grounding

C

Crosstown

Member
Location
indiana
Occupation
electrician
I am installing a 3000 amp service, with (10) 350mcm copper conductors to feed the main disconnect, and then a 3000 amp sub panel is getting fed off of the main disconnect. Getting fed with the same amount of parallel conductors. If i am looking at this correctly, It looks like acording to 250.66, the main panel would get fed with one 3/0 copper gec, and the sub panel ,according to 250.122 would have to be fed with a 400 mcm in each one of the 10 conduits feeding the sub panel. Am i sizing this correctly? If so, does this make sense to anybody why i would have to put this many grounds from the main switch to sub panel?
question 2: i will have zero neutral load, can i just use a 1/0 for my neutral for both sides since this is the minimum?
 
Crosstown said:
It looks like acording to 250.66, the main panel would get fed with one 3/0 copper gec
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Yes the largest GEC ever required is #3/0 so that would be the correct size depending on the type of electrode.

Crosstown said:
the sub panel ,according to 250.122 would have to be fed with a 400 mcm in each one of the 10 conduits feeding the sub panel. Am i sizing this correctly?
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According to T250.122 a 400 kcmil copper EGC in each raceway is the correct size for 3000 amps.
Crosstown said:
question 2: i will have zero neutral load, can i just use a 1/0 for my neutral for both sides since this is the minimum?
Click to expand...
Both sides? Do you mean ahead of and after the servcie disconnect? For the feeder after the service disconnect no neutral is required if there are no neutral loads.
 
You could actually have a #6 if rods were your only GE. Remember the GE doesn't really do much at our voltages.
 
Crosstown said:
does that make sense, i have to have 10 400mcms for 1 3/0 feeding the whole thing?
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Not sure why you're saying that you have "1 3/0 feeding the whole thing?" The GEC is not feeding anything it is just connecting the service to the grounding electrode system.
 
I guess feeding is the wrong term. I dont see why i would need to connect with only 1 3/0 for the main service and i would need 10 400mcms to connect the main service to the sub panel
 
Crosstown said:
could you explain this further and point to code ?
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250.66(A)

(A) Connections to Rod, Pipe, or Plate Electrodes.
Where the grounding electrode conductor is connected to
rod, pipe, or plate electrodes as permitted in 250.52(A)(5)
or (A)(7), that portion of the conductor that is the sole connection
to the grounding electrode shall not be required to be
larger than 6 AWG copper wire or 4 AWG aluminum wire

​

Click to expand...
 
Crosstown said:
I guess feeding is the wrong term. I dont see why i would need to connect with only 1 3/0 for the main service and i would need 10 400mcms to connect the main service to the sub panel
Click to expand...
All metal parts on the line side of the service disconnect are bonded directly to the neutral as there are no EGC's on the line side of the service disconnect. At the service disconnect the main bonding jumper (MBJ) connects the load side EGC bus to the neutral creating the EGC for everything after the service disconnect. The GEC has nothing to do with fault clearing so the size is not relevant to what happens after the point where the MBJ is installed. As Roger stated the GEC to a rod electrode can be as small as #6 even for a 3000 amp service.
 
The neutral conductor cannot be smaller than the grounding electrode conductor. Is 3/0 is the grounding electrode conductor size, then 3/0 is the minimum neutral size
 
Crosstown said:
I dont see why i would need to connect with only 1 3/0 for the main service and i would need 10 400mcms to connect the main service to the sub panel
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Because the grounding electrode conductor, the 3/0, is not part of the fault clearing path, and the 400 kcmil conductors are the fault clearing path.
 
Dennis Alwon said:
The neutral conductor cannot be smaller than the grounding electrode conductor. Is 3/0 is the grounding electrode conductor size, then 3/0 is the minimum neutral size
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He has 10 sets of 350kcmil feeding the service disconnect assuming that they're in 10 raceways wouldn't the minimum neutral in each raceway need to be #1/0?
 
Dennis Alwon said:
The neutral conductor cannot be smaller than the grounding electrode conductor. Is 3/0 is the grounding electrode conductor size, then 3/0 is the minimum neutral size
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So If i bring in one ground with a size of 3/0 from the ground rods to my main service, would i have to have 10 3/0 neutrals going from the transformer to the main switch?
 
Crosstown said:
thats what i have, and thats what i thought
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If that's what you have then it's code compliant. The neutral can be sized based on the conductors in each parallel raceway. From Table 250.102(C)(1) even though for 350 kcmils a #2 is large enough the smallest neutral you can parallel is #1/0.

The conductor from a ground rod electrode is a GEC. The largest required for a rod electrode is #6 but you can certainly use #3/0.
 
infinity said:
If that's what you have then it's code compliant. The neutral can be sized based on the conductors in each parallel raceway. From Table 250.102(C)(1) even though for 350 kcmils a #2 is large enough the smallest neutral you can parallel is #1/0.

The conductor from a ground rod electrode is a GEC. The largest required for a rod electrode is #6 but you can certainly use #3/0.
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so just to be certain, even though i only have 1/0 paralled neutrals from my transformer to my main disconnect, I dont have to pull a neutral to 3000 amp sub panel, but i have to pull paralled 400 mcm grounds to my sub panel, Is that correct?
 
Crosstown said:
so just to be certain, even though i only have 1/0 paralled neutrals from my transformer to my main disconnect, I dont have to pull a neutral to 3000 amp sub panel, but i have to pull paralled 400 mcm grounds to my sub panel, Is that correct?
Click to expand...
Yes, between the service disconnect and the sub-panel that 3000 amp feeder must have an EGC in each parallel raceway but since a neutral is not being used you do not need to install any. One the line side of the service disconnect the neutral is required, load side not required.
 
infinity said:
Yes, between the service disconnect and the sub-panel that 3000 amp feeder must have an EGC in each parallel raceway but since a neutral is not being used you do not need to install any. One the line side of the service disconnect the neutral is required, load side not required.
Click to expand...
what i am having a hard time unerstanding and wraping my mind around is, the fault path is supposed to go back to the neutral on the transformer. If the neutral to the main service is only fed with paralled 1/0, why would I have to have my ground kicked up to paralled 400 mcm from my sub panel to my main switch. Wouldn't the 1/0 be undersized
 
Crosstown said:
so when does table 250.66 come into play if its not reffering to the ground rod?
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T250.66 is used to size the GEC. Some electrodes have maximum sizes that are smaller than the larger ones in T250.66. For example for a ground rod electrode #6 is the largest required. For a CEE a #4 is the largest required.
 
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