Getting close (finally) in relocating panelboard. Currently, 100 amp fed with #2 AL THW. House built in 1966. Copper is now allowed and desire to use it. Local electric supply suggested #3-not sure of the type, i.e. THHW, etc. In referencing table 310.16, I'm a bit confused in the wire sizing and ampacity rating.

1. A- What is the difference between a #3 wire and a #3/0, or any wire with a /0 designator?

B- Which would be acceptable for 100 amp service—3 or 3/0?

C- Any particular type (THW, etc) to look for?

2. I grounded the system with #6 copper per table 250.66. It was originally grounded with #10 copper. According to the table a 3/0 copper requires a # 4 copper ground conductor, correct? Or is the amp rating also taken into account?

3. When tying a utility ground, such as the phone, into the electrical ground, should ground wire be consistent in size with system ground conductor?

Common wire sizes from smallest to largest: 14, 12, 10, 8, 6, 4, 3, 2, 1, 1/0, 2/0, 3/0, 4/0, then the MCM's(Million Circular Mils) 3/0 is pronounced 3 Odd.
#3 is what you need.
I use THHN/THWN
# 4 won't hurt for ground.
Many utilities use #10

For a residential 100A service entrance #4CU is standard and typical. You can go to #3CU if you like, although there is no reason to.

We DO NOT use T310.16 in figuring residential service conductor size, we use T310.15(B(6).

When you say you grounded the system, what do you mean? Ground rod? Water pipe ground? Water bond?

A grounding electrode conductor run solely to a ground rod never needs to be larger than #6CU. Although any smaller needs physical protection.
A water bond for a 100A service needs to be no larger than #8CU according to T250.66.

I hate to say it, but I think you are quite mixed up about all this and REALLY need to get things straight.

Here is a very good read for you to look over:
Wiring a Meter / new service panel / grounding system (http://www.selfhelpandmore.com/homewiringusa/2002/maindwelling/meter/index.htm)

I hate to say it, but I think you are quite mixed up about all this and REALLY need to get things straight.



Sorry stan, I'm not a licensed electrical contractor such as yourself. That's why I ask questions. The last time I looked this was still a HELP forum!

Why the heck are you getting offended?
I simply suggested you get things sorted out before going any further.

I could have said don't do this at all. Hire a professional, which IS your best bet.
I just know how folks are, and they get really miffed when someone suggests something that they don't want to hear.

common wire sizes from smallest to largest: 14, 12, 10, 8, 6, 4, 3, 2, 1, 1/0, 2/0, 3/0, 4/0, then the MCM's(Million Circular Mils) 3/0 is pronounced 3 Odd.
#3 is what you need.
I use THHN/THWN
# 4 won't hurt for ground.
Many utilities use #10

Strat,
Where are you getting #3cu from?



We DO NOT use T310.16 in figuring residential service conductor size, we use T310.15(B(6).True


When you say you grounded the system, what do you mean? Ground rod? Water pipe ground? Water bond? A DIY may not have any idea on what you are talking about. Heck, most electricians are struggling with grounding/ bonding!


A grounding electrode conductor run solely to a ground rod never needs to be larger than #6CU. Although any smaller needs physical protection. True



A water bond for a 100A service needs to be no larger than #8CU according to T250.66. true


MCM's(Million Circular Mils) sure it doesn't mean Multi chip modules. :D This is an older abbreviation.


sorry stan, I'm not a licensed electrical contractor such as yourself This is why I do not advertise my title! Side note: When you advertise, you are saying you know, and people "may" expect you to know! Yet, what you don't know, is that you don't know! :D

Yet, what you don't know, is that you don't know! :DVery true. This is why I never state things I am not sure about, or things that "I was told...".
Of course I will make mistakes every now and then, but this is not for lack of facts. Usually it's just plain stupidity.:p

If you mean wire sizes like 3, 4, 5, 6, 7, and so on:
Wire Sizes (http://www.tpub.com/neets/book4/11c.htm)

A few comments I have on this post:

Seems to me that this comment misses the point, purpose, and spirit of Ask Me Help Desk:



I hate to say it, but I think you are quite mixed up about all this and REALLY need to get things straight.

That is exactly why AMHD exists, to give someone a place to ask questions, and hopefully get a straight and accurate answer from an expert or someone knowledgeable about the topic, without being condescending or disrespectful.

Had I not been busy the last few days with an ice storm, a power outage and my son's wedding, I would have deleted this unnecessary comment promptly, but that opportunity has passed.

Shader has been diligent in asking for help with his ongoing project, while he may not know as much as a licensed electrician, he certainly has had the foresight to ask very good, detailed questions seeking accurate answers to help him complete his project properly.

Now onto other details that need clearing up:

MCM stands for Thousand Circular Mils, as in 250 thousand circular mils, now more accurately called KCMIL.

The American Wire Gauge ends at 0000, or 4/0, also called 4 Ought in some areas, and continues onto the actual KCMIL measurement of wire, rather than the wire gauge numbers assigned to wire sizes.

Table 250.66 of the NEC makes no reference to amperage or the rating of a service. A grounding electrode conductor is sized according to this table that lists the largest size service entrance conductor(s) installed.

If I have a 100 amp service, that I can use #4 copper, as allowed by Table 310.15 (B) (6), and Table 250.66 allows for #8 grounding electrode conductor, but for some reason I use #1, #6 copper wire must be used for the GEC.

The increase in GEC wire size is required due to the lower impedance of the larger service entrance conductor, and the resulting higher available fault current that can occur.

If you mean wire sizes like 3, 4, 5, 6, 7, and so on:
Wire Sizes (http://www.tpub.com/neets/book4/11c.htm)
Ok, but what does this have to do with my question?
Please note that #4cu is allowed. The reason I asked, is because some electricians do not know when to use the tables in subject.
Just trying to help! I rest!

Seems to me that this comment misses the point, purpose, and spirit of Ask Me Help Desk:

That is exactly why AMHD exists, to give someone a place to ask questions, and hopefully get a straight and accurate answer from an expert or someone knowledgeable about the topic, without being condescending or disrespectful.

Had I not been busy the last few days with an ice storm, a power outage and my son's wedding, I would have deleted this unnecessary comment promptly, but that opportunity has passed.
No. It seems you and the OP missed my intent of that statement.

Where did I say he should go elsewhere?
Where did I say he should not do this himself?
Where did I say he should not post questions?

ALL I said was he should get his facts and information in order before doing this work.
You call THAT "condescending or disrespectful"?????? You cannot be serious.

Man, why do folks have to read into things in order to find stuff that is not there.

Is that how you deal with things around here? A post that you ASSUME is "unnecessary" gets "promptly" deleted??
I am a mod of a couple of boards myself. If I acted all superior like that I'd get run out of town.



PLEASE go back and re-read that post. Heck, I even included a very good DIY link to help him in the right direction.

I know # 4 is good for 100 Amps, he was asking #3 or 3/0.
I have only seen #3 Rarely.
1,000,000 circular Mils = 1,000 kcmil's

Tk, thanks for the info. THe NEC has a lot of tables/info and just wanted to make sure I was using the right ones. Your grounding answer explained the "theory" behind different GEC/SEC combos, which is what I was looking for...