Parrallel 4/0 in 2 2-1/2" conduit runs.

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You would have to follow 250.122(F) for parallel conductors. Since the cables are in separate conduits, the EGC would be required to be 3 AWG for a 400 amp breaker.

Not sure if this applies to your situation, but you may want to also check out 250.122(B). Assuming no derating is required, two sets of 4/0 would give you 460 amps and two sets of 3/0 would give you 400 amps per Table 310.15(B)(16). If the minimum ampacity for the intended installation is 400 amps, then two sets of 4/0 would be a size above what is required. The EGC would then need to increase proportionately.
 
MyBeardAndMe said:
You would have to follow 250.122(F) for parallel conductors. Since the cables are in separate conduits, the EGC would be required to be 3 AWG for a 400 amp breaker.
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Can i use single parrallel 4/0 conductors (8 wires and 1-#3AWG egc THHN) in one 3" RMC raceway? Of course i would need a 9" cable tray.
 
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acoron01 said:
MyBeardAndMe said:
You would have to follow 250.122(F) for parallel conductors. Since the cables are in separate conduits, the EGC would be required to be 3 AWG for a 400 amp breaker.
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I knew it, it blows my mind how cable manufactures don't already have this is stock. Thank you,
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How would they be able to to that?...there are hundreds of different combinations of parallel conductor set ups.

There will be some help for this issue in the 2017 code, but only where the complete run is installed in cable tray. It will still be an issue where the run is part cable tray and part conduit like yours is.
 
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acoron01 said:
MyBeardAndMe said:
You would have to follow 250.122(F) for parallel conductors. Since the cables are in separate conduits, the EGC would be required to be 3 AWG for a 400 amp breaker.
Click to expand...

I knew it, it blows my mind how cable manufactures don't already have this is stock. Thank you,
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They probably built that multiconductor cable with a 225A circuit in mind.
 
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Carultch said:
They probably built that multiconductor cable with a 225A circuit in mind.
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So i cant use 600kcmil, because i cannot find a supplier that sells it without a 12-14 week lead time and in bulk of 2000'. The breakers i ordered have lugs with provisions only up to 500 kcmil. Why would a manufacture make a breaker that doesn't protect the cable? So special lugs, long lead time, and in bulk of 2000-5000' minimum. I tried getting away with 4/0 in parallel in 2-2-1/2" RMC but the vendors egc is too small. How is this not a common problem?
 
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That's why I prefer single conductors for parallel circuits. You don't have to special order an EGC in a multiconductor cable.

As long as you aren't feeding a VFD that requires a VFD cable, I would go with tray rated single conductor if possible.
 
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MyBeardAndMe said:
That's why I prefer single conductors for parallel circuits. You don't have to special order an EGC in a multiconductor cable.

As long as you aren't feeding a VFD that requires a VFD cable, I would go with tray rated single conductor if possible.
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So the feeder is for a 300kw, 480v standby diesel with kirk key interlock. The diesel supplies power to our HVAC equipment which has several VFDs' - 4 to be exact. So can i run all of the single conductors in one conduit, i thought there was a rule you cannot have the same phase in the same conduit run?
 
You should be fine with single-conductor. You only have to worry about VFD rated cables from the VFD to the motor. Depending on the VFD, you'd have to look at the manual to see if the manufacturer recommends a VFD rated cable. Anyways...

Reason I say you should be fine is dependent on your cable tray. So I would check out Article 392 if I were you. For example, 392.10(B)(1)(a) states for industrial establishments that the maximum allowable rung spacing in ladder cable tray is 9 inches when installing single-conductor.

As for parallel cables in the same conduit, that is fine as long as you derate appropriately based on the number of current-carrying conductors in the same conduit. See Table 310.15(B)(3)(a). I would route them in separate conduits to avoid upsizing the cable due to derating.
 
acoron01 said:
MyBeardAndMe said:
You would have to follow 250.122(F) for parallel conductors. Since the cables are in separate conduits, the EGC would be required to be 3 AWG for a 400 amp breaker.
Click to expand...

