Subpanel to Main panel wiring

[davidforhim]

I've installed a subpanel for my attic renovation project, 100A service. I have XHHW-2 2AWG aluminum cable to connect with.

Connecting the hots to the main is done through a 100A breaker, but how do I connect the neutral and ground?

Do I split up the "surrounding" neutral wires into bunches that will fit into the holes in the neutral bus bar in the main?

Can I connect a ground cable from the ground bus in the sub to the ground bus in the main? Is a certain size wire required for this?

David

[mdshunk]

I've installed a subpanel for my attic renovation project, 100A service. You say "service". This word has a very specific meaning, and I don't think that's what you intended to say. Do you mean that you fed this subpanel at 100 amps, to make it a 100 amp subpanel? The word "service" refers to the size of the main wiring entering the dwelling.

I have XHHW-2 2AWG aluminum cable to connect with.I seriously doubt that you have XHHW-2. XHHW is single, insulated concuctors. The conductors inside of the cable that you're using may say XHHW on them, but the jacket of the cable likely says something like "TYPE SE". You can't wire up a subpanel with single, insulated conductors unless you put them in conduit, or unless they are the consist of a manufactured cable.

Connecting the hots to the main is done through a 100A breaker, but how do I connect the neutral and ground? The neutral and ground wires may indeed be too large for the holes in the ground bar. Typical ground bar holes will normally only accomodate up to a #4 gauge conductor. You'll need to get a "ground bar lug kit" which is a lug that bolts on top of the ground bar in place of 2 or 3 of the ground bar screws. These are available from the manufacturer of your panel, and are normally a stock item at an electrical supply house. Don't fray out the large cable to go into 2 or 3 holes. This is a flaming voilation.

Do I split up the "surrounding" neutral wires into bunches that will fit into the holes in the neutral bus bar in the main? No, see above comments.

Can I connect a ground cable from the ground bus in the sub to the ground bus in the main? Is a certain size wire required for this? HUH? You mean the ground wire is not in the subfeed cable? Perhaps you bought the wrong type of cable. If you purchased a cable that has two insulated hot wires with the neutral concentrically wrapped around the hot wires, you got the wrong stuff. You need "100 amp subfeed cable", which has two hots, a neutral, and a ground wire all in the same cable. The ground and neutral both originate on the same bar in the main panel, but each terminate on their own bars in the sub panel.

[davidforhim]

You say "service". This word has a very specific meaning, and I don't think that's what you intended to say. Do you mean that you fed this subpanel at 100 amps, to make it a 100 amp subpanel? The word "service" refers to the size of the main wiring entering the dwelling.Correct, I am using a 100A rated panel and plan to power it through a 100A breaker in the main panel.

I seriously doubt that you have XHHW-2. ... but the jacket of the cable likely says something like "TYPE SE".As you surmised. Here's what's on the outside sheathing "E39723 S AA-8000 AL TYPE SE CABLE STYLE U XHHW-2 600V 2 CDRS 2 AWG 1 CDR 2 AWG"

You'll need to get a "ground bar lug kit" which is a lug that bolts on top of the ground bar in place of 2 or 3 of the ground bar screws.Will do.

HUH? You mean the ground wire is not in the subfeed cable? Perhaps you bought the wrong type of cable. If you purchased a cable that has two insulated hot wires with the neutral concentrically wrapped around the hot wires, you got the wrong stuff. You need "100 amp subfeed cable", which has two hots, a neutral, and a ground wire all in the same cable. The ground and neutral both originate on the same bar in the main panel, but each terminate on their own bars in the sub panel.I got the wrong stuff.
Unless there's a bus I can't see, my main panel has a separate bar for ground and neutral which don't appear to be connected. So would I connect the sub-ground to main-ground and sub-neutral to main-neutral in this case?
About cable - Looking at the article "Calculation of Conductor Ampacity" on homewiringandmore.com, I tried to figure what I need. Unfortunately I only have NEC1996 on hand so I couldn't find NEC Article 310.15.B.2.A. The cable will run from the sub (located in an attic space) to the main (in the attached garage) through a wall for a total length of less than 10 feet. If I use XHHW-2 conductors, these are 90C rated. This must be derated by 0.91 if ambient in the attic stays at or below 104F. I don't have NEC Article 310.15.B.2.A, but based on the article I don't think I need to derate based on this. Correct?
So using the Raceway/Cable/Earth table, I think I can use either 1AWG ALU (115A*.91 = 104A) or 3AWG copper (110A*.91 = 100A). The Free Air table comes out to 4AWG ALU and 4 Copper. Which table should I use?
Is this correct, or assuming a balanced load on both phases, can I halve the 100A to 50A required on each phase? In other words, does the 100A breaker mean each phase can source 100A, the total of both phases can source up to 100A or each phase alone can source up to 50A?

