Table250.66 parallel AREA

[dnem]

For a 500a main supplied with 1250kcmil Al, the building steel and watermain electrode conductor size from Table250.66 is 3/0 Cu.

For a 500a main supplied with parallel 300kcmil Al, the building steel and watermain electrode conductor size from Table250.66 is sized according to the instructions at the column heading which says, “Size of Largest Ungrounded Service-Entrance Conductor or Equivalent Area for Parallel Conductors”. The equivalent area of a double 300 is 600kcmil so the electrode conductor is 1/0 Cu.

The equivalent ampacity of a double 300 would be 1250kcmil which requires the 3/0 Cu.

Why would Table310.16 and Table250.122 and so many other parts of the code be based on ampacity and parallel conductors in Table250.66 be based on area ?

David

 

[bphgravity]

310.16 and 250.122 are based on system operations internal to the premise wiring. These systems must be sized to carry the normal operating and fault currents subjected to those conductors without exceeding the insulation rating or withstand rating of the conductors.

The grounding electrode system is a different animal. It is subject to surge currents outside of the system. The ability of the system to inject these surge currents more reflects how much capacity in physical size the system can be subject to.

The mere fact that 1250 kcmil is likely to be able to carry more surge current than paralleled 300 kcmil conductors mandates the increase in GEC size even though both arrangements can carry the same total normal current without exceeding any one conductor insulation rating.

At least, this is what the grounding experts seem to indicate. To be honest, I don't think anyone alive today can clearly identify why the GEC needs to be sized per 250.66... I intend to have this table removed from the NEC once I can prove it has no basis. My initial try at this was rejected for the 2008 NEC.

 

[dnem]

At least, this is what the grounding experts seem to indicate. To be honest, I don't think anyone alive today can clearly identify why the GEC needs to be sized per 250.66... I intend to have this table removed from the NEC once I can prove it has no basis. My initial try at this was rejected for the 2008 NEC.

So if the table is removed, what would you propose be used to size the GEC ?

 

[bphgravity]

I propose the size of the GEC shall not be required to be larger than #2 awg cu. or 1/0 al. and any bonding jumpers shall not be required to be larger than #6 awg cu or #4 al.

I base this on the minimum requirements for Class 1 materials used on structures for the main down conductors of a lightning protection system.

A #2 cu GEC on ANY service would be sufficient to carry the surge current from lightning to ANY grounding electrode of ANY earth resistance. Bonding of other metallic parts and systems with a #6 cu is sufficient in preventing the rise of voltage on any non-current carrying metal parts.

This would satisfy the minimum requirements of the NEC, namely 250.4(A)(1). Anything above this minimum, like Table 250.66 is mandating design in my opinion.

 

[dnem]

Building steel and watermain electrode conductor size from Table250.66 for a 500a main supplied with parallel 300kcmil Al is equivalent area of 600 which is 1/0 Cu.

Building steel and watermain electrode conductor size from Table250.66 for a 500a main supplied with 1250kcmil Al is 3/0 Cu.

The mere fact that 1250 kcmil is likely to be able to carry more surge current than paralleled 300 kcmil conductors mandates the increase in GEC size even though both arrangements can carry the same total normal current without exceeding any one conductor insulation rating.

OK
How about ?

Building steel and watermain electrode conductor size from Table250.66 for a 500a main supplied with 1250kcmil Al is 3/0 Cu.

Ground rod electrode conductor size from 250.66(A) for a 500a main supplied with 1250kcmil Al is 6 Cu.

David

 

[bphgravity]

That's my point. The table is useless and does not accurately provide minimum requirements to meet the scope of the code. By requiring a larger grounding electrode conductor based only on the type of grounding electrode is design.