3/3/4/6 for 100 amp feed?

[cory58]

I'm building a 30x50 garage and was speaking to the electrician who did the basic install to get the permit closed (meter, 200a main panel, 2 lights, 2 outlets). I'm installing a 100a sub-panel on the opposite side of the garage from the main panel (approx. 90 foot run), and asked him about the feed wire. The electrician said he would run 3/3/4/6. From my research, I thought 3/3/3/8 would be correct for this feed. I wouldn't mind running a smaller neutral wire, but also don't want to undersize the ground.

So what do the GJ experts think?

Thanks, Cory

 

[sberry]

You can use 8 for the ground with 3 and 100, right now am having a brain fart and don't recall the rule for the neutral here.

 

[pattenp]

If using copper SER it will be 3-3-3-5 or aluminum is 1-1-1-3 for 100A.

 

[cory58]

If using copper SER it will be 3-3-3-5 or aluminum is 1-1-1-3 for 100A.

Sorry for showing my ignorance. I was planning to use individual conductor stranded THHN wire. That is not the same as SER, correct?

 

[tyme2par4]

Sorry for showing my ignorance. I was planning to use individual conductor stranded THHN wire. That is not the same as SER, correct?

Correct. THHN would be the way to go if you are going underground in conduit. SER is not rated for underground use.
SER is a bundled conductor with nonmetalic jacket similar to NM-B.

 

[cory58]

Correct. THHN would be the way to go if you are going underground in conduit. SER is not rated for underground use.
SER is a bundled conductor with nonmetalic jacket similar to NM-B.

Thanks. This feed is all within the garage. I'm running the feed in EMT from the main panel within the garage to a sub-panel on the other side of the garage.

Cory

 

[pattenp]

Running EMT is fine. The SER is another option if you don't want to run EMT.

 

[wyliesdiesels]

The cost difference between EMT and THWN vs. no conduit and SER is probably a wash when u factor in both materials and labor.

If u would rather not run conduit, then go with the SER...

 

[matt_i]

The economics might be better to buy a 500' spool of the #3 CU THH/WN, run 4 conductors and then craigslist the rest. But, you can figure that out pretty easily. I ran a #2 ground for same reasons although I had to buy 2 lugs to attach to the ground bars since they don't have a hole size appropriate for #2.

What size conduit are you running?

 

[cory58]

I'm using 1.5" EMT, half of which I've already installed.

Thanks for all the responses! I'm planning to run THHN wire. Any more thoughts on whether I can run #4 for the neutral, instead of #3?

Cory

 

[pattenp]

Technically you are to do load calculations for the neutral to show that a reduced neutral can be used. Personally I'd forgo the reduced neutral and just use #3 for all the current carrying conductors, that way there's no question down the road. The #3 should not cost that much more than the #4 for just the one wire. Maybe $20 more?

 

[bjcouche]

If it were me, I would use use 4 individual THWN/THHN wires in conduit just as you are proposing. However, the electrical supply house by me sells THWN ALUMINUM individual conductor 2awg cable. Thus I would pull 2-2-2-4 aluminum and breaker it at 90A on the feed breaker. It's doubtful you'd ever know the difference between 90 and 100A and my local supply house doesn't stock 1awg THWN aluminum. They only have it in black, so you'd have to put white and green tape on the neutral and hot at each end. This wire is made by Southwire and is of the simpull family meaning it comes PRE lubed. The conductors are also "compact" meaning that the strands are squished together, and actually this 2awg aluminum is about the same OD as the same ampacity non compact copper wire.

http://www.southwire.com/documents/Aluminum_SIMpull_THHN_black_revised_11-08.pdf

No, I don't sell the stuff, but I use it whenever I have to run wire in conduit larger than 6awg. The cost savings is there, and being pre lubed does help. For those familiar with direct burial mobile home cable, this is NOT the same stuff, it's way different.

Brian

 

[Bert_]

I would not hesitate to use a reduced neutral. But in my area it is generally accepted to reduce 1-2 sizes before anyone will request a calculation.
From a practical standpoint it would be impossible to overload a neutral reduced by this amount. You would have to have 100A worth of 120v loads all on one phase with nothing on the other. It just won't happen.

If it were me, I would use use 4 individual THWN/THHN wires in conduit just as you are proposing. However, the electrical supply house by me sells THWN ALUMINUM individual conductor 2awg cable. Thus I would pull 2-2-2-4 aluminum and breaker it at 90A on the feed breaker. It's doubtful you'd ever know the difference between 90 and 100A and my local supply house doesn't stock 1awg THWN aluminum. They only have it in black, so you'd have to put white and green tape on the neutral and hot at each end. This wire is made by Southwire and is of the simpull family meaning it comes PRE lubed. The conductors are also "compact" meaning that the strands are squished together, and actually this 2awg aluminum is about the same OD as the same ampacity non compact copper wire.

http://www.southwire.com/documents/Aluminum_SIMpull_THHN_black_revised_11-08.pdf

No, I don't sell the stuff, but I use it whenever I have to run wire in conduit larger than 6awg. The cost savings is there, and being pre lubed does help. For those familiar with direct burial mobile home cable, this is NOT the same stuff, it's way different.

Brian

I agree 100%. In many situations aluminum will perform just as well as copper, with a substantially lower price tag.

I have mostly used XHHW since it is stocked more around my area. Probably not as slick as THHN but, as you said, it will pull into conduit much easier than a twisted cable that is intended for direct burial.