Number of conductors in cable tray

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skyline77

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Hi,
I am designing the electrical supply to an electric steam boiler of 1250 Kw, 380 Volts, 3 Phase, 60 Hz system

So,
FLA = 1899.23
The maximum size we use here usually is 600 MCM, thus, per Table 310.15(B)(16)
number of conductors per phase required is
1899.23/475 = 4

Since there are more than three current carrying conductors in a cable raceway or cable, in this case it's 4 X 3 = 12

per NEC Table 310.15(B)(3)(a), the derating factor would be 50%, so,
Ampacity = 1899.23/0.5 = 3798.46

Applying ambient temperature correction factor (considering ambient temperature of steam boiler room to be 50 C)
3798.46/0.82 = 4632

4632/475 =9.75

Per TABLE 250.122
Ground required would be 2 X 350 MCM
Thus,
the conductors required would be 10 of 600 MCM per phase. Or 40 conductors 600 MCM plus 2 X 350 MCM :jawdrop:

Do you agree with this?
 
Nope, don't agree.

If you select the right width of your tray, assuming ladder type open top, the cables will be installed triplexed together, then maintain a spacing as described in 392.80(A)(2)(d), the ampacity is per 310.15(B)(20).

Per 310.15(B)(2)(b) the temperature adjustment is 0.89 on a 90 deg C rated cable to go from 40 to 50 deg C ambient.

600 KCMIL is good 647A @ 40 deg C, which de-rates to 575A, but that is greater than the 553A at 75 deg column, so the value you have to work with is 553A @ 50 deg C.

This means 4 x 600 KCMIL // conductors per phase would work just fine, and actually I think you can get away with 4 x 500KCMIL // conductors but you need to do math.
 
kingpb said:
Nope, don't agree.

If you select the right width of your tray, assuming ladder type open top, the cables will be installed triplexed together, then maintain a spacing as described in 392.80(A)(2)(d), the ampacity is per 310.15(B)(20).

Per 310.15(B)(2)(b) the temperature adjustment is 0.89 on a 90 deg C rated cable to go from 40 to 50 deg C ambient.

600 KCMIL is good 647A @ 40 deg C, which de-rates to 575A, but that is greater than the 553A at 75 deg column, so the value you have to work with is 553A @ 50 deg C.

This means 4 x 600 KCMIL // conductors per phase would work just fine, and actually I think you can get away with 4 x 500KCMIL // conductors but you need to do math.
Click to expand...

I support you messenger.
 
As usually ,I am late. I agree with kingpb, of course.
I think according to NEC Art. 392.17 d) if distance between groups is 2.15*[o/d one cable] then the cable ampacity will be according to 310.15(B).20[it does not referring to the title note " Not More Than Three Single Insulated Conductors "] .
In this case 600 mcm 75 dgr.C insulation will be 553 A. Multiplying by derating factor of 0.85 from table 310.15.(B).2 for 50 dgr.C ambient will be 470 A.
5 parallel cables [5 parallel groups ] in 2 ft. width open top cable tray will be enough.
This is the maximum permissible current according to DIN VDE 0276-1000 and 0276-603 for PVC cable [70 dgr.C insulation]-one lay of groups in a cable rack. However, there are two remarks:
1) the cable rack is a support structure in which the supporting area is no more than 10% of total area.
2) the clearance required is only 2*d[instead of 2.15 as per NEC].
3) The derating factor is 0.71 for 50 dgr.C ambient [from 30 dgr.C rated]
So the ampacity will be only 557*.71=395.5 A
Then take NEC it is better!:D
 
skyline77 said:
Hi,
I am designing the electrical supply to an electric steam boiler of 1250 Kw, 380 Volts, 3 Phase, 60 Hz system

So,
FLA = 1899.23
Click to expand...

I understand how you calculated the 1899A from the 1250kW.
This infers that the power factor is unity. For the application, it may well be.
If you don't know with certainty that this is the case, it would be worth checking.
 
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