EQUIPMENT GROUND CONDUCTORS IN PARALLEL

Question

EQUIPMENT GROUND CONDUCTORS IN PARALLEL

ALEVIN (Electrical)

(OP)

4 Apr 01 11:06

THE NATIONAL ELECTRICAL CODE, ARTICLE 250, GROUNDING, PARAGRAPH: 250-122(f),(1): STATES " EACH PARALLEL EQUIPMENT CONDUCTOR SHALL BE SIZED ON THE BASIS OF THE AMPERE RATING OF THE OVERCURRENT DEVICE PROTECTING THE CIRCUIT CONDUCTORS IN THE RACEWAY OR CABLE IN ACCORDANCE WITH TABLE 250-122.

THE APPLICATION OF THIS CRITERIA IS TO PREVENT OVERLOADING AND POSSIBLE BURN-OUT OF THE GROUNDING CONDUCTOR SHOULD A GROUND FAULT OCCURS ALONG ONE OF THE PARALLEL BRANCHES.

HOWEVER, CONSIDERING ALL PARALLEL BRANCHES WILL CONTAIN THE SAME LARGEST SIZED GROUND CONDUCTOR SEEMS A BIT EXCESSIVE WITH REGARDS TO THE TOTAL GROUNDING "CSA" OF THE REST OF THE CONDUCTORS INSTALLED WITHIN THE PARALLEL SETS OF CONDUCTORS.

I WOULD LIKE TO KNOW WHY THE LARGEST GROUND CONDUCTOR SIZE MENTIONED IN TABLE 250-122 "MINIMUM SIZE EQUIPMENT GROUNDING CONDUCTORS FOR GROUNDING RACEWAY AND EQUIPMENT" CAN'T BE PROPORTIONATELY REDUCED IN SIZE SO THAT THE SUM OF ALL THE INDIVIDUAL CONDUCTORS CONTAINED WITHIN THE PARALLEL SET OF FEEDERS WILL EQUAL OR EXCEED THE MINIMUM CONDUCTOR SIZE MENTIONED IN THIS TABLE.

REDDOG (Electrical) · Answer 1 · 2001-04-04 13:59:34

The ground return path must be of sufficiently low impedance to faciliate the operation of the overcurrent devices 250-1-d. The size of this conductor is based upon the size of said device. The load side conductors in parallel is based upon the ampacity of the circuit, and the environment temperature. For example parallell 600 mcm rated at 420 amps 75 degree C these are connected to a 800 amp OCD. The 1/0 grounding conductor is rated at 150 amps.
Short time withstand rating 28000 amps for 4 cycles. The return path would be 2 1/0 , the impedance would depend upon the location of the ground fault. The idea is to escalate the current as quick as possiable so the OCD will clear the fault. Also recommended for parallel feeder are cable limiters to disconnect the damage cable, and allow operation under limited conditions.

jbartos (Electrical) · Answer 2 · 2001-04-06 19:08:52

Artsand88, please, would you clarify the origin of your paragraph:
HOWEVER, CONSIDERING ALL PARALLEL BRANCHES WILL CONTAIN THE
SAME LARGEST SIZED GROUND CONDUCTOR SEEMS A BIT EXCESSIVE
WITH REGARDS TO THE TOTAL GROUNDING "CSA" OF THE REST OF
THE CONDUCTORS INSTALLED WITHIN THE PARALLEL SETS OF
CONDUCTORS.
Namely, why would all parallel branches contained the same largest sized ground conductor?

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EQUIPMENT GROUND CONDUCTORS IN PARALLEL