Sub-Panel Installation guide line drawings

[Bruto]

As requested I am attaching some images of mine to assist those searching and reading these forums to learn about sub-panel installations. These images will show panels installed in the same structure as the service equipment (aka where the main disconnect is located) and detached building separate from main disconnect (service equipment).

Use care in applying them as all installations are not necessarily the same and always get your installations inspected. Note that as of 2008 any jurisdiction using the 2008 NEC will require 4 wires to any sub-panel regardless of where it is located and regardless of parallel metallic paths to detached structures...3 wire feeders are not allowed. Some areas local and state do not allow 3 wires to sub-panels period no matter the code cycle your on.

You may copy these images, I don't care, they are not copyrighted just be sure a qualified person agrees with them for your application... it's not my fault if you don't use common sense...


NOTE : THE THREE WIRE FEEDER IS NOT ALLOWED ON 2008 NEC CODE CYCLE



This is allowed in 2008 NEC code and it is required for new subpanel installment that including replaceing the old subpanels

[french277V]

Myself and Tolyn do have extra drawing on hand if you readers request it.

One of us will make more drawing in here any other question please just ask any Moderators in this fourm we will be more than happy to answer the question.

Merci,Marc

[fernandof]

If the subpanel is going to be installed in the same structure, would a grounding bar still be required?

[french277V]

Originally Posted by fernandof

If the subpanel is going to be installed in the same structure, would a grounding bar still be required?

The answer is yes the grouding bar still required.


Merci.
Marc

[rwreuter]

Note that as of 2008 any jurisdiction using the 2008 NEC will require 4 wires to any sub-panel regardless of where it is located and regardless of parallel metallic paths to detached structures...3 wire feeders are not allowed.


Can someone please explain to me why the 4th grounding wire is needed, what is the reasoning behind this? I mean from a safety standpoint....

I have a house (not a trailer) that will have a remote meter and it looks like I will have to run Mobile Home Wire (4/0-4/0-2/0-4) and it is ridiculously more expensive (almost $2.0 a foot more) than the standard URD 4/0-4/0-2/0.

If it isn't a safety issue and just someone hair brain idea I may just go with the old method that has been in use for as long as I can remember.

[ray2047]

Can someone please explain to me why the 4th grounding wire is needed, what is the reasoning behind this? I mean from a safety standpoint....

Soil conductivity can vary. By having a fourth wire you insure a low resistance path back to where the neutral and ground are tied together.

[rwreuter]

and the hazard would be.....for example.....(do you have one)

i mean, if there is a fault at the building (house), it will either go to the water main or the ground rods, not back through the neutral and energize the main disconnect and meter.

is that what they are trying to prevent, the possibility of a fault going back through the neutral because the resistance is lower at the pole with a ground rod there?

i'm just trying to think this through.

[ray2047]

The breaker wouldn't trip because of no low resistant path. That could leave the metal shell of equipment dangerously charged if you provivded another path to ground by touching something such such as communication coax cable.

[rwreuter]

ok, i guess i am just tracking here. are you saying that in the structure that i am building there may not be sufficient low resistance (with 2 ground rods, water main) to cause the breaker to trip and without that 4th grounding conductor to carry the fault back to the other ground rod it would cause the service in the house to be hot?

[crokett]

This might be an "ask your local inspector" question but are two ground rods required at the subpanel if it is in a separate structure? I am being advised that it is 2 rods, spaced at least 8' apart. I find it a little odd since when I did my main panel replacement last week the inspector said 1 was ok but I had to drive it in all the way - the original install there was 24" or so sticking out.

[Bruto]

Originally Posted by rwreuter

ok, i guess i am just tracking here. are you saying that in the structure that i am building there may not be sufficient low resistance (with 2 ground rods, water main) to cause the breaker to trip and without that 4th grounding conductor to carry the fault back to the other ground rod it would cause the service in the house to be hot?

Unfortunately you have a major misunderstanding about water pipes and ground rods versus the effective fault path that allows a circuit breaker to open. Your water pipes (metal) and ground rods compromise your grounding electrode system (GES) and have absolutely nothing to do with fault currents and causing breakers to trip. They are for property protection from high voltage and amperage events like lightning or a downed high voltage utility line energizing your service drop.

The equipment grounding system (EGS) is the bonded metal and equipment ground wires likely to be energized in the event of a ground fault and are for human safety to protect from electrocution. This path lets fault currents get to the service neutral and then to the transformer center tap over an extremely low impedance path, and therefore allows enough current (amperage) to flow through the breaker to cause it to trip and clear the fault.

System current and fault current seek the source not earth. If earth was the only path to the transformer ( source ) for fault current the resistance would be so high a breaker would not have enough current go through it to trip and all metal in the fault path would come to line voltage.

Please study these drawings below........ there are specific requirements in order to run a 3 wire feeder to a detached building from the service equipment. You need to know what those are before you run 3 wires, if you are not on 2008 code and meet all those conditions for allowing a 3 wire feeder then you can do so. As for a 4th ground wire there isn't one nor is there a 'ground' wire in a 3 wire feeder. The neutral in a 3 wire feeder is not ground or a grounding wire!! It is a current carrying wire that is 'grounded' by virtue of the service neutral having a connection to earth at the service equipment and at the center tap of the utility transformer. It is used as the ground fault path for 3 wire feeders to sub -panels because there isn't an equipment grounding wire to carry the fault current back to to the main panel where it can use the service neutral to get to the transformer center tap. Fault current will not go to earth unless it is the only path available to it and then the resistance will be so high that your fault will not clear because the breaker will not trip.