Garage Sub-panel Wiring Confusion - Please Help?

[salvatore giordano]

Hello all – I’m looking for some clarification on wiring my newly-built detached garage.


At the outset, I’ll say I am not a complete novice with electrical, as I wired half of my house myself and installed an automatic standby generator myself as well. I’ve done a lot of minor electric work, though never major stuff like service entrance work with large aluminum wire. My setup is as follows:


I have 200 amp service in my house (2 main panels), with plenty of empty spots for circuits and breakers. I would need a little over 100’ of wire to connect the main service panel in the house to the subpanel in the garage. I already have 1 ¼” grey pvc conduit buried between the house and the garage site. I do wish now I have gone for something bigger, depending on what size wire I end up having to pull. AND, for what it’s worth, it is considered a farm building by my county, so it will not get inspected (which does NOT mean I want it to be unsafe, however!)


The garage is going to have two garage door openers, some general use receptacles, some lights, and a workshop. The biggest load I’d have in the workshop would be a compressor (just larger than a pancake one, but NOT one of the big stand-up ones) and a table saw (not a real industrial one, basic Lowes model). Only one or two tools would be used at the same time – no welders or anything like that.


My plan was to install a 60-amp breaker in my home’s main service panel, a 100-amp subpanel in the garage, and to connect to the two with 6-3 copper wire (NM-B/Romex). In reading about the question of a ground wire, it seems that my sub-panel in the garage should be the SE-R type and have a separate ground and ground rod, right?


My questions are about 1) the breaker in the house main panel, 2) the sub-panel in the garage, and 3) the most confusing part of it all seems to be the wire. I’ll start there.


1) Will 6/3 copper wire suffice for this project? AND, how much of a bear will it be to pull 100’ of that through the 1 ¼” conduit?

2) I am very confused about THHN wire – are they copper or aluminum or either? Why would I choose THHN over either NM-B/Romex or aluminum?

3) If I needed something more than 6/3, should I consider aluminum? If I went with aluminum would I need #2, #4, or #6? If it’s being fed from a 60amp breaker in my main panel, it seems like I could use #4 wire (smaller and easier to pull I imagine)

4) If I really needed to, I could dig and use Direct Burial Romex if that makes the process easier (although it seems they also make direct burial aluminum).


It’s really the wire that there seems to be a lot of debate about. Perhaps 6/3 is too small?


Any help greatly appreciated!

 

[bud16415]

I’m not a pro and they should be along shortly.


The calculators show 240v, 60a with 3% voltage drop over 100’ to require #4 copper #3 aluminum.


You don’t want to try pulling Romex and it would not be to code even if you could get it thru your conduit.


I believe you want individual #4 THHN as to the bonding and the secondary ground rod I will let the pros explain.


I have pulled a few long runs like this and it can be a job. Did you put any pull rope in the conduit yet. I have blown a string thru with air and also sucked a string thru with a vacuum, then pull a stronger cord or rope to pull with. They also have lube you coat the wires with by the person feeding them in.


That’s my early advice stick around for the pro version.

 

[salvatore giordano]

Bud16415 - I do have a pull rope in there now, so that's good!

 

[Michael Armstrong]

This is one of the most confusing and discussed topics in electrical work. I am also not a pro, and even if I were I'd consult with my local AHJ (inspector) to get his or her blessing. But what I'd start with is (3) #4 copper THHN conductors for the 2 hots (black or red) and neutral (white), and #10 copper THHN for the equipment ground (green). At the garage, I'd wire the hots and neutral normally, drive 2 ground rods at least 6' apart, and connect the equipment grounds to them. I would NOT bond the neutral bus to ground at the garage. BTW, I hope you buried your conduit at least 18" deep.

Your mileage may vary...

 

[bud16415]

The last project I did involving pulling THHN wires was my hot tub install. When I went to buy the wire I was planning on doing what @MichealArmstrong suggests and going with color coded wiring. When I started pricing the per foot cost in doing that compared to buying a roll of just black the price was much higher along with buying a roll I had a lot of left over wire. I bought the black and marked the wires with color tape on each end. I used the larger wire as the green ground as well.


It is nice when a commercial installer does this work as they have spools of each on a pipe frame and they can attach and pull different colors and sizes easily. I had to string out my lengths one at a time and cut them then recoil them back up as a bundle after marking them.


At a buck a foot or so it might be worth doing it that way though. Just an idea.


It is also a good idea to at least talk to the AHJ as mentioned above even though you are sure you wont need an inspection. It can’t hurt as rules change.

