I am installing a 100 amp sub panel in a newly constructed detached garage. I want to run the wiring from the 200 amp main panel to the garage through the floor joists of my basement (about 50') and then through conduit to the garage (about 20'). I have a copy of the NEC codes and I believe that 2-2-2-6 copper is appropriate for this application. I was just told by a local reputable shop that 2-2-2-4 SER aluminum will work.
What gauge and class of cable do I need for this run? (100 amps about 70' run through the house and then through conduit) Thanks in advance.

You are going at this backwards.

1 st. step is to decide on the connected load and then the "Calculated Load". The Calculated load determines the sizing of the conductors.

For example, NEC 2008 - Tale 310.15 (60C) you will need #1 AWG for the hot conductors. You may be able to downsize your neutral depending on the types of loads in the garage.

If you want to use Aluminium, then the size would be 1/0.

You must run 1/3 cable from the main panel to the sub panel. At the sub panel, you will need to install at least 1 ground rod and connect a Bare #6 conductor from the rod to the ground bus in the sub panel. Ground from the service panel will connect to the ground bus. Neutral from the service panel will connect to the Neutral bus. The Neutral bus MUST NOT BE BONDED TO THE SUB PANEL.

The two hot conductors will connect to the two phase bars.

Since the distance is 70' +/- you will not be affected by voltge drop.

Don, Why is Table 310.15 being used? Is that a typo?

And why are you using the 60 Deg C column?

Lastly, do not discuss reduced neutrals with laypeople.

Fljo, even thou the Code will not tell you this, #2 Al SER cable will most likely be allowed.

Check with your local inspector on this.

Certainly the #2 copper SER cable would be more than adequate.

Quote:

---

Originally Posted by tkrussell

Don, Why is Table 310.15 being used? Is that a typo?

And why are you using the 60 Deg C column?

Lastly, do not discuss reduced neutrals with laypeople.

Fljo, even thou the Code will not tell you this, #2 Al SER cable will most likely be allowed.

Check with your local inspector on this.

Certainly the #2 copper SER cable would be more than adequate.

---

Is the SER rating appropriate for use inside the house and buried in conduit?

Does not matter where, in general, where the SER is located.

I certainly would not pull SER through buried conduit, unless the conduit is grossly oversized, even then, I would not.

Quote:

---

Originally Posted by tkrussell

Does not matter where, in general, where the SER is located.

I certainly would not pull SER thru buried conduit, unless the conduit is grossly oversized, even then, I would not.

---

NEC 2008 338.12 A2 states:

SE cable cannot be used underground with or without a raceway.

Does this apply to SER cable and/or a conduit underground?

SER cable is SE cable.

Hi:
I just talked to one of the Inspectors here. This is a Washington State Inspector. Not local They are available to answer questions from 8:00AM to 8:30AM which is better than nothing. He said (which is all that really matters) that I can run the SER 2-2-2-4 Al cable through the floor joists of my basement AND through buried conduit if I wanted. He suggested (which I will probably do) to put a junction box where the conduit starts and pull individual cables from there to the garage. He also said that he would only rate the SER 2-2-2-4 Al with a 70' run at 80 amps at the main panel which is fine for me.
Thanks for the help. The project is progressing.

T.K.

Yes, T.310.15 was a typo. T. 310.16 was the table I was referring to.

I used the 60c column because I had no information on the connection design.

I know that most panelboards are listed for 60 or 75c. But since I did not specifically know, I chose to default to the 60c column.

Can you pm and tell me why we don't discuss reducing neutral.

As it is right now, we are studying Article 230 and I am having fits over the "Largest Motor" formula. I opened a post about that.

All terminations in standard everyday distribution equipment will be rated 75 Deg C.

Assuming 75 Deg C as a min will be fine.

As far as discussing reduced neutrals, we can do it here.

Before just reducing a neutral, calculations should be done.

It is allowed, but, this topic should be kept to professionals to deal with, not DIY'ers.

There are a few specific situations that reducing a neutral is permitted, I chose to not get into that discussion with laypeople.

They can easily misunderstand, and be installing smaller neutrals when not permitted, or too small than permitted.

For the purpose of this forum, I believe this is best.

Quote:

---

Originally Posted by donf

You are going at this backwards.

1 st. step is to decide on the connected load and then the "Calculated Load". The Calculated load determines the sizing of the conductors.

