100A Service to Detached Garage

[col*klink]

I've read several threads about wire size, conduit size, rating, etc, and I think I've managed to convince myself I need to start another for clarification.... So:

Details:
I'm planning electric to a sub panel on my new garage (120ft). I'm certain that 60A (maybe even 40A) would be plenty, but I want to make sure that I have the cable to support 100A if needed.

I also plan to run CAT5, RG5, and a light switch circuit back to the house.

When I had the slab poured, I put in 2, 1-3/4" conduit so I wouldn't have to come through the outside of the wall.

I already decided copper is too expensive, especially when I can get it done w/ AL.

My questions are:
1. What gauge AL wire? I have figured 2/2/2 SEU for direct burial @ 2-ft.
2. Do I need a 4th for ground?
3. Will it fit through 1-3/4" Conduit?

I've see charts that tell me 2/2/2 is good for 90A.... and 120 for direct burial. I'm hoping that the latter is correct.

Thanks for the help! This site is awesome!

 

[Synergy]

Just know that AL may be cheaper, but using copper allows you to use 2 sizes smaller gauge for the same amps and it corrodes less than AL.

You also need 4 conductor cable: Hot, Hot, Neutral, Ground.

I borrowed this drawing from another site The 100 amp sub is fed from a 100A double pole breaker on the main panel. You can run a sub with or without a main breaker.

 

[bochnak]

http://www.westernextralite.com/resources.asp?key=23

http://www.westernextralite.com/resources.asp?key=47

 

[larry4406]

Regarding aluminum vs copper and the corrosion aspect - keep in mind that the power company's buried drop to your house is aluminum, at least it is today in Northern VA. Thus, the power company may well be the weak link. The copper will let you use a smaller diameter wire so possible benefit on conduit size.

I ran 4 conductor aluminum, can't recall size - Line 1, line 2, common, and ground all tied back to the house panel via 3 inch conduit. My detached garage was built per IRC 2000 and did not require ground rods at the detached structure. Make sure at the sub panel not to bond the ground and the common.

 

[Bevis]

I'm having a time grasping something. I'm doing a sub panel from the house (200a) to the garage (100a), maybe 60' apart.
Now for the panel in the garage, do I get a main breaker panel and use it as a sub, or do they make a sub panel?? that's got me kinda confused or my mind is just overloaded from work.

 

[sberry]

You are not installing a sub, you are installing a new service to the building. If is more than 6 spaces you need a main breaker. http://www.selfhelpandmore.com/home...o-dwellings/wiring-a-detached-garage-2002.php

 

[tfi racing]

Good link there,sberry27,it should be made a sticky and required reading before anyone asks an electrical question.A few rules and interpretations are slightly different than what can be done in Canada,but nothing that would get anyone into any trouble with the authorities.

 

[Charles (in GA)]

You are not installing a sub, you are installing a new service to the building. If is more than 6 spaces you need a main breaker. http://www.selfhelpandmore.com/home...o-dwellings/wiring-a-detached-garage-2002.php

As noted in another thread, NEC defines a service as that provided by the PoCo. If the garage panel is fed off of a house panel or disconnect fed from the house meter, the cabling to the garage is not a service, but rather a feeder.

Common sense tells me that there should be a disconnect at the garage subpanel or on the outside of the building, for simplicity and safety.

Charles

 

[Matti]

#2 Al or #3 copper is good around here for 100 amps up to 95' run IIRC. The stuff I used is 2 conductor with a concentric bare neutral which is another story. Typically you have to have a branch breaker to feed the garage panel i.e.60 amp breaker to the garage. If you run direct burial cable then you could use the conduit for the other cables. Perhaps 1" or 1 1/4" would suffice. It might help with the shielding.

 

[Bevis]

You are not installing a sub, you are installing a new service to the building. If is more than 6 spaces you need a main breaker. http://www.selfhelpandmore.com/home...o-dwellings/wiring-a-detached-garage-2002.php

That makes more sense to me now...thanks for the link too. Yep, it's gonna be more than 6 spaces.

