Egc

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muhandas

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It's a routine situation but my brain is refusing to work this AM and I'd appreciate your help. I apologize if this has been answered before, I couldn't find it.
Consider the rather usual situation. A feeder from an OCPD in a main distribution panel is connected to a 480V:208Y/120V transformer. The transformer secondary feeds a subpanel. Assume that the grounding connection of the separately derived system is made at the transformer (it isn't clear from the plans) and that it is sized correctly in accordance with 250.66. My question relates to the EGC that runs from the transformer to the subpanel. Should that be sized in accordance with 250.66 as well or should it be sized in accordance with 250.122 using the OCPD rating in the main panel multiplied by the transformer ratio?
Many thanks for your help.
Heinz R.
 
Tx. for reply, Pete. To size the Equip. Bonding jumper in accordance with the phase conductors (as indicated in 250.102) was also my first reaction but the entire separately derived system section ignores any OCPDs ahead of it. It could be argued that the Equip. bonding jumper only needs to be large enough to alow any ground fault current to trip the OCPD in the primary of the transformer. But I couldn't find support for that anywhere in the NEC. Absent that permission I would say that sizing the Bonding jumper related to the phase conductors make sense; it's consistent with other service related requirements. Do you agree?
Heinz R.
 
(2) Equipment Bonding Jumper Size. Where a bonding
jumper of the wire type is run with the derived phase conductors
from the source of a separately derived system to
the first disconnecting means, it shall be sized in accordance
with 250.102(C), based on the size of the derived
phase conductors.

then sends you to below :

(C) Size ? Equipment Bonding Jumper on Supply Side
of Service. The bonding jumper shall not be smaller than the
sizes shown in Table 250.66 for grounding electrode conductors.
Where the service-entrance phase conductors are larger
than 1100 kcmil copper or 1750 kcmil aluminum, the bonding
jumper shall have an area not less than 12 1⁄2 percent of the
area of the largest phase conductor except that, where the
phase conductors and the bonding jumper are of different materials
(copper or aluminum), the minimum size of the bonding
jumper shall be based on the assumed use of phase conductors
of the same material as the bonding jumper and with
an ampacity equivalent to that of the installed phase conductors.
Where the service-entrance conductors are paralleled in
two or more raceways or cables, the equipment bonding
jumper, where routed with the raceways or cables, shall be run
in parallel. The size of the bonding jumper for each raceway
or cable shall be based on the size of the service-entrance
conductors in each raceway or cable.

Just for those following along....;)
 
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