BONDING SIZE

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bphgravity

Senior Member
Location
Florida
250.102(D) states that the equipment bonding jumper shall be sized per table 250.122. Table 250.122 is sized per the OCD ahead of the equipment,ect. However, isn't the idea of bonding jumpers to ensure effective FAULT current path. The OCD rating in 250.122 is for normal operational current. 250.4(5) states that the fault path must be capable of carrying the maximum fault current likely to be imposed on it. I know that fault current can be several thousand amps and Table 250.122 size requirements may not be enough to cover this, however, there is no calculation or table that shows what fault current could be imposed, and I never see this being enforced. Any thoughts?
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Re: BONDING SIZE

The Table is based on the equipment grounding/bonding conductor being able to conduct enough current to cause the OCPD to open under normal fault conditions. This is a minimum conductor size and may have to be increased under special conditions. An example of a code required size increase is found in 250.122(B).
You can view some conductor withstand ratings at another Bussman publication . Look at page 74 and you will find that #12 copper can carry 2700 amps for one cycle without damaging the 75?C insulation on the wire. A 2700 amp fault on a 20 amp OCPD should cause the OPCD to open the circuit in about 1 cycle or less. The 2700 amp number is on the conservitive side as it limits the conductor temperature to 75?C.
Don
 
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