 

[Charles (in GA)]

#2 Al or #3 copper is good around here for 100 amps up to 95' run IIRC. The stuff I used is 2 conductor with a concentric bare neutral which is another story. Typically you have to have a branch breaker to feed the garage panel i.e.60 amp breaker to the garage. If you run direct burial cable then you could use the conduit for the other cables. Perhaps 1" or 1 1/4" would suffice. It might help with the shielding.

Running more than three current carrying wires (neutral and two hots) in a single conduit, causes all wires in the conduit to be derated, thus necessitating larger cables in some instances.

#2 AL as USE-2 is good for 100 amps at 90c. However, there are no breakers rated at 90c. Connecting a 90c wire to a 75c breaker makes the wire a 75c wire, and at 75c the #2 AL wire is only good for 90 amps.

Indeed, #3 CU is good for 100 amps at 75C

See 310.16 of the NEC. 310.15(B)(6) is N/A to this as we are not discussing dwellings.

Charles

 

[Torque1st]

#2 AL as USE-2 is good for 100 amps at 90c. However, there are no breakers rated at 90c. Connecting a 90c wire to a 75c breaker makes the wire a 75c wire, and at 75c the #2 AL wire is only good for 90 amps.

Why would connecting a wire to a breaker change the insulation covering the wire or it's rating?

 

[Charles (in GA)]

Why would connecting a wire to a breaker change the insulation covering the wire or it's rating?

NEC 110.14(C)

Temperature Limitations. The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.

This is the weak link of the chain theory. The lowest temperature rating of the various components of the circuit, ie. wire, switch, receptacle, circuit breaker, etc. form the temperature rating for the entire circuit. You are, however, allowed to use the published higher temp/amp rating for adjustments such as high ambient temp, or more than three current carrying conductors in a conduit, etc.

Others may correct me, but this is what I understand it to be, as clearly written in the code.

Charles

 

[Torque1st]

NEC 110.14(C)

Temperature Limitations. The temperature rating associated with the ampacity of a conductor shall be selected and coordinated so as not to exceed the lowest temperature rating of any connected termination, conductor, or device. Conductors with temperature ratings higher than specified for terminations shall be permitted to be used for ampacity adjustment, correction, or both.

This is the weak link of the chain theory. The lowest temperature rating of the various components of the circuit, ie. wire, switch, receptacle, circuit breaker, etc. form the temperature rating for the entire circuit. You are, however, allowed to use the published higher temp/amp rating for adjustments such as high ambient temp, or more than three current carrying conductors in a conduit, etc.

Others may correct me, but this is what I understand it to be, as clearly written in the code.

Charles

Now that has to be a vague reading piece of code. It looks like the bold section in red allows a person to use the 90°C wire rating. I could not imagine a reason for changing the ampacity rating for a wire just because it is connected to a lower rated device as long as the connection point is rated for the ambient temperature in the area and the ampacity. The temperature rating on wires and devices is for the environment they are used in like inside a conduit to prevent insulation deformation and subsequent shorts. Otherwise there would be no use for ANY higher temp rated wire anywhere if it was limited by any breaker temp ratings in the circuit.

 

[Matti]

Running more than three current carrying wires (neutral and two hots) in a single conduit, causes all wires in the conduit to be derated, thus necessitating larger cables in some instances.

#2 AL as USE-2 is good for 100 amps at 90c. However, there are no breakers rated at 90c. Connecting a 90c wire to a 75c breaker makes the wire a 75c wire, and at 75c the #2 AL wire is only good for 90 amps.

Indeed, #3 CU is good for 100 amps at 75C

See 310.16 of the NEC. 310.15(B)(6) is N/A to this as we are not discussing dwellings.

Charles

Perhaps this applies for single wires. I used USEB which has the wires encased in a single jacket so it's rating takes into consideration that there are 2 hots and a neutral. Canadian Codes are likely different that the NEC anyway.

 

[Torque1st]

Canadian Codes are likely different that the NEC anyway.

Thank your lucky stars for small favors.

The NEC and UL standards are nightmares.

