculations to New Switchgear

Riley,

Select 400 kcmill @ 75c = 335a x 3 runs = 1005a

If you do this, then 250.122(B) will require a 2/0 EGC in each raceway.
Don

 

[riley]

Re: Conductor Calculations to New Switchgear

Don,
Thank you for correcting my mistake.

 

[bajan]

Re: Conductor Calculations to New Switchgear

with the length of your run and the 800A of your load the difficulty of your pull has to be taken into account here. Personnally I would use 4" and parallel the cables. The cable size would be 95mm sq . Do you intend to use armored cable or single. I always prefer Armored as it can be run in ceilings without being placed in conduit.
N.B I am from Barbados and our systems maybe different. Apart from NEC we are also gov. by IEE