Understanding Parallel Cable Run Impact on Calculations

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TacoEngineer

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New member here with a question on Parallel Cable Runs

Let's say we have a 480VAC, 3-Phase, 15HP Motor, 550ft Cable Length, 75C Cable Rating, P.F. of 0.85, Copper Cable, and Temp Correction Factor of 1.
Using this information, and NEC430.250 I am seeing an FLA of 21[A].
Multiplying the FLA * 1.25 and using TableNEC240.6 I see a recommended upstream breaker size of 30A.
For Cable size based on Ampacity: Multiplying (UpstreamBreakerSize * TempCorrectionFactor) = (30A * 1) = 30 and using TableNEC310.16 i see a cable size of 10AWG
For Cable size based on 3%VoltageDrop: ( Sqrt(3) * 12.9 * CableLength * FLA) / CopperCM ) It appears 6AWG gives me 2.05%
Chosen cable size is the larger of the two above, so 6AWG.
Sorry for the long setup.

My question is a 2-parter:

1. Would having two parallel runs (same length and cable size per parallel run) change the formula for Cable Sizes as shown below?
New Cable Size Based on Ampacity: (UpstreamBreakerSize * TempCorrectionFactor) / NumberofParallelRuns and use TableNEC310.16 to get 14AWG per run
New Cable Size Based on 3%VD: ( Sqrt(3) * 12.9 * CableLength * FLA / NumberofParallelRuns) / CopperCM ) to get 10AWG at VD 2.59%

2. How would this affect upstream breaker size? Normally the cable needs to be sized to handle the full breaker ampacity. I would think that lowering the gauge of the cable would break this rule.
 
TacoEngineer said:
My question is a 2-parter:

1. Would having two parallel runs (same length and cable size per parallel run) change the formula for Cable Sizes as shown below?
New Cable Size Based on Ampacity: (UpstreamBreakerSize * TempCorrectionFactor) / NumberofParallelRuns and use TableNEC310.16 to get 14AWG per run
New Cable Size Based on 3%VD: ( Sqrt(3) * 12.9 * CableLength * FLA / NumberofParallelRuns) / CopperCM ) to get 10AWG at VD 2.59%
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Yes...but parallel sets of #10awg are not allowed by the NEC.
TacoEngineer said:
2. How would this affect upstream breaker size? Normally the cable needs to be sized to handle the full breaker ampacity. I would think that lowering the gauge of the cable would break this rule.
Click to expand...
It wouldn't. Two sets of #10 have an ampacity of 70. But for a motor the ampacity of the conductor does not need to match the breaker size.

If voltage drop was not a concern, the NEC would allow #10awg conductors on a 60A c/b for the 15hp, 460V motor that you described.
 
The conductors ares sized to 125% of the motor FLC from T430.250. The OCPD's for inverse time breakers can be up to 250% of the motor ampacity and is not based on the conductor size. Parallel conductors in general do not begin until the conductors are size #1/0 and larger.
 
infinity said:
The conductors ares sized to 125% of the motor FLC from T430.250. The OCPD's for inverse time breakers can be up to 250% of the motor ampacity and is not based on the conductor size. Parallel conductors in general do not begin until the conductors are size #1/0 and larger.
Click to expand...
Understood on the parallel run size requirement, no issues there.
If we were to size the conductor based on 1.25% of FLC from NEC430.250, how would you size the upstream breaker? Could i use the same rule?
 
TacoEngineer said:
Understood on the parallel run size requirement, no issues there.
If we were to size the conductor based on 1.25% of FLC from NEC430.250, how would you size the upstream breaker? Could i use the same rule?
Click to expand...
The OCPD needs to be large enough to allow the motor to start. 125% may or may not be larger enough. I'm unsure if engineers use a rule of thumb or there is some hard percentage that they use. Maybe someone will chime in. From the NEC's perspective you could use 250% of the motor current.
 
TacoEngineer said:
Understood on the parallel run size requirement, no issues there.
If we were to size the conductor based on 1.25% of FLC from NEC430.250, how would you size the upstream breaker? Could i use the same rule?
Click to expand...
The breaker is not the overload protection for the motor branch-circuit cable. The starter overloads do that job. That is why the breaker is permitted (but not required) to be up to 250% of the FLA. It is required to be able to hold the starting current and 125% of FLA may not. YMMV.
 
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