When I buy anything I look for the CSA and/or VDE labels rather than UL which I consider to be a junk standard or a standard for junk.

 

[JakeD]

You are not installing a sub, you are installing a new service to the building. If is more than 6 spaces you need a main breaker. http://www.selfhelpandmore.com/home...o-dwellings/wiring-a-detached-garage-2002.php

Very helpful link, thanks for this.

 

[col*klink]

Thanks for the info from all....

So, from what I read, I DO NOT need a grounding conductor between the house and garage based off of the following from the selfhelpandmore link:

Directly under the "Pictorial Example of a Garage Panel with 3 Or More Circuits 240 Volt 60 Amp, 100 Amp, 200 Amp Branch Circuit with Underground Conductors with an Equipment Grounding Conductor Ran with the Feeder Supplying the Detached Structure" Picture

"Special Note: Any detached structure that has an existing non-current carrying metallic path such as water pipes installed between the main structure and the detached structure must have an equipment grounding conductor installed with the feeders installed between the two buildings. Article 250-32-B-1 & 2"

I don't have any "Non current carrying conductors", so 3-wire should be fine...

I also heard the same from a local supply co... Why does this have to be so confusing?!? I understand that part of the reason is to keep the average Joe safe, but when you get several "pro's" with different interpretations....??

I left a message with the inspector... I'll try to remember to post an update with his 'ruling'

 

[Charles (in GA)]

Best thing you can do is run four wires, to include the grounding wire. More expensive? yes. But this allows you more flexibility in the future, as you may wish to install a "non-current carrying metallic path" between the house and garage, and not having that ground will preclude you from doing so. It also is additional safety to have it.

Non-current carrying conductors is considered by many to include low voltage or signal type wires, such as phone, coax, alarm, door bell, intercom, etc.

Its always best to spend money up front and save yourself hassles later on. I always look way ahead to determine what should be done now.

Charles

 

[Aceman]

My questions are:
1. What gauge AL wire? I have figured 2/2/2 SEU for direct burial @ 2-ft.
2. Do I need a 4th for ground?
3. Will it fit through 1-3/4" Conduit?

I've see charts that tell me 2/2/2 is good for 90A.... and 120 for direct burial. I'm hoping that the latter is correct.

Answers:

1. You cannot use SE/SEU cable underground. Ever. USE yes, SE no. I realize it's the same 3 letters changed around but they're different cable types with different limitations. I'd recommend USE.
#2 AL-90 amp
#1 AL-100 amp
#1/0 AL-120 amp
2. You need 4 wires.
3. Double check your conduit size, 1 3/4" isn't a standard size.

"Special Note: Any detached structure that has an existing non-current carrying metallic path such as water pipes installed between the main structure and the detached structure must have an equipment grounding conductor installed with the feeders installed between the two buildings. Article 250-32-B-1 & 2"

I don't have any "Non current carrying conductors", so 3-wire should be fine...

As stated above you need 4 wires. It's good practice, it's required under the 2008 code and you're creating other potential metallic paths between the building when you install the low voltage wiring.

If you want to use the USE-2 splice a 1' long piece of #3 alum with a but splice before it attaches to the breaker. I think you will be happier with the heavier wire instead so the lights will not dim as much when things like your compressor starts.

Check with your AHJ about the disconnect. Mine believes the 6 throw exception applies only to Service entrances and not detached garages.

Why would someone **** splice a wire in the panel before landing it on the breaker? That's not even mentioning the fact you want him to buttsplice a smaller #3 AL 75 amp rated wire onto a #2 AL 90 amp rated wire. Sure that's fine, but it's taking a step backwards because now your whole feeder has been reduced to a 75 amp rating.

You need a disconnect at a detached garage. Simplest way is to purchase a 100 amp main breaker panel.

Don't forget, it's always best to check with your inspector about electrical requirements as different cities often have different rules and amendments to the code.

 

[JBurgess]

Should have been #1 alum or #3 copper to use the 90 Degree rating on the #2 alum with a 75 degree breaker.

