320/400 amp GEC to water size

S

sparkympk68

Member
Location
Omaha,Ne,USA
Occupation
electrician
I'm installing a Residential service with a 320 /400 amp meter combo disconnect . poco will switch to underground service in my 3" pvc trench ..I have a 200 amp breaker to the house and a 150 amp breaker to a new Garage . My question : is the water GEC requirement for 400 amps? (1/0) or 320 continuous amps ?(#2) copper? ......... of course the water meter is 80' away across the basement. Thanks !!
 
I believe the service entrance conductors are factory sized busbar where poco attach their underground service lateral to lugs . Busbars feed (2) breakers up to 200 amps each .In this particular meter/combo disconnect . I only need (1) 200 amp and (1) 150 amp in this installation . I.m guessing the available ampacity is 400 so I need size GEC accordingly . The service equipment is listed as 320 continuous amps . I think this is a common size for large homes above 200 amps.
 
Note 2 to 2023 NEC Table 250.66 says "If there are no service-entrance conductors, the grounding electrode conductor size shall be determined by the equivalent size of the largest service-entrance conductor required for the load to be served."

So if there are no SECs, you need to do a load calculation. The load will come out to some number not more than 350A, assuming the two breakers are sized properly. Then size the GEC per the 75C column of Table 310.16 and Table 250.66. E.g. 151-200A = 2/0 or 3/0 Cu = #4 Cu GEC. 201-310A = 4/0-350 MCM = #2 Cu GEC. 311-350A = 400-500 MCM = 1/0 Cu GEC.

Since the GEC is running to the water meter, none of the smaller sizes allowed by 250.66(A), (B), and (C) apply.

Cheers, Wayne

P.S. FWIW Note 2 is actually a bit ambiguous, due to the possibility of parallel conductors. For example, if the load calc came out to 350A, you could use two parallel 2/0 Cu (each of area 133.1 MCM, or 266.2 MCM total) or a single 500 MCM. Those correspond to different sizes in Table 250.66, so you'd use #2 Cu for 2 sets of 2/0 Cu, and #1/0 Cu for a single 500 MCM. Meaning arguably if the load calc is 350A and there are no SECs, you could go for #2 Cu instead of #1/0 Cu. For simplicity I assumed one conductor in the response above.
 
wwhitney said:
Note 2 to 2023 NEC Table 250.66 says "If there are no service-entrance conductors, the grounding electrode conductor size shall be determined by the equivalent size of the largest service-entrance conductor required for the load to be served."

So if there are no SECs, you need to do a load calculation. The load will come out to some number not more than 350A, assuming the two breakers are sized properly. Then size the GEC per the 75C column of Table 310.16 and Table 250.66. E.g. 151-200A = 2/0 or 3/0 Cu = #4 Cu GEC. 201-310A = 4/0-350 MCM = #2 Cu GEC. 311-350A = 400-500 MCM = 1/0 Cu GEC.

Since the GEC is running to the water meter, none of the smaller sizes allowed by 250.66(A), (B), and (C) apply.

Cheers, Wayne

P.S. FWIW Note 2 is actually a bit ambiguous, due to the possibility of parallel conductors. For example, if the load calc came out to 350A, you could use two parallel 2/0 Cu (each of area 133.1 MCM, or 266.2 MCM total) or a single 500 MCM. Those correspond to different sizes in Table 250.66, so you'd use #2 Cu for 2 sets of 2/0 Cu, and #1/0 Cu for a single 500 MCM. Meaning arguably if the load calc is 350A and there are no SECs, you could go for #2 Cu instead of #1/0 Cu. For simplicity I assumed one conductor in the response above.
Click to expand...
thank you wwhitney , very informative .I appreciate the example service sizing to help see #2 is acceptable for 350A calc load
 
wwhitney said:
P.S. FWIW Note 2 is actually a bit ambiguous, due to the possibility of parallel conductors. For example, if the load calc came out to 350A, you could use two parallel 2/0 Cu (each of area 133.1 MCM, or 266.2 MCM total) or a single 500 MCM. Those correspond to different sizes in Table 250.66, so you'd use #2 Cu for 2 sets of 2/0 Cu, and #1/0 Cu for a single 500 MCM. Meaning arguably if the load calc is 350A and there are no SECs, you could go for #2 Cu instead of #1/0 Cu. For simplicity I assumed one conductor in the response above.
Click to expand...
Typically more sets will yield a smaller GEC since the smaller conductors give you more ampacity per circular mil. So if you are trying to minimize wire size, calculate with more sets (of course not going smaller than 1/0). However, in these situations I have always just sized things based on what service entrance conductors I would have used if they existed.
 
infinity said:
How did you come up with 350 amps as the calculated load? How will you run the GEC to the disconnects?
Click to expand...
GEC size per 250.66 , service conductors based on table 310.12 . The max load served should be 200amp house panel ,plus 150amp garage panel. The water GEC will be installed across basement, penetrate exterior wall ,connect to intersystem bond terminal , then terminate in meter combo at grounded lug. supplemental #4 ground will connect to this same lug to a ground rod , then ufer rebar at new garage . I expect this all to pass inspection . thanks for reviewing for accuracy
 
sparkympk68 said:
I will do the load calc . for accuracy . The 320 amp service is probably oversized ,2500'sq house and 1000'sq garage . Customer may want Garage car fast charger ability, and maybe split unit A/C heat pump, and compressor for tools . thanks
Click to expand...
I was just asking because per post #4 the GEC could be smaller than #2 if the load calculation is actually less than 350 amps.
 
You must log in or register to reply here.
Top