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07-18-2005, 04:17 AM #1
Does anyone have a chart for ground rod and ground wire size for a 400 amp service?
The meter base wire specs:
07-18-2005, 10:43 AM #2
Australian Status - a go by only.
As3000 = AUS and NZ Wiring rules - Federal law here.
Table B4.1 of AS 3000 - 2000 version standard only slightly changed in last 4.5 years.
protective device rating:
200 A - maximum value of fault loop impedance at 230 VAC (this is the impedance from the DB , through the fault and back to the earth point at the MEN link ) MCB = moulded case breaker
Type B MCB .29R - trips at 4 x rated current
Type C MCB .15R
Type D MCB .09R - trips at 12.5 x rated current
BS88 type fuse (HRC) 0.13R for a computed 400ms disconnect time.(you have to have the mafr fuse curve and know the source impedance to calculate the disconnect time.)
Yessir that was for 200A - 400A is not even covered. Resistance values should be halved for your current of 400A.
Minimum recommended earth conductor size in table B 5.1 is 25mm2 for 200A so make it 50mm2 for 400A Maximum circuit length (out and back) is 84m for and HRC fuse and 60m for a type C MCB. This would be based on 140mm2 active conductors for the 400A feed. B and C MCB types allow longer route lengths.
AS 3000 our federal wiring code does not say how you are to achieve the resistance required to cause fault clearance - that is your problem.
Clause 188.8.131.52 suggests 12mm dia non ferrous rods driven to a depth of 1.2m. Note it says rods but not how many. If you want to know the R to earth of the rod alone you clearly need 2 independent rods and a source of say 20A to run through the circuit to check the volt drop.
The other thing to watch is the type of connection made (how well the Cu is clamped to the rod and the metal prep) and how well the joint is protected - there are specific paint sprays electricians use to protect the joint after construction.
Santos, who are in oil and gas in the Aust desert and have serious earth resistance problmems from time to time, drive a couple of 2m Cu clad stainless steel rods as minimum and put them in pre-augured holes filled with Bentonite - which is sandy clay material that retains moisture well and is used in the metal casting industry - this could get you out of gaol if you come up with a high test with conventional rod drives.
Only a go by and should get you started - none of this will protect you from your electrical inspector if the US NEC requirements are more stringent. Probably not. Many of the commercial type US standards are pretty Half_A_d compared to the Australian/NZ ones. If this does not make much sense then get a qualified Elec to look at it because if it is done wrong and the fault impedance is too high the machine and operator will light up if there is a serious "bolted to earth" type of fault.
07-18-2005, 01:19 PM #3
Thanks for the information.The soil is red clay with plenty of iron in it so there shouldn't be a problem there.The local building code chart guide only goes to 200 amp.It doesn't specify ground rod size just a depth of at least 4' in the ground and using #4 copper for the ground wire.The circuit length to the rod and back is 20'.The typical ground rod size that is sold locally for a 200 amp service is steel with heavy copper plating 5/8" x 8'.
07-18-2005, 03:33 PM #4
The NEC does not appear to differentiate ground
electrodes according to service capacity - only
types and methods. Basically, electrodes should
be 8' long, buried (driven to 8'), at least 5/8"
dia. if steel or iron, 1/2" if copper or copper-
clad, and 3/4" if pipe or conduit. Or a 2 sq. ft.
plate. Total resistance less than 25 ohms for
"made" electrodes. Other stuff about water pipes,
metal buildings, rebar in concrete, etc.
As far as the conductor, there is much noise about
wire protection, connection points, bonding, etc.
but it seems to boil down to at least a No.4 copper
conductor, no straps on the electrode, and some
sort of covering where exposed (EMT, jacket, etc.)
Practical bottom line, an 8' copper rod, using at
least AWG4 with no splices, clamped to the rod, in
decent soil, should pass inspection. In functional
terms, the more buried surface, the better; the
larger the conductor the better; and always with
a minimum of bends, length, and bonding resistance.
Most of that last is lightning related; a safety
ground mainly just needs a low resistance.
I have never seen a 2 8' electrodes, buried up to
the last 3", 6' apart, with AWG2 solid copper
clamped or cad-welded, not pass. Most small service
entrances use only one rod, AWG6 or AWG8 conductors
plus the distribution ground.
07-19-2005, 06:51 PM #5
I must take issue with HTS, because if the earth resistance is 25 Ohms, at 110V it will only allow an earth current of 110/25 = 4.4 Amps - not enough to clear the 400A fuse!
What I believe is that if a full fault occurs from the supply to a frame, under these conditions,
1. the frame must not exceed 50v to any local metalwork/ground to protect personel.
2. the fault current must be high enough to clear the supply fuse. So the earth conductor must be capable of carying the fault current BUT for only the time it takes the fuse to clear, so it is of smaller size.
I helped some electricians with an earth rod problem, They needed less then 1 ohm, using 2 metres of propriety gunmetal earth conductor into chalk, they got about 10 to 15 ohms, a bucketful of salt in a little water got it to better then .1 Ohm! (don't know for how long for , but it is tested annually ).
In the installations, I worked on with these sorts of power, they were ALWAYS fed from a transformer and the Protective earth always went back to it. The ground rod was used for the transmission equipment/lightning protection.
P.S. Doing some rough sums, your load = 110V/400A= .28 ohms, for incoming 2% regulation, supply impedance = .28/50 = .0056 Ohms, therefore fault current = 110/5.6 kA = 20 kA
If we say that the frame can't be any greater then 50V, this then inplies an earth resistance of 50/20 =2.5 milliOhms including EVERTHING(rods,leads,terminations etc.)
07-21-2005, 12:25 PM #6
Please note that I did not make up the 25 ohm thing,
nor do I recommend it in any instance. It IS a bone
of contention. But it is in the code. Read this:
"Ground Resistance-It's Not What You Think"
There is an excellent general treatise on worldwide
07-24-2005, 03:50 AM #7
It may sound silly but I would check with the
local buliding inspector, I don't know where
you are from, but here in So Cal regulations
vary from county to county. On new construction
20' of rebar running through the foundation,
attaching 4 gauge wire, clamping to the plumbing,
copper/steel rods into the earth no longer
07-25-2005, 12:18 AM #8
Yes Dualkit,it is really silly.The building inspector had a handout with a chart on wiring and ground rods,but it only went up to 200 amp service.I have decided to use two 5/8"x8'copper plated ground rods with #4 Cu wire and I don't believe that there will a problem.
Thanks to all for the help and the mikeholt.com link.