 

[hornetd]

If you have not purchased your Garage panel yet then let me make a suggestion even though it may cost a few more dollars. Buy a "Main Lug Only" (MLO) panel for the garage, Install a 60 Ampere Breaker with a breaker hold down kit as the main breaker, and install the largest breaker that the size of the feeder conductor allows in the house panel. If you use the #4 American Wire Gage (AWG) that bud16415 has suggested you can install an 80 Ampere Breaker in the Service Equipment panel at the house to protect the feeder to the detached garage. You would do this because it insures that, when you are working in the garage, if you overload the supply from the house, that the main breaker at the garage will open on that overload prior to the Feeder breaker at the house. You then can reduce the load, reset the garage main breaker, and resume working without the hike to the house.

The panel you install at the garage will be the Building Disconnecting Means. It must be listed as Service Equipment by an electrical testing laboratory. Do not install the green screw, copper strap, or aluminum jumper that serves as the main bonding jumper in the Garage panel. If a main bonding jumper is preinstalled you must remove it. Those bonds are to connect the Neutral Conductor of the Supply to the Equipment Grounding Conductors of the supplied wiring, and the Grounding Electrode System of the garage. That is not permissible under any recent addition of the National Electric Code. The reason is that the neutral currents of the supply must be kept on the insulated neutral conductor of the feeder to avoid creating possibly dangerous touch potentials on the non current carrying metal parts of the garage electrical system.

You do have to install a Grounding Electrode System at the garage. The absolute minimum is two eight foot long driven rods that are six feet apart. Those would be connected to the Equipment Grounding Conductor buss bar of the Garage panel by a #6 AWG copper Grounding Electrode Conductor (GEC) which is protected from physical damage. If you up-size the GEC to #4 AWG it need only be protected if it is exposed to extreme physical damage such as direct contact by moving vehicles, landscaping equipment, gardening implements, or farm machinery.

To protect any electronic equipment, including the electronic controls of electrical equipment, from damage by lightning, switching surges, voltage spikes, and other electrical disturbances you should consider running 20 feet of solid #2 AWG copper Grounding Electrode Conductor that is buried, in a 20 foot long trench, at a depth of at least 30 inches, connected to two driven rods which are twice the length of the longest driven rod apart. That will lower the impedance to earth of the Grounding Electrode System (GES) considerably and, according to the National Institute of Standards and Technology (NIST), it will make the wiring system much more resistant to such damage. In actual practice the GEC is buried at the maximum practical reach of any mechanical equipment used to dig the trench because deeper is better.

WARNING /DANGER Do NOT enter the trench to place the GEC or drive the ground rods unless there is another person right there to raise the alarm in case of accident. Also the torso of your body must NEVER be below the top of the trench OR you must be completely protected by shoring which is strong enough to prevent any risk of the trench collapsing.

--
Tom

 

[JoeD]

You can't use the NM-B romex in the conduit. It is not rated for wet locations. All conduit outside or underground is considered a wet location.

 

[salvatore giordano]

If you have not purchased your Garage panel yet then let me make a suggestion even though it may cost a few more dollars. Buy a "Main Lug Only" (MLO) panel for the garage, Install a 60 Ampere Breaker with a breaker hold down kit as the main breaker, and install the largest breaker that the size of the feeder conductor allows in the house panel. If you use the #4 American Wire Gage (AWG) that bud16415 has suggested you can install an 80 Ampere Breaker in the Service Equipment panel at the house to protect the feeder to the detached garage. You would do this because it insures that, when you are working in the garage, if you overload the supply from the house, that the main breaker at the garage will open on that overload prior to the Feeder breaker at the house. You then can reduce the load, reset the garage main breaker, and resume working without the hike to the house.


You do have to install a Grounding Electrode System at the garage. The absolute minimum is two eight foot long driven rods that are six feet apart. Those would be connected to the Equipment Grounding Conductor buss bar of the Garage panel by a #6 AWG copper Grounding Electrode Conductor (GEC) which is protected from physical damage. If you up-size the GEC to #4 AWG it need only be protected if it is exposed to extreme physical damage such as direct contact by moving vehicles, landscaping equipment, gardening implements, or farm machinery.

To protect any electronic equipment, including the electronic controls of electrical equipment, from damage by lightning, switching surges, voltage spikes, and other electrical disturbances you should consider running 20 feet of solid #2 AWG copper Grounding Electrode Conductor that is buried, in a 20 foot long trench, at a depth of at least 30 inches, connected to two driven rods which are twice the length of the longest driven rod apart. That will lower the impedance to earth of the Grounding Electrode System (GES) considerably and, according to the National Institute of Standards and Technology (NIST), it will make the wiring system much more resistant to such damage. In actual practice the GEC is buried at the maximum practical reach of any mechanical equipment used to dig the trench because deeper is better.

