Going Goofy....Is 2-2-2-4 OK for 100 Panel?

[Matt M, PA]

Here's the deal...we have constructed a garage about 100 feet from our house.

I do have a 220v compressor, blast cabinet, and one day would like heat, maybe A/C, etc in this garage, so we decided 100amp was the right thing. I did not want to be short in the future.

The house has 200amp service, and I can add a 100amp breaker to feed this new garage panel.

Here's the dilema. The inspector said I should use 2-2-2-4 for this panel, and I got some 2-2-4 UDR, as well as the same length in 2 USE to make up what we need. (Aluminum)

Now, I have a few friends (who likely know better than I) who say I should be running 1 to make up for the drop with this length of cable.

They are coming to trench on Friday....so I can go back to the electric supply house on Friday morning and hopefully swap this out...or is the 2 okay?

I'm not sure how much loss I will have in 120 (or so) feet.

Thanks!!!

[sberry]

This all assumes the wire will be fully loaded, unlikely in a small garage. You probably never use more than 60A and they make 70A breakers.

[Matt M, PA]

Perhaps I should add that this garage is 32x26 (seen in another post), and I have installed (but could certainly change as no wiring is done) a 100amp panel with main 100 amp breaker.

I was planning on spltting up the breakers pretty well, overhead lights, one wall's outlets, back wall's outlets, etc. So, I liked the idea of more slots in the panel.

I look forward to comments...I'm rreally against the wall here.

[Charles (in GA)]

The size of the panel and its main breaker really won't matter. You are using the 100 amp main breaker as a disconnect in this panel, and it will (might) protect against major shorts in the panel itself. The key is what you use in your house panel to supply the 2-2-2-4 to the shop, it is the one protecting the cable.

Depending on your panel brand, you can get virtually any size breaker, and I'm thinking you should use something on the order of a 90 amp unit in your house panel.

Charles

[Stuart in MN]

Here's a voltage drop calculator: http://www.csgnetwork.com/voltagedropcalc.html This is just one of many out there, but they all work about the same. Enter your parameters (normal conditions, aluminum wire, 2AWG, 240vac single phase, 120 feet, 100 amps,) and it calculates a voltage drop of 2.3%. You want to keep voltage drop at 3% or less, so 2AWG aluminum wire is not a problem.

However, it sounds like you're wanting to run two cables in parallel, to get the proper number of conductors? You better check to see if the inspector will allow that, I'm not sure it's kosher.

[Aceman]

The house has 200amp service, and I can add a 100amp breaker to feed this new garage panel.

Here's the dilema. The inspector said I should use 2-2-2-4 for this panel, and I got some 2-2-4 UDR, as well as the same length in 2 USE to make up what we need. (Aluminum)

Not UDR, URD.:)


Now, I have a few friends (who likely know better than I) who say I should be running 1 to make up for the drop with this length of cable.

I'm not sure how much loss I will have in 120 (or so) feet.

Thanks!!!

I calc'ed it: #2 al @90 amps=2.3%
#1 al @90 amps=2.9%

Your under 3%, so you're good to go.

I'd stick with the #2 along with a 90 amp breaker. Code says 90 max unless local amendments allow 100.

Here's a voltage drop calculator: http://www.csgnetwork.com/voltagedropcalc.html This is just one of many out there, but they all work about the same. Enter your parameters (normal conditions, aluminum wire, 2AWG, 240vac single phase, 120 feet, 100 amps,) and it calculates a voltage drop of 2.3%. You want to keep voltage drop at 3% or less, so 2AWG aluminum wire is not a problem.

I manually calc'ed it and my numbers don't match yours. I get 3.2% at 100 amps with #2 al.


However, it sounds like you're wanting to run two cables in parallel, to get the proper number of conductors? You better check to see if the inspector will allow that, I'm not sure it's kosher.

He's adding the required 4th wire to the 3-wire cable assembly by using a single USE conductor.

[Stuart in MN]

I manually calc'ed it and my numbers don't match yours. I get 3.2% at 100 amps with #2 al.

I went back and double checked the results from that calculator, and it actually came up with 2.7% (I don't know how I mistyped it the first time around.) I ran the numbers through several other online calculators, and some of them also came up with 2.7% while others came up with 3.2% like you calculated. I'm not sure what's going on there.

Also, I double checked my code book and #2 AL is indeed rated for 90 amps (at 75 degrees C) so you're right, a 90 amp breaker is as big as you should go with that size wire. That's what I get for not verifying things before I type...I rarely deal with AL wire, so I'm not as familiar with its ampacities as I should be.

[Matt M, PA]

Thanks guys...I really want to make sure I'm doing this right. I'm okay with re-buying this run of cable in another size should it be needed.

If this is not going to be enough...I'll bite the bullet and get the #1 wires first thing Friday morning...I only want to do this once. But, if what I have will do the job...well...at least I'm done spending.

[Charles (in GA)]

I didn't want to get into the argument about 90°C vs 75°C wire and which rating you should use based on the "weak link in the chain", so I just suggested the 90 amp breaker and left it at that.

If you take a 90°C wire and connect it to a breaker rated 60°/75°C like virtually all are (this is the weak link in the chain), then the wire has to be considered to be 75°C wire, which is only good to 90 amps in this case. The only way to take advantage of the higher amp rating of the 90°C rated wire is to drive the temp above 75°C due to loading, and this over temps the lower rated breaker and other connections. You ARE allowed to use the higher temp rating/higher amp rating of the wire for calculating the de-rating of wire due to high ambient temp (above 86°F/30°C) installations (attics, hot climates, etc), and this is in large part, the advantage of having the higher temp rating wire, as virtually any wire you run in a non-air-conditioned shop or building, especially in the south, will exceed the 86°F limit and require derating. In cases like #12 THHN or NM-B Romex which is also a 90°C wire, which are good for 30 amps, you can derate them, say, for an attic installation which can easily go to 150°F/65°C you would use a de-rate factor of .58 and get 17+ amps, so you still have a 15 amp wire. If you used older NM (not -B) which is I think, 75°C rated you would use a de-rate factor of .33 and end up with a rather useless 9.9 amps. This is why the code has grown more sophisticated and detailed and at the same time, products such as wire insulation, have grown better in quality and operating extremes.

How did I get this far off track for an amateur?

Charles

[Aceman]

Thanks guys...I really want to make sure I'm doing this right. I'm okay with re-buying this run of cable in another size should it be needed.

If this is not going to be enough...I'll bite the bullet and get the #1 wires first thing Friday morning...I only want to do this once. But, if what I have will do the job...well...at least I'm done spending.

Gaining 10 more amps by going to #1 isn't worth it IMO. I think 90 amps will suit you just fine.

[sberry]

Gaining 10 more amps by going to #1 isn't worth it IMO. I think 90 amps will suit you just fine. I agree with Ace on this. I haven't seen many small garages or shops being underpowered with 100A service. Seen a lot of them make it on 60. The only real advantage with bigger wire is not really for the "10 amps" but more to do with voltage drop when and if this wire has significant load, example, all the lights running, AC on hi welding and the air comp kicks on, really for motor starts. I know Warren at the self help site doesn't rally consider a garage to be a significant added load to a residential service, I would tend to agree.
Its one of the reasons there is some minor bickering about the allowed use of a 100A breaker on this wire, its allowed for service entrance due to the nature of the load, garage would be similar. Its not like a dedicated circuit to a continuous piece of equipment, like the loads in a house they come on and off, probably more so in garage than residence where there may be AC, cooking, electric heat or water heater. Unless you have electric pottery kiln out there very few sustained loads in a home garage.