Grounding electrode/ Grounding conductor sizing

Spencer125

Member
Location
New Hampshire
can anyone really dumb down the grounding of services and transformers. im wondering when i use which table, i remember one is for service and one is for equipment ground but can you dumb it down? when pulling wires to feed a new panel which table? when pulling wires to a meter socket or to feed a meter socket which panel? when do you change to the equipment table? after a breaker? but what if there is a main breaker? how do you size the primary and secondary ground for a transformer and the bonding jumper and the wire for building steel? i know its a very broad and extensive question but i could use some help understanding this. thanks alot
 

infinity

Moderator
Staff member
Location
New Jersey
Occupation
Journeyman Electrician
T250.66 is for service main bonding jumpers and grounding electrode conductors (GEC).
T250.122 is for EGC's which are sized based on the OCPD ahead of the branch circuit or feeder.
T250.102(C) is for transformer system bonding jumpers (SBJ), supply side bonding jumpers (SSBJ)and GEC's.

Take a look at the heading and notes for each table.
 

augie47

Moderator
Staff member
Location
Tennessee
A somewhat simple rule I tell guys to follow is to ask yourself if there is an overcurrent device directly protecting the associated phase wires. If the wires are protected by a fuse or breaker, then 250.122 is used based on that fuse/breaker size. If there is no overcurrent protection, such as the secondary of a transformer or service wires from a meter socket, then 250.66 is your guide.

Not 100% fool-proof but pretty close.
 

highlegdelta

Member
Location
US
Generally the grounding electrode conductor (table 250.66) is not ran with your feeders. It is ran from the service/system neutral to a grounding electrode (building steel, cold water, ufer, ground rod,....)

Equipment grounds (table 250.122) are the ground wire you pull with your feeders.

You generally won't have either with your service conductors from the utility.

Exceptions may apply when dealing with bonding jumpers.

Sent from my SAMSUNG-SM-G930A using Tapatalk
 

jumper

Senior Member
A somewhat simple rule I tell guys to follow is to ask yourself if there is an overcurrent device directly protecting the associated phase wires. If the wires are protected by a fuse or breaker, then 250.122 is used based on that fuse/breaker size. If there is no overcurrent protection, such as the secondary of a transformer or service wires from a meter socket, then 250.66 is your guide.

Not 100% fool-proof but pretty close.
Gud enuf fer gubmint werk.:)

(I think I strained a few brain cells typing that.)
 
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