 

[sberry]

The panels in separate buildings are not really subpanels as far as code is concerned. They are the service equipment for the structure. You can have a subpanel from the service equipment in each building if you want. Between buildings, section 250.32 applies and if there are metal interconnections between buildings such as water piping, or air lines, or any metal interconnection at all, then you must install an equipment ground wire so if you have a 120/240 system, you would have four wires. The neutral would be separated in each building and a grounding bar would be installed, just like a subpanel. Then the code requires a grounding electrode conductor (GEC) to a grounding electrode (usually a ground rod). The GEC will be connected to the equipment ground bar at each building. This is not to clear overcurrent devices, this is for two reasons. One is lightning, the more important one is to put the equipment ground at the same relative potential as the earth. This is for step potential or touch potential voltages so that what you touch in the building is at the same potential as what you are standing on. Now the tricky part. If you do not have any interconnecting metal between buildings, the code allows you to install three conductors between buildings. When you do this you bond the neutral and ground the neutral just like a new service. Some inspectors think that every panel in a separate building must be treated as a subpanel, but this is not true. In past codes (1996 and older) these rule were in section 250-24 and Exception 2 addressed the grounding bus.

 

[col*klink]

This is great stuff!

Aceman... That's more like it! 3 questions, 3 answers!

So far, I have confirmed w/ an inspector (from another city), that '08 code requires 4 wires. He said that they deal with many issues because of the change.

Now for the type of wire... USE is acceptible (not SEU). This brings up more confusion (probably from trying to get answers from Menards). They went out back to cut me 140' of 2-2-4 UD cable. While I waited I got into a conversation with another gentleman who explained that UD cable was meant to run between the transformer and the meter... can't run it into a panel. Can anybody confirm? Needless to say, they have a 140' roll of remnant right now.

So... Where can I find 4-wire USE cable??? Seems like the 3-wire stuff is falling off the trees?! Can I run a bare #6 GND with the cable? My main goal here is to walk in to a store, tell them what I need, and be done with it.

Still waiting to hear from my local inspector... If he can tell me what I need, I guess that's as good as gold!

Thanks to all for the great input!!! I figure we'll get this ironed out soon!

btw... Oshkosh, WI... Getting cold.

 

[Charles (in GA)]

From Article 100 - Definitions
2008 NEC

Service - The conductors and equipment for delivering electric energy from the serving utility to the wiring system of the premises served.

Service Point - The point of connection between the facilities of the serving utilities and the premises wiring.

Service Equipment - The necessary equipment usually consisting of circuit breaker(s) or switche(s) and fuse(s) and their accessories, connected to the load end of service conductors connected to a building or other structure or otherwise designated area, and intended to constutite the main control and cutoff of the supply.

Feeder - All circuit conductors between the service equipment, the source of a separately derived system, or other power supply source and the final branch circuit overcurrent device.

Branch Circuit - The circuit conductors between the final overcurrent device protecting the circuit and the outlet(s).

 

[Charles (in GA)]

Funny, I have a 150 ft of USE-2, 2-2-2-4 AL on the shelf in my shop right now. Around here, they refer to the stuff as "mobile home" wire. it is on a spool, all four spiraled together (open, no jacket), designed for direct burial and other uses. It also has different classifications on the jacket other than USE-2. It was a remnant of a spool. Bought it real cheap from HD for a neighbor lady to run to an outbuilding to get rid of a second meter and ongoing cost associated with it. Just never have got around to doing it. Stuff is dang difficult to work with however, real stiff.

No idea right off hand what UD is. it is not mentioned in table 310.13(A) thru (E) which describes the different identifiers and specifications.

Charles

 

[dwilliams35]

What's with the fixation on cable? It's a whole lot easier to just run individual runs of THHN or similar: We've already established that there's conduit in place: it's just a matter of what fits in it at that point. You're digging a ditch anyway, and some PVC conduit in the ditch with THHN doesn't usually cost any more than DB cable: It'll be a lot easier to deal with than any cable, you can put just whatever conductors you come up with needing, and it's a lot easier to terminate. Once the aluminum corrodes in two in ten years, that conduit will sound like a really good idea.