Question about Grounding Electrode conductors

Status
Not open for further replies.
JJWalecka

JJWalecka

Senior Member
Location
New England
I have a few question concerning the Grounding Electrode conductor.
Hypothetically, I have a 200 amp service. Table B.310.3 Ampacities of multi conductor cables with no more than three insulated conductors type USE, states that 2/0 AWG copper THHN @90 degrees Celsius is rated at 215 amperes. It is my understanding that there is no breakers or equipment rated for 90 degrees Celsius. 3/0 AWG copper at 75 degrees Celsius is rated at 213 amperes.
Table 250.66 Grounding Electrode Conductor for Alternating Current Systems states that copper 2/0-3/0 AWG the Grounding Electrode conductor is sized at 4 AWG.
250.64 Grounding Electrode Conductor installation
250.64(F)To Electrode(s)
A Grounding Electrode Conductor shall be permitted to be run to any convenient grounding electrode available in the grounding electrode system or to one or more grounding electrode(s) individually. The grounding electrode conductor shall be sized for the largest grounding electrode conductor required among all the electrodes connected to it.
250.53 Grounding Electrode System Installation
250.53(E) Supplemental Electrode bonding Connection Size.
Where the supplemental electrode is rod, pipe, or plate electrode, that portion of the bonding jumper that is the sole connection to the supplemental grounding electrode shall not be smaller than 6 AWG copper wire or 4 AWG aluminum wire.
If I'm interpreting this correctly the bond to the water meter street and load side would be 4 AWG copper, because the ground rod is the supplemental ground a 6 AWG is sufficient. If the water into the house was installed with pvc it couldn't be used as the main ground. The ground rods, pipe or plate electrodes would then be the primary ground? I would then have to use 4 AWG?
250.56 Resistance of rod, pipe, or plate electrodes.
A single electrode consisting of a rod, pipe, or plate that does not have a resistance to ground of 25 ohms or less shall be augmented by one additional electrode of any of the types specified by 250.52(A)(2) through (A)(7). Where multiple rod, pipe, or plate electrodes are installed to meet the requirements of this section, they shall not be less than 1.8m (6ft) apart.
It was my understanding that if 25 ohms or less was not apparent then you had to install more electrodes till 25 ohms or less was reached. Unless I'm reading this wrong, if 25 ohms or less is not reached than one additional electrode, specified in 250.52(A)(2) through (A)(7), would be installed?
Please enlighten me upon my misunderstanding. Thank you for your time and effort.

Justin
 
Re: Question about Grounding Electrode conductors

I know there are questions in there some place. :D

The ground rods, pipe or plate electrodes would then be the primary ground? I would then have to use 4 AWG

No. They would never be a primary ground and #6 is all that is required. Simply remember that they cannot conduct more current than a #6 can handle.

It was my understanding that if 25 ohms or less was not apparent then you had to install more electrodes till 25 ohms or less was reached. Unless I'm reading this wrong, if 25 ohms or less is not reached than one additional electrode, specified in 250.52(A)(2) through (A)(7), would be installed?

Yup, or just install two electrodes to begin with unless you have the gear to measure ground resistance.

-Hal
 
Re: Question about Grounding Electrode conductors

Hal even if you have the gear to measure it, there is no requirement to go beyond two rods as the condition of the soil is that poor that adding more rods would not improve the connection by very much. So if 25 ohms can't be met with one rod then install one more and go home. :D

[ February 23, 2005, 07:18 PM: Message edited by: hurk27 ]
 
Re: Question about Grounding Electrode conductors

Originally posted by justinjwalecka:
Table B.310.3 Ampacities of multi conductor cables with no more than three insulated conductors type USE, states that 2/0 AWG copper THHN @90 degrees Celsius is rated at 215 amperes.
Click to expand...
You probably don't need to reference the ANNEX B tables. Just use the Tables in 310. You are correct about no 90? ratings. In most applications, the conductor amapacity cannot exceed the 75? rating.


If the water into the house was installed with pvc it couldn't be used as the main ground. The ground rods, pipe or plate electrodes would then be the primary ground? I would then have to use 4 AWG?
Click to expand...
No, #6 is the largest conductor required for all situations and condictions.


Unless I'm reading this wrong, if 25 ohms or less is not reached than one additional electrode, specified in 250.52(A)(2) through (A)(7), would be installed?
Click to expand...


Thats correct. If 25-ohms is acheived with one rod electrode, your all done. If 25-ohms is not acheived, drive another and your all done.
 
Re: Question about Grounding Electrode conductors

Originally posted by justinjwalecka:
If the water was not avalable to use a a grounding electrode all that would be rtequored would be a 6 AWG copper regardless of which electrode it was installed to.
Don't forget that even if the water pipe is plastic comming into the house and the house is pipped in metal you still need a #4cu to bond the interior water pipe.

[ February 23, 2005, 08:03 PM: Message edited by: electricmanscott ]
Click to expand...
Click to expand...
 
Status
Not open for further replies.
Top