For example, NEC 2008 - Tale 310.15 (60C) you will need #1 AWG for the hot conductors. You may be able to downsize your neutral depending on the types of loads in the garage.

If you want to use Aluminium, then the size would be 1/0.

You must run 1/3 cable from the main panel to the sub panel. At the sub panel, you will need to install at least 1 ground rod and connect a Bare #6 conductor from the rod to the ground bus in the sub panel. Ground from the service panel will connect to the ground bus. Neutral from the service panel will connect to the Neutral bus. The Neutral bus MUST NOT BE BONDED TO THE SUB PANEL.

The two hot conductors will connect to the two phase bars.

Since the distance is 70' +/- you will not be affected by voltge drop.

---

Hi again:
Getting close to finishing this project. Questions about grounding now related to above statements.
On my main panel in the house the neutral and ground are bonded. I understand that I cannot do this on the sub-panel in the detached garage. The neutral bus in the sub panel must be isolated from the sub panel and the ground bus. The ground bus is attached to separate grounding rods at the garage.
To what in the sub panel do I connect the grounding wire that I ran from the main panel. The wire I ran was SER 2-2-2-4 Al with the #4 as a ground. Does the #4 ground simply connect to the neutral bus in the sub panel in the garage?
Thanks

NO!

Since this is a separate building, you must install a grounding system at this building.

You MUST install a ground rod at the new building and connect the EGC from the sub panel to ground rod.

If the resistance from the ground rod to earth is 25 Ohms or more you must install a second ground rod no closer than six feet from the first ground rod and bond the two ground rods together.

Neutral has to remain isolated from ground in a sub panel. That means, do not connect any neutral conductors to the ground bus and absolutely no ground conductors connected to the neutral bus.

Quote:

---

Originally Posted by donf

NO!

Since this is a separate building, you must install a grounding system at this building.

You MUST install a ground rod at the new building and connect the EGC from the sub panel to ground rod.

If the resistance from the ground rod to earth is 25 Ohms or more you must install a second ground rod no closer than six feet from the first ground rod and bond the two ground rods together.

Neutral has to remain isolated from ground in a sub panel. That means, do not connect any neutral conductors to the ground bus and absolutely no ground conductors connected to the neutral bus.

---

Thanks for the quick reply.
I will go with two grounding rods. The reason that I asked the question is because the neutral and the ground are bonded at the main panel. I do not really see what difference it makes having them not bonded at the sub panel 75' away. I would like someone to explain that to me but that is OK.
All that matters is that it meets code and the inspection is approved.
Thanks again.

The neutral is only grounded at the Main Service Disconnecting Means to create a single point of grounding for the system neutral.

Once a feeder leaves the Main, the neutral is considered as a current carrying conductor, and is treated as such.

Equipment grounding conductor is not current carrying, under normal conditions, and is reserved solely for the purpose of fault currents.

Quote:

---

Originally Posted by tkrussell

The neutral is only grounded at the Main Service Disconnecting Means to create a single point of grounding for the system neutral.

Once a feeder leaves the Main, the neutral is considered as a current carrying conductor, and is treated as such.

Equipment grounding conductor is not current carrying, under normal conditions, and is reserved soley for the purpose of fault currents.

---

Slightly different subject but realted.
I am going to install a 50 amp 240 volt socket in the detached garage for later use if I get some welding gear. The socket has two live wires and what I was told was a ground. Does the ground connect to the ground bus or the neutral bus in my sub panel?
Thanks in advance.

NEVER connect a Ground conductor to a Neutral Bus in a sub-Panel! Ground and Neutral must be isolated from each other.

If it is a straight 240 VAC welder, than all you need is the two hots and a ground.

Neutral is not required. If you needed 120/240 VAC at the welder, than you would need to supply the Neutral.

Quote:

---

Originally Posted by donf

NEVER connect a Ground conductor to a Neutral Bus in a sub-Panel! Ground and Neutral must be isolated from each other.

If it is a straight 240 VAC welder, than all you need is the two hots and a ground.

Neutral is not required. If you needed 120/240 VAC at the welder, than you would need to supply the Neutral.

---

Thanks for the quick reply.
Your answer is what I figured from the information that I had. I just wanted to verify.

You will need to purchase and install a ground bar kit for your sub-panel. Square D part # would be PK-15 for a 15 space ground bar. That bar will be attached to provided areas in the sub-panel with a self threading (not sheet metal or "Tek" screw).