parallel service

[balco]

I recently ran 2 secondary conduits for a service to a new building. I realized that ( after the building was almost done ) I was given 1200 Amp. switch gear . The two sets of 600 mcm would not do the job. I am now told to split up another set and pull two more conductors through each conduit. Derating aside is this legal to do? Yes or no where does it specify in the NEC?

 

[infinity]

Could you be more specific? What size conduit? How long are the runs? Wire type?

 

[iwire]

Do I have this right?

You have a 3 phase 4 wire service fed with two sets of 600 kcmil each in their own raceway.

Each raceway has A, B, C and N in it.

That gives a capacity of 840 amps.

Now you being told to;

to split up another set and pull two more conductors through each conduit

as in, add A, B to one raceway and C, N to the other raceway.

If I have all of that correct than what you are being told to do is a violation of 300.3(B) and 300.3(B)(1).

It is also a very bad idea.

 

[ramsy]

Nice call Bob. As required, adding another parrellel set in it's own conduit, with all conductors together, may still be cheeper than two 1500 MCM sets. Why panic if you can use the same trench, with another run of 600 in PVC. Call their supplier and get the different price points to CYA.

 

[don_resqcapt19]

Balco,
Assuming that this is a service, if there is more than one service disconenct the wire only has to be sized for the calculated load, and not the total of the service OCPDs.
If you need the full 1200 amps, assuming 4" raceways and that the neutral is not a current carrying conductor, 8 250 kcmill THWN in each raceway will do the job.
Don

Note the wire size and type should be 400 kcmil THWN-2.

 

[Pierre C Belarge]

Don
Would you mind showing the math for your last post?

 

[don_resqcapt19]

Pierre,
I was working on a number of possibilities and posted the wrong size wire. It should say 400kcmil THWN-2 with a 90?C ampacity of 380 amps. Thanks for catching that. Three sets of 250 kcmil in each raceway will also work if the nuetral is not a current carrying conductor.
Don

 

[Pierre C Belarge]

Don
I was not trying to catch you. I know you are very intelligent and have great code skills, maybe you knew something I needed to learn.

 

[ramsy]

If you need the full 1200 amps, assuming 4" raceways (more wire) ..in each (x2) raceway will do the job.

Don, if "600 mcm would not do the job", because this 1200A service will be 1200A per phase / 2 pipes, I get 1500cu@75? or 600cu@75?C in 3 pipes. I also figure, derating won't permit more conduit fill between 2 pipes in cu or al, a 3rd conduit is needed for mcm below 1500cu, and excluding the neutral here didn't help, regardless of 315(B)4.

____ Target__Voltage ________ Table Data Or Field Measurement
1) I  = P/E              1200 Get load Amps. or Clamp meter
2) Z  = TotFt/1k*Ohm  0.00858 NEC Tbl.8/9 or Meter Z=(E-Ec)/I
3) Ec = (Nominal-Z*I)     470 Get choked Volts
4) Pc = Ec*I          563,645 Get choked power
5) Ic = Pc/E             1174 Get choked Amps
____ Check__Wire__Size ______
6) CM   = KIL/VD      1503497 [email]Kcu@75?C=12.9,L=1000.Ft[/email],VD=E-Ec 
7) De   = Ic/@Max?C      ?2 = 587/705=0.83 Max Derating @90?C
8) PF   = Ec/E  -No Motors-   Tbl.430-150 See Motor.or Metered
9) ASCC = E/Z           55944 Avail.Short.Ckt.Cur.

If 5% xfrmr (FCBN) taps raise Target Voltage output, 3-pipes work with 900mcm (AL), but less efficiently at 1000.ft.Tot.

____ Target__Voltage ________ Table Data Or Field Measurement
1) I  = P/E              1200 Get load Amps. or Clamp meter
2) Z  = TotFt/1k*Ohm   0.0235 NEC Tbl.8/9 or Meter Z=(E-Ec)/I
3) Ec = (Nominal-Z*I)     452 Get choked Volts (480 w/taps)
4) Pc = Ec*I          542,160 Get choked power
5) Ic = Pc/E             1130 Get choked Amps
____ Check__Wire__Size ______
6) CM   = KIL/VD       899895 [email]Kal@75?C=21.1,L=1000.Ft[/email],VD=E-Ec 
7) De   = Ic/@Max?C      ?3 = 377/480=0.78 Max Derating @90?C
8) PF   = Ec/E  -No Motors-   Tbl.430-150 See Motor.or Metered
9) ASCC = E/Z           20426 Avail.Short.Ckt.Cur.

What am I doing wrong here?

 

[ramsy]

Any ideas on improving this template?

____ Target__Voltage ________ Table Data Or Field Measurement
1) I  = P/E          =        Get load Amps. or Clamp meter
2) Z  = TotFt/1k*Ohm =        NEC Tbl.8/9 or Meter Z=(E-Ec)/I
3) Ec = (Nominal-Z*I)=        Get choked Volts
4) Pc = Ec*I         =        Get choked power
5) Ic = Pc/E         =        Get choked Amps
____ Check__Wire__Size ______
6) CM   = KIL/VD     =        [email]Kcu@75?C=12.9,L=xxxx.Ft[/email],VD=E-Ec 
7) De   = Ic/@Max?C  =        xxx/xxx=x.xx Max Derating @90?C
8) PF   = Ec/E       =        Tbl.430-150 See Motor.or Metered
9) ASCC = E/Z        =        Avail.Short.Ckt.Cur.

Circular Mills (6) arn't accurate with larger changes in power factor.

 

[don_resqcapt19]

Roger,
I have no idea what all of that is!! A 400 kcmil copper has a 90?C ampacity of 380 amps. If I put two sets in a raceway and the neutral is not a current carrying conductor, I have an 80% derating factor. 80% of 380 is 304 amps. That would give us 608 amps in each raceway and 1216 amps for the total circuit.

Don

 

[iwire]

Circular Mills (6) arn't accurate with larger changes in power factor.

What has that got to do with the question at hand?

Why must we make it complicated?

Don seems to have it nailed down with some simple math.

I would never attempt to use 1500 Kcmil for this as it would be difficult to find a 1200 amp breaker that would accept two 1500 Kcmils

 

[ramsy]

Nevermind, found several problems with my template. The formula's ignored 3-phase systems, and data came from the wrong tables. The question at hand provided a good excersise in getting the math and the big picture all in one place, but ultimately it had to go in a spreadsheet and won't fit in my hip pocket anymore.