Multiconductor Cable Ampacity

[petersonra]

The "well" is about 8' x 8' x 18'. The pump is in the sump, but the motor is 45' above the pump, and a very long shaft connects them.

575V motors are not real hard to get. my guess is that given the nature of the project it is likely they will get a spare motor anyway.

 

[Julius Right]

In my opinion, 3*#4 [compact stranded conductor,0.64 sq.in] is the maximum power cable
allowed to pass through 1-1/4" RMC.
If the conduit is embedded under 3 ft in concrete and 90 RHO soil 3*#4 copper conductor could withstand 103 A as per IEEE 835/1994 IEEE 835/1994- 0.6 to 5 kV Unshielded Single Conductor Extruded Dielectric Cable in Underground Duct Bank - Triplexed - Single Circuit
25ºC Earth Ambient 90oC insulation rated table.
Let's say the pump works 1 hr and 0.5 hr pause.Then Iav=sqrt(1*124^2/1.5)=101.25 A
Another solution: a parallel cable of 3*#4 copper along the same path.
It is not a beautiful solution but it could work for a while.

 

[augie47]

In my opinion, 3*#4 [compact stranded conductor,0.64 sq.in] is the maximum power cable
allowed to pass through 1-1/4" RMC.
If the conduit is embedded under 3 ft in concrete and 90 RHO soil 3*#4 copper conductor could withstand 103 A as per IEEE 835/1994 IEEE 835/1994- 0.6 to 5 kV Unshielded Single Conductor Extruded Dielectric Cable in Underground Duct Bank - Triplexed - Single Circuit
25ºC Earth Ambient 90oC insulation rated table.
Let's say the pump works 1 hr and 0.5 hr pause.Then Iav=sqrt(1*124^2/1.5)=101.25 A
Another solution: a parallel cable of 3*#4 copper along the same path.
It is not a beautiful solution but it could work for a while.

Not only would it not be beautiful, it would not be legal (can't parallel less than 1/0)
'

 

[Julius Right]

I agree with you, augie47.A better solution, if there is a free path, to cancel the 3*#4 cable and run along the same path a suitable cable for ampacity and for voltage drop reason.:happyyes:

 

[Sahib]

In my opinion, 3*#4 [compact stranded conductor,0.64 sq.in] is the maximum power cable
allowed to pass through 1-1/4" RMC.
If the conduit is embedded under 3 ft in concrete and 90 RHO soil 3*#4 copper conductor could withstand 103 A as per IEEE 835/1994 IEEE 835/1994- 0.6 to 5 kV Unshielded Single Conductor Extruded Dielectric Cable in Underground Duct Bank - Triplexed - Single Circuit
25ºC Earth Ambient 90oC insulation rated table.
Let's say the pump works 1 hr and 0.5 hr pause.Then Iav=sqrt(1*124^2/1.5)=101.25 A

Thanks, JR. As for any unequal spacing in the triplex cable, any voltage unbalance at the 100 hp pump motor?

 

[Julius Right]

As far as I know, triplex means that single-core cables are tied together so the center-to-center distance is the same. I think you intend to say separately laid single-core cables inegal distanced. The voltage drop depends on resistance and reactance -differences it could be if the phase length is different.The center line distance difference does not produce phase voltage drop difference.
X=2.π.f(0.1404log10(S/r)+0.0153)x10^(-3) ohm/1000'
Equivalent spacing S=(A*B*C)^(1/3) it is the common equivalent S for any of the phases.

 

[winnie]

Using compact stranded copper conductors, the minimum EGC for the available circuit size, and 600V, what is the maximum power that could be delivered in a 1-1/4 rigid conduit?

I think you get 3x 1/0 AWG + 1x 6AWG

This gives 150A of ampacity, and continuous loading of 120A. 125 kW

If you use larger gauge tails and 90C splices (to get the 90C ampacity of the wire in the conduit) and use 100% rated circuit protection, then you get 170A and 177kW capacity.

There has to be some way to work the requirements to operate a 75kW motor into conductors with more than double that capacity!

-Jon

 

[kwired]

Using compact stranded copper conductors, the minimum EGC for the available circuit size, and 600V, what is the maximum power that could be delivered in a 1-1/4 rigid conduit?

I think you get 3x 1/0 AWG + 1x 6AWG

This gives 150A of ampacity, and continuous loading of 120A. 125 kW

If you use larger gauge tails and 90C splices (to get the 90C ampacity of the wire in the conduit) and use 100% rated circuit protection, then you get 170A and 177kW capacity.

There has to be some way to work the requirements to operate a 75kW motor into conductors with more than double that capacity!

-Jon

Use the raceway as EGC and you can get 3 -2/0 compact THHN in 1.25" RMC.

 

[Julius Right]

If you intend to use XHHW cables-the insulation thickness it is only 55 mils- then you are right.
3*1/0 it could pass through. 3 conductor cable it is not a NEC cable but a UL 44 and it is provided with insulation of 55 mils and overall jacket of 80 mils. I am not sure XHHW may be use for pump supply.
However 3*#2 conductor cable-it seems to me- may run through 1-1/4 RMC.
For this cable IEEE 835/1994 allows 136 A[90oC insulation,90 RHO,25oC Earth, copper conductor,100% L.F.]