Another SubPanel Question

[Bart]

Want to run 3-#4 or maybe 3-#3 copper conductors to my subpanel. What size ground wire is required?

[Chris Lewis]

According to Bart :

#6 is usually code-proper up to a 200A service at least. But I recommend consulting your inspector before buying anything. The other wires you mention are brutally expensive, and it's best to nail it all down exactly with the guy who gets to say yea or nay before shelling out the bucks.

Furthermore, it's cheaper (plus legal and safe if done properly) to use aluminum conductors for big feeds. If the wires are longer than a few feet, I recommend considering it. It's MUCH cheaper.

Going to aluminum saved me about $400 on my subpanel feed.

Ask the inspector about aluminum too.

[Tom Horne]

The size of the Equipment Grounding Conductor (EGC) is based on the rating of the Over Current Protective Device (OCPD) rather than on the size of the conductors. What size OCPD were you planning to use to protect the feeder?

-- Tom H

[Bart]

OK. The subpanel to receive power off a 100A breaker in my main panel.

[Tom Horne]

For a one hundred ampere breaker a number eight copper or number six aluminum Equipment Grounding Conductor (EGC) is required. If you do use number three copper for the feeder conductors you must use number six copper or number four aluminum for the EGC. This last is because the code requires that the size of the EGC be increased in proportion to the increase in size of the current carrying conductors.

-- Tom

[Mark or Sue]

Tom,

While what you say is true, I'm curious at what point you determine a wire is oversized. If using table 310.15(b)(6), then #4 cu is all that is required. If using 75C wire in table 310.16, then you need #3 cu. If using

60C wire (or NM cable), then you need #2 cu. I can't see how the equipment grounding would care about what insulation heat rating the wire has. So at what point must you begin to increase the EGC size?

-- Mark Kent, WA

[Bart]

Thanks. I was hoping #6 would be OK. Now I have another problem. In my main panel, which is 200A, the neutral buss bar is full. Actually it's the ground bus bar. I can see the entrance service cable attached to the lug on the neurtal bar. The ground bar is only about 1" away and an inch in front of the neutral bar. It appears the neutral bar wasn't even used. The white wire & bare ground from each circuit attach to the same screw on the ground bar. (easier access for whoever wired the house years ago. The rear bar is really inaccessable. And I assume they are bonded together anyway)..... I can't double up any of the wires because they're already doubled up. Well, I could double up 1 of them and install a 2-pronged lug adaptor for my #3 conductor. That would solve the problem for my new neutral conductor. But where to attach my new ground wire? Can I attach another ground bus bar directly to the main panel sheet metal? I could them attach a couple of the ground wires to this to free up room on the neutral bar. (Opps, I mean the ground bar.)

[hornetd]

Your panel was improperly wired during installation. Since the white coded neutral conductors carry current during normal operations they must be terminated singly with only one under each clamp or screw. The label in your panel may indicate that the neutral bar can accept more than one wire per terminal but that is only permissible when the wires in question are Equipment Grounding Conductors only. Since EGCs do not carry current during normal operations there is little possibility that the unequal loading of the multiple conductors will cause the terminal to loosen. With neutral conductors this is an issue so each neutral wire must be the only one in each terminal.

What are you planning to use the number six wire for?

-- Tom

[Tom Horne]

I made a bad assumption that the reason for the #3 conductors was to compensate for voltage drop. 310.15 (b) (6) only applies to service conductors and to feeders that supply the entire supply to a dwelling unit so it would not apply to the situation as outlined in the original posting. I would use the smallest wire size that the code would permit for the OCPD in question as the starting point. If the wire selected is larger than that minimum then the only rational reason is to avoid voltage drop. In any case if you increase the wire size to avoid voltage drop you must increase the size of the equipment grounding conductor proportionately. I don't have an NEC here so I can't check the wire size for the one hundred ampere feeder. If the number three is the minimum that the code would allow for the 100 ampere feeder then a number eight EGC would be all that is needed.