Am I headed in the right direction here? Thanks a bunch for your help.
David

[suemarkp]

I'm not so sure your "main" panel is a main either. A main panel has the neutral and ground bars connected somehow, usually with a green screw involved. If you don't have that, you may have an outside disconnect where this is the case. One you're past the main panel, neutral and ground must be kept separate within the same building. The attic panel must be fed with 4 wires (3 of them insulated) regardless. Matching ground to ground and neutral to neutral would be the safe thing to do in your "main" panel, and then it won't really matter if its a main or not.

Your wire size is another issue. While it is rated to 90C, the panel lugs usually are not. So you can't exceed te 75C ampacity. This will allow for some hot ambient temperature derating (this can be applied to the 90C ampacity) which I think you should expect in an attic.

Normally, you use table 310.16 for 3 insulated wires in cable or raceway. This one is safe if you stick to the 75C column. However, in a dwelling, you can use table 310.15(B)(6), but some places won't allow you to apply it to a subpanel feeder. This table says #4 copper or #2 aluminum for a 100A feeder. Table 310.16 requires the next size up for each metal type (#3 copper or #1 aluminum). In a hot attic, I'd definitely go with the larger wires.

Finally, your 100A feeder will provide 100 amps from each leg. This is 200A worth of 120V loads, or 100A of 240V, or some combination in between. Generally, you can run about a whole house off of a 100A feeder unless you have 40A to 50A equipment (ranges, electric furnaces, AC compressors, etc).

[davidforhim]

When using ALU conductors for my main panel to sub panel connection (2 hots, 1 neutral and one ground), do I need to coat the conductors with some material to protect from oxidation at each point of connection?

Specifically-
1) Into the bus bars in main (both ground and neutral)
2) Into the breaker in the main
3) Into the terminal lugs in the sub (both hots and neutral)
4) Into the ground bus bar in sub

David

[suemarkp]

Yes to all four. And apply it immediately after stripping the wire and running a wire brush over the freshly exposed wire.

[bobm]

>And apply it immediately after stripping the wire and running a wire brush over the freshly exposed wire.<
What is the name of this product and will instructions be on the can?

Thanks bob

[mdshunk]

Some brand names are Noalox, Penetrox, and Ox Gaurd.

It goes by the trade slang "wire grease", "de-ox", and "goose ****".

Since this stuff if specially forumlated to keep aluminium from oxidizing, it is of absolutely no value when placed on copper wire. It MUST be placed on all aluminium wire terminations for a long lived connection.

If you choose to wire brush the connection, it should be with a stainless steel bristled "toothbrush" that is commonly used to clean aluminium connections before TIG welding. Don't use a steel or brass bristled brush. I forget exactly why, but I had a fellow give me an elaborate explanation why one time and I was compelled to only use stainless steel bristled brushes.

[Ohm1]

Should have posted this in the electrical forum

[polaris]

If you choose to wire brush the connection, it should be with a stainless steel bristled "toothbrush" that is commonly used to clean aluminium connections before TIG welding. Don't use a steel or brass bristled brush. I forget exactly why, but I had a fellow give me an elaborate explanation why one time and I was compelled to only use stainless steel bristled brushes.

"Brushing Stainless Steel or Aluminum: Use of carbon steel wire on stainless steel or aluminum will contaminate the surface and leave an "after-rust" effect. To prevent "after-rust", always use a stainless steel wire on stainless steel, and use stainless steel or aluminum wire on aluminum."

"...the softer the material to be brushed, the more likely that a steel wire brush will cut into the material and remove base material in addition to the targeted coating or contamination."

Source: http://www.spiralbrushes.com/steel-wire.html
* I'm not recommending using a power tool to brush aluminum wire.