I may have to do a 400A service with a CT can feeding two 200A panels located next to each other. Each panel will have its own Main bonding jumper. Do I have to run two # 4 wires (one from each panel) to Grounding Electrode or I can use one # 1 looped through each panel instead? Do I run two # 6 wires to the two rods or I can run just one # 4 looped through both panels? I have done many 200A service but this is the first 400A. I appreciate your input. Thank you.
1. The GEC must be sized per 250.66 2. Although it must not be spliced, it may be tapped onto by the GEC from each 200 amp panel. 3. I would connect it into one panel and then run a # 4 from the 2nd panel to tap onto it. 4. The #6 GECs should also be able to just tap onto it.
Disclaimer-- as always, if there is significant costs invloved , check with AHJ first!
[This message has been edited by Redsy (edited 09-19-2002).]
Table 250-66 is based on "the largest ungrouded service-entrance conductor". Now which one is considered service-entrance conductor, the 500 kcmil coming into the CT or the 3/0 coming into the individual 200A panels. I can see your point TE. Along the same line, what if we go out of CT can right into a 400A Main disconnect and then from load side split off into the two 200A panels - one next by and the other in basement. Now the Main bonding jumper will have to be at the Main disconnect with 4 wires feeding each panel (GND bus separated from Neutal bus). According to 250-66 we need 1/0 for EGC based on 500 kcmil coming into the Main disconnect. # 4 or 1/0 ...that is the question...
UP to the 200A Mains' they are Service Entrance Conductors. 250.102(C) would apply.
Note: Senerio 2) has two 3/0 Cu per ungrounded conductors from the CT cabinet to the Pull Box so the Metal Raceway ( conduit ) bonding bushing jumper would be based on two 3/0 Cu ( 167800 CM each ), 335,600 CM. Table 250.66 = 2 Cu.
Thus: Line side of CT bonding bushing jumper = 1/0 Cu.
CT to pull box bonding bushing jumper = 2 Cu.
Pull box to each 200A Main bonding bushing jumper = 4 Cu.
After some research I found that "Soares book on grounding", 7th edition, page 102 & 103 covers this GEC issue really well. The method of sizing GEC is a matter of choice: 1. an individual GEC may be installed from each service disconnecting means enclosure to GE ( in our case one # 4 out of each 200A panel) 2. a single GEC is permitted to serve several enclosures. The main GEC is sized for the main service-entrance conductors ( 500 kcmil in our case). The NEC does not use the term main GEC to identify the GEC that connects directly to GE. It is only used here to distinguish from the grounding electrode tap conductor. Taps that are sized for the individual service-entrance conductors ( 3/0 in our case)are connected from each service disconnecting means enclosure to the the main GEC. ( one # 4 cu from each 200A panel will tap into 1/0 cu which is our main GEC) TE I always use 2 rods not that they are required but just me being practical.
If your 2 200 amp panels are close together you might want to try a 320 meterbase and nipples right into both panels ! less waiting to fire it up from the city and faster ! "just be carefull when you cut your ko's in the back"
Attach your 1/0 from the water to the point where the 500's terminate in the CT cabinet and run one #6 from the rods to the same place. If the utility doesn't allow that, then I'd still run one 1/0 to both panels neutral buss and one #6 from the rods to both panels neutrals as well.
I have always considered it kind of funny that you base the size of the GEC from the largest set of service conductors, but there is no requirement to attach the GEC to the point where the largest set of service conductors terminate.