--
Tom

Thanks for detailed response. I do understand what you are saying about using the Main Lug panel in the garage, having a larger breaker in the house than in the garage, so as to not have to go back to the house. I might do it that way. Although I think I am only going to use #6 awg copper XHHW-2 wire, I'm not sure I can fit 4 awg (4 conductors total) in that conduit. Maybe I will buy a few feet from Lowes, bundle 4 pieces together and see if it fits. I'd obviously RATHER have the larger capacity if possible.

One other question is about the grounding - If I'm having separate ground rods at the garage, why do i need to run a ground conductor from the house to the garage panel? If I didn't have to do that, then I could definitely run #4 awg to the garage due to only needing three wires, and not four.

Lastly - I'm still a little confused about the sub-panel type. I'm seeing some that are labelled "plug-on neutral ready." Is that what i am looking for?

Thanks again!

 

[Michael Armstrong]

If memory serves, 4 #4 THHN conductors in a 1 1 4/" conduit fill to under 25%. Should be no problem if you use yellow slime when pulling them:

 

[hornetd]

Unless I have this question crossed up with another one you have a Schedule 40 PVC Conduit in the 1&1/4 inch size so you are allowed to have all 4 #4 AWG conductors in there as long as they are THWN. The code allows you to decrease the size of the Equipment Grounding Conductor (EGC); which actually functions as an Equipment Bonding Conductor; down to #10 AWG for a 60 ampere circuit. Since you have increased the size of the conductor to compensate for voltage drop you must increase the size of the EGC proportionately. That brings the required EGC up to #8 AWG. So what you actually run between the two buildings would be 3 #4 AWG and 1 #8 AWG conductors. The reason that you are allowed to use a smaller EGC is that it only has to carry fault current long enough for the Over Current Protective Device (OCPD) to open thus clearing the fault. That period is so brief that there is not enough time for the EGC to heat to the point were the insulation will be damaged.

Your question about why you have to have both an EGC and a Grounding Electrode System (GES) is that they serve 2 different purposes. The EGC provides a low impedance path for fault current back to the source of supply to facilitate the operation of the overcurrent OCPD. The GES and the Grounding Electrode Conductor (GEC), which is part of the GES, connect the electrical system to the earth so as to meet the requirements in 250.4 (A) (1)

250.4 General Requirements for Grounding and Bonding.
The following general requirements identify what grounding and bonding of electrical systems are required to accomplish. The prescriptive methods contained in Article 250 shall be followed to comply with the performance requirements of this section.
(A) Grounded Systems.
(1) Electrical System Grounding. Electrical systems that are grounded shall be connected to earth in a manner that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher-voltage lines and that will stabilize the voltage to earth during normal operation.

As you can see the GES does not have a roll in clearing internal faults in the electrical system of the building.

The EGC is installed so as to provide the Effective Ground-Fault Current Path.

(5) Effective Ground-Fault Current Path. Electrical equipment and wiring and other electrically conductive material likely to become energized shall be installed in a manner that creates a low-impedance circuit facilitating the operation of the overcurrent device or ground detector for high-impedance grounded systems. It shall be capable of safely carrying the maximum ground-fault current likely to be imposed on it from any point on the wiring system where a ground fault may occur to the electrical supply source. The earth shall not be considered as an effective ground-fault current path.

Take careful notice of the sentence which is underlined and typed in bold. The impedance of a GES is quite variable because of seasonal and long term changes to the moisture content and the chemical composition of the soil in which it is built. It is still the best we can do to limit the voltage to Ground to levels which will not destroy the systems insulation, kindle fire, and impose deadly touch potentials or step potentials (That being the difference in voltage between the feet of a person or animal when walking and in the case of four legged animals while standing still.) The impedance of the EGC, on the other hand, will remain fairly constant once it is installed thus serving to carry enough fault current to open the fault and limit the damage the fault can do.

You install 2 different systems to achieve different objectives.

As an aside I hope to see the EGC renamed the Equipment Bonding Conductor in my life time and being fully retired I really hope the code making panel gets moving on the issue. The EGC is deliberately Grounded in one place per building. That place is in the Service or Building Disconnecting Means were it connects to the GEC.

About the panel. A plug on neutral is one were the Circuit breaker assembly includes a termination for the neutral conductor of the circuit which the Circuit Breaker protects. The circuits current carrying conductors are then of equal length and run closely together which has some benefits in limiting electrical noise but the major gain is in ease of installation. With that type of circuit breaker the neutral is plugged on when the breaker is installed.