I knew it, it blows my mind how cable manufactures don't already have this is stock. Thank you,
Click to expand...
Stock items go with what the demand is. There are many other cables out there with similar issues if you want to use them in parallel.
 
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MyBeardAndMe said:
You should be fine with single-conductor. You only have to worry about VFD rated cables from the VFD to the motor. Depending on the VFD, you'd have to look at the manual to see if the manufacturer recommends a VFD rated cable. Anyways...

Reason I say you should be fine is dependent on your cable tray. So I would check out Article 392 if I were you. For example, 392.10(B)(1)(a) states for industrial establishments that the maximum allowable rung spacing in ladder cable tray is 9 inches when installing single-conductor.

As for parallel cables in the same conduit, that is fine as long as you derate appropriately based on the number of current-carrying conductors in the same conduit. See Table 310.15(B)(3)(a). I would route them in separate conduits to avoid upsizing the cable due to derating.
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i am in two runs of 2-1/2" RMC, with a #3gnd in each. You caught me on the rung spacing, thanks a million. Just ordered everything.:happyno:
 
acoron01 said:
Can i use single parrallel 4/0 conductors (8 wires and 1-#3AWG egc THHN) in one 3" RMC raceway? Of course i would need a 9" cable tray.
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Maybe. There is no question you can do that if the neutral is not required to be counted as a current carrying conductor. If the neutral is a current carrying conductor you can do it as long as the load is less than 364 amps.
 
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don_resqcapt19 said:
Maybe. There is no question you can do that if the neutral is not required to be counted as a current carrying conductor. If the neutral is a current carrying conductor you can do it as long as the load is less than 364 amps.
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Curious as to where the 364 Amps came from. But we oversized the generator purposely to power the entire building. We will never use 300 amps in this building. However, i would like to know your logic. We are using phase voltages so the neutral will be a current carrying conductor.
 
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acoron01 said:
... We are using phase voltages so the neutral will be a current carrying conductor.
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The fact that you have line to neutral loads means that the neutral will carry current, namely the unbalance of the three line currents. That makes the neutral a CCC. But the code is very clear that even though it is a CCC you do not count it as a CCC for adjustment calculations. The logic is that to get a particular current on the neutral you have to reduce the current in one or two of the hot wires by a corresponding amount, so the total heat dissipation is no greater than it would be for the three hots alone.
 
Read 310.15(B)(5) to see if your neutral counts. On a 3 phase, 4 wire circuit, the neutral only counts if the majority of the loads are nonlinear.

Without the neutral counting you would have 6 current-carrying conductors (2 sets of 3). Per Table 310.15(B)(3)(a), routing 6 current-carrying conductors in one conduit would require you to derate by 80% of the ampacity in Table 310.15(B)(16). If your conductors are only rated at 75C, two sets of 4/0 would give you 460 amps total. Derating by 80% would be 368 amps. That is how Don is getting 368.

If the conductors are rated 90C, you can use the 90C column to derate. 4/0 at 90C gives you 260 amps. Two sets would give you 520 amps. Derated 80% is 416 amps.
 
acoron01 said:
Curious as to where the 364 Amps came from. But we oversized the generator purposely to power the entire building. We will never use 300 amps in this building. However, i would like to know your logic. We are using phase voltages so the neutral will be a current carrying conductor.
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My understanding is that you have a 400 amp overcurrent protective device. The ampacity of 4/0 copper in the 90°C column is 260 amps, after the application of the 70% adjustment factor, the ampacity becomes 182 amps. That gives you a total ampacity for the parallel set of 364 amps. The rule in 240.4(B) lets you use the next standard size OCPD which is 400 amp, but only if the ampacity of the conductors is equal to or exceeds the load current.
 
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