If you are feeding a subpanel(50-125 amp) from a main service panel(200 amp) , do you need to have 4 wires to the sub panel? The grounding and grounded conductors are terminated at same place (theoretically) in the sub panel. therefore you would need to use the supplied "bonding" screw (green).right?
Please clear this up for me, as I have heard /seen many different methods on this. I have installed may subpanels with 3 wires and I REALLY want to know if I am doing this wrong.

If you are feeding a subpanel(50-125 amp) from a main service panel(200 amp) , do you need to have 4 wires to the sub panel?

Yes

The grounding and grounded conductors are terminated at same place (theoretically) in the sub panel...

No. They are in the disconnect or main load center, whichever is first in line from the meterbase.

therefore you would need to use the supplied "bonding" screw (green).right?

No, not it the sub-panel, but in the first panel in line from the meterbase.

Note: You will need to aquire an equipment ground bus bar for your sub-panel.

Also note: You also must size the sub-feeder conductors according to Table 310.16, for 310.15(B)(6) is for "...conductors that serve as the main power feeder to a dwelling..."

[This message has been edited by sparky66wv (edited 09-16-2002).]

so in the SP , you would need a ground bar? and it must not be elctrically continuous to the neutral bar?

It definitely is not bonded in any way to the neutral bar (except via the EGC subfeeder back to the main disco or panel, then through the bond to the grounded conductor then back to the sub panel).

No bond screw or bonding jumper in the sub-panel at all. To do otherwise would create a parallel path for the neutral current.

ok , Thank you for the plain english. So, it is continuous , but at the main panel? ground/neutral bar at sub are seperate, but bonded at main?

Yep, you got it!

Thank You very much you may be able to help a little situation I have here on this post https://www.electrical-contractor.net/ubb/Forum7/HTML/000185.html

definately a neutral problem, i've seen this scenario many times.

A neutral from main to sub? or in the building? Im going out there right now.

turns out they kernied the neutral at service drop for the barn.there are three weather heads on the pole and it looks like a birds nest up there.I continued to have same problems today, so I am going to run a new underground service tomorrow for the barn and discontinue the overhead service.
Its either an insulation breakdown in the ungrounded conductors feeding the barn (which are 30+ years old) or I have lost a neutral at the bird nest.
Either way, this was definitely a learning experience.
I need to invest in a megger, whats a good name to get ??

Is there ever an occasion where a ground rod should be installed at a sub panel in a remote building (and attached to the ground wire from the main panel)? I haven't found any reference to it (yet) and I would think it should be done only because of ground potential differences between where the two panels are mounted.
Thanks-Michael

was not attached to ground from main panel. There was no ground to the sub panel in barn. with all the welding/compressor loads, its a good idea.:O)

MJR,
250.32(A) requires a grounding electrode at the second building any time there is more than a single branch circuit feeding the second building.
Don

Thanks, Don - got it!

Not to open any worms but... you may be able to use your bonding screw depending on what type of panel you use. For example, GE has a true split neutral buss which allows you remove the MBJ with two bolts. Thus splitting the bus into a isolated neutral and ground bus. Then you could use the pretty green screw on the ground side (the left hand bus)

Do not confuse this with the short bus, which is what I rode to school in.

arseegee, yep... I've only removed the bar once in an installation, the rest of the time I've always installed a bus.

But you are indeed correct, especially with GE (and I consider myself somewhat of an expert with GE since that's what I use the most often!).

The important thing is to bond the EGC and GC only once and to provide a bond to the EGC to all metal enclosures, and to use an approved means to do so.

(quote)"If you are feeding a subpanel(50-125 amp) from a main service panel(200 amp) , do you need to have 4 wires to the sub panel?"

Correct me if this is wrong here guys, but my continuing ed instructor often illistrates a sub-panel in a seperate building (garage) using a 3-wire feed from the main panel(no equipment ground). The heading under this illistration reads "permitted but not recommended".

The illistration clearly shows the feeder wires originating at a 2-pole breaker inside the main panel, not from a meter socket. It also shows the main bonding jumper installed in the sub. I'd like to clear this up.

Can anyone quote an article number where it states that "all" sub-panels must be fed with a 4-wire feed? Can anyone quote an article where the code even uses the term "sub-panel"? Matt

Wow... In fact, 250.32(B)1 & 2 elude to the option as well... (if it is to a seperate structure...)


Hmmm... My bad, Matt... Jeez, back to the books I go!

Now how come y'all didn't catch that!

[This message has been edited by sparky66wv (edited 09-24-2002).]

I was talking to someone again Today about this he swears you Do run a ground back from main and DO NOT bond at sub for the simple fact that your GFCI protection will function more accurately .
Thats the best reson so far

The only ground fault protection that will function more accurately is the ground fault protection of equipment and that is already covered by condition three of section 250.32 (B) (2).

(2) Grounded Conductor. Where (1) an equipment grounding conductor is not run with the supply to the building or structure, (2) there are no continuous metallic paths bonded to the grounding system in both buildings or structures involved, and (3) ground-fault protection of equipment has not been installed on the common ac service, the grounded circuit conductor run with the supply to the building or structure shall be connected to the building or structure disconnecting means and to the grounding electrode(s) and shall be used for grounding or bonding of equipment, structures, or frames required to be grounded or bonded. The size of the grounded conductor shall not be smaller than the larger of
(1) That required by 220.22
(2) That required by 250.122

If ground fault protection of equipment is installed the building disconnecting means and panels must be wired as if they are in the same building as the service supplying them. This is because a bonding connection to the neutral at the second building will cause some current to flow through the earth back to the service and transformer grounding electrodes. This can cause nuisance tripping of the ground fault protection.
--
Tom