--
Tom Horne

 

[Eriksat1]

I also am running a new feed to my detached garage subpanel. I bought a older tig welder Miller Dialarc 250 HF and also plan on running a big standup type air compressor. I have 100 ft of aluminum direct burial wire but it only has 3 wires 2 - #2 hot wires, and 1 - #4 neutral. I guess the new codes require gfci in a detached garage so now I need to add a 4th underground wire. So
I have 2 questions what type and gauge wire for the 4th ground wire. And where it hooks up to at the house panel I assume the neutral / ground bar they are bonded together at the main house box. But in the garage where does it hook to the neutral is isolated from the ground block. Ground block at garage has 2 - 8 ft ground rods. Does it hook to the neutral or the ground?

 

[hornetd]

This should really have been posted as a new thread! I have a few questions for you. Have you read this entire thread very carefully? If not then please do so and then ask whatever questions you still have. If you have read it thoroughly ask questions about the parts which you do not understand. Two things which I don't believe were covered in this thread are: the size of wire in the United States is given as American Wire Gauge (AWG). AWG sizes run numerically inverse to the size change in that a #14 is smaller than a #12. At any given current flow it takes a larger Aluminum wire to carry the same number of Amperes. The advantage of Aluminum is it's lower price per ampere of capacity. The nominal Ampacity; which is the current carrying ability stated in Amperes; is 95 Amperes. You are allowed to round up to the next higher breaker or fuse size of Standard Overcurrent Protective Device. To carry the same 95 Amperes on aluminum wire would require #2 AWG. Your 1&1/4 inch rigid schedule 40 PVC Conduit is more than large enough to carry 4 of either.

--
Tom Horne

 

[Eriksat1]

My aluminum direct burial 2,2,4, AWG is not to be used with conduit it uses the earth as a cooling medium. As for the 4th wire that would run from house panel ground bar to the ground bar in garage panel I’m not to clear on what AWG gauge that wire needs to be? I plan on a 100 amp 2 pole 240v breaker in the house panel to feed the garage subpanel 2 hot feed wires. I have read this whole thread closely. I plan on having a electrician look at everything before I hook it up. I just want to do all the hard work myself to save money.

 

[hornetd]

The US NEC allows an ampacity of 119 Amperes on your triplex if listed as UF cables and 139 Amperes if it is listed as USE cable when it is direct buried. The listing type or types can be found embossed or printed every 2 feet on the jacket. In triplexed cables the insulation is also the cable jacket. A #6 American Wire Gauge (AWG) Aluminum (Al) conductor is required for the Equipment Grounding conductor (EGC) of a 100 Ampere circuit. Do not install the main bonding jumper; probably a Green colored screw; in the panel at the building being supplied. If the panel design does not make provision for separating the Neutral busbar into two electrically isolated sections; many do not; install one or more add on Equipment Grounding Conductor (EGC) busbars in the outbuilding panel cabinet. The add on busbars mounting screws go into tapped holes in the back of the cabinet or are self tapping and go into unthreaded predrilled holes. The mounting screws are tightened to a prescribed torque value to insure conductivity between the add on busbars and the conductive metal of the cabinet. Since you did not install the green screw; or other form of Main Bonding Jumper; the neutral busbar that was already built into the panel will be insulated from the panel cabinet and thus from the add on EGC busbars. Connect the Grounding Electrode Conductor and the #6 AWG Al EGC to one of the add on EGC busbars. Connect the #4 AWG Al Neutral Conductor to the Neutral Conductor busbar using the Lug that is provided between the two lugs of the outbuilding's main circuit breaker.

The following is not required by the US National Electric Code. As a matter of convenience only you may want to do the following. Protect the feeder cable at the nearest standard breaker rating for it's ampacity. That would be 120 Amperes for type UF triplex and 140 amperes for type USE triplex. Using a Building Disconnecting Means breaker which has a lower rating than the breaker which is protecting the feeder will make it unlikely that the Feeder breaker in the house would open on an overload before the Building Disconnecting Means breaker in the garage opens. That will save you having to go to the house to reset the feeder breaker after an overload because the Garage's Building Disconnecting Means breaker will have opened first.

If you want emergency power in the garage the approach you take will depend on how your emergency power source is connected to the loads it will carry. If your supply transfer mechanism transfers the entire service of the house to generator supply then you will not need a separate emergency circuit. If your transfer arrangement will transfer a separate emergency panel to the generator then you will want a separate Emergency Circuit from the emergency panel in the house to the garage. Install a #10 AWG 4 wire + Ground Underground Feeder cable or cables in the same trench as the your feeder cable. That would supply 1 20 Ampere circuit for one receptacle, the outside lights for the pathway from the house to the garage and minimal interior lights. The 4 current carrying conductors of the UF cable/s would need to be wired as a still code compliant variation on the now outlawed Carter System 3 way switch arrangement. If you plan on installing a separate emergency circuit to the garage then ask for instructions on how to wire the #10 UF cable to control the outside lights from either end and still provide the constant current needed for interior lighting and a single receptacle outlet.

--
Tom Horne