lpalmer
Member
- Location
- California
I am referencing the 2005 NEC, as this is what California is still using.
When you look at the requirements for equipment grounding for example; a 400 Amp paralleled feeder consisting of 3 runs of 4 ea. 1/0 AWG Al. conductors, 2005 NEC Section 250.122(F)(1) requires the equipment grounding conductor to be full size; #3 AWG for each raceway. Where there isn't overcurrent protection, such as in the case of transformer secondary conductors, and where interpreting the size of the equipment bonding jumper is based on the size of the secondary conductors of each raceway, the equipment bonding jumper size from Table 250.66 would be #8 AWG, which is three sizes smaller than the required equipment grounding conductor size.
Given the separately derived system equipment bonding jumper carries ground fault current without secondary overcurrent protection and 2005 NEC Table 250.66 heading states "Size of Largest Ungrounded Service-Entrance Conductor or Equivalent Area for Parallel Conductors"; is the substantiation used for determining the size of separately derived system equipment bonding jumpers. Therefore the size of the equipment bonding jumper is based on the equivalent area for the parallel conductors: 1/0 AWG = 105,600 circular mils x 3 runs = 316,800 circular mils. Referring to Table 250.66 based on 350 kcmil Al. the equipment bonding jumper size is #2 AWG for each raceway, which is one size larger than the equipment grounding conductor required for a 400 amp feeder.
Where 250.30(A)(2) Equipment Bonding Jumper Size is based on Section 250.102(C) on the size of the derived phase conductors. And 250.102(C) refers the size to Table 250.66. The last two sentences in Section 250.102(C) SHOULD NOT be used to determine the size of a separately derived system equipment bonding jumper, where this part of the section is referring to service entrance conductors which are not applicable to transformer separately derived system phase conductors (transformer secondary taps). Service entrance conductors typically have a grounded conductor (neutral) that are grounded both at the supply and the first service disconnecting means. The service entrance grounded conductor carries ground fault current back to the source, whereas the grounded conductor for a separately derived system is typically grounded at the source (transformer) and not grounded at the first disconnecting means. The equipment bonding jumper for a separately derived system carries ground fault current back to the source (transformer). Therefore the separately derived system equipment bonding jumper is required to be sized based on the equivalent area of parallel conductors and required to be full size for each raceway.
Any thoughts on this?
When you look at the requirements for equipment grounding for example; a 400 Amp paralleled feeder consisting of 3 runs of 4 ea. 1/0 AWG Al. conductors, 2005 NEC Section 250.122(F)(1) requires the equipment grounding conductor to be full size; #3 AWG for each raceway. Where there isn't overcurrent protection, such as in the case of transformer secondary conductors, and where interpreting the size of the equipment bonding jumper is based on the size of the secondary conductors of each raceway, the equipment bonding jumper size from Table 250.66 would be #8 AWG, which is three sizes smaller than the required equipment grounding conductor size.
Given the separately derived system equipment bonding jumper carries ground fault current without secondary overcurrent protection and 2005 NEC Table 250.66 heading states "Size of Largest Ungrounded Service-Entrance Conductor or Equivalent Area for Parallel Conductors"; is the substantiation used for determining the size of separately derived system equipment bonding jumpers. Therefore the size of the equipment bonding jumper is based on the equivalent area for the parallel conductors: 1/0 AWG = 105,600 circular mils x 3 runs = 316,800 circular mils. Referring to Table 250.66 based on 350 kcmil Al. the equipment bonding jumper size is #2 AWG for each raceway, which is one size larger than the equipment grounding conductor required for a 400 amp feeder.
Where 250.30(A)(2) Equipment Bonding Jumper Size is based on Section 250.102(C) on the size of the derived phase conductors. And 250.102(C) refers the size to Table 250.66. The last two sentences in Section 250.102(C) SHOULD NOT be used to determine the size of a separately derived system equipment bonding jumper, where this part of the section is referring to service entrance conductors which are not applicable to transformer separately derived system phase conductors (transformer secondary taps). Service entrance conductors typically have a grounded conductor (neutral) that are grounded both at the supply and the first service disconnecting means. The service entrance grounded conductor carries ground fault current back to the source, whereas the grounded conductor for a separately derived system is typically grounded at the source (transformer) and not grounded at the first disconnecting means. The equipment bonding jumper for a separately derived system carries ground fault current back to the source (transformer). Therefore the separately derived system equipment bonding jumper is required to be sized based on the equivalent area of parallel conductors and required to be full size for each raceway.
Any thoughts on this?
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