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I have 200 amp service at the house. I want to install a DP 60 amp breaker in that panel and run # 6 wire approximately 150 feet (10% added for swag) overhead to a dog rescue building. The building will have a window AC installed (7.8A@208V), plus a number of 20" box fans (0.8A each) , and LED lighting for a 480 square foot building. The AC unit is the only thing pulling a significant amount of power.

It is my understanding that the grounds and the neutrals need to be separated. Does that mean I should not install the bonding screw which I think is meant to tie the ground and the neutral together?

Next, can I run 6/3 without the additional ground as long as I sink 2 grounding rods at the sub-panel, at least 6 feet apart? This is not my house! The sub-panel (which had been fed by an extension cord) is either from HD or Lowe's (Homeline?). It has a factory installed 100 amp main breaker which does not appear to be replaceable.

Since the main panel has a DP 60 amp breaker this 100 amp breaker will basically be a switch to kill the sub-panel. If I use AL wire for the 150 ft should I drop the breaker in the main panel to a 50amp?

  • How many square feet is the building, and how much current does the A/C actually pull? – ThreePhaseEel Jul 15 '18 at 20:10
  • We would need the total load , I have many 3' diameter fans (large commercial grade) to cool our show horses you may be talking 18" house type fans so we don't know what the voltage drop will be and if dropping the feeder breaker will be enough. – Ed Beal Jul 15 '18 at 21:16
  • The building is 480 sq ft with a 7 foot ceiling, The AC united is rated at 7.8 amps @208 volts. This building houses about 40 dogs that were rescued from the needle. The importance of this is the building rarely uses heat during the winter due to the heat from the dogs. 4 fans are the 20x20 box fans from Walmart, .8 amps each, I think. 1 36" barrel fan from Tractor Supply. – Jim Edwards Jul 16 '18 at 0:41
  • The barrel fan is 2.38 amps. The ac is 7.8 amps. 4 20 x 20 fans ,8 or 3.2 amps. A total of 15 amps including LED lights. – Jim Edwards Jul 17 '18 at 0:08
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Neutral and ground being separated means you must remove the neutral-ground bonding device (commonly, a screw) from the neutral bar. You must also have a separate ground bar, which the cheapie big-box panels don't always come with.

You absolutely need both local grounding rods and a ground wire from the main panel. They do different jobs. Remember, electricity travels in loops and wants to return to source. The local grounding rod returns ESD and lightning, which is sourced from earth and should be returned right at the shed, not carried 150' on #6 wire to your house. Human electricity is sourced from the neutral bus on your transformer; the ground wire returns fault current to the neutral in your supply at the main panel. Dirt doesn't conduct well enough to do this job, all you will do is electrify all the grounds in your building.

Since you are doing a pole line, you really ought to use a "messenger wire" optimized for carrying the physical weight of the wire. This messenger can double as the ground wire, in fact you will want to earth it!

You are correct that a subpanel main breaker is simply a shutoff switch, which is mandatory in outbuildings. Its ampacity does not matter at all. Playing games with breaker sizes to try to get the nearer breaker to trip first, doesn't really work.

If you use aluminum wire use #4Al instead of #6Cu. They have the exact same current capacities and you will not need to change your calculations at all.

Also stop buying at big-box stores when you get as sophisticated as a pole line. Go to a real electrical supply, they have what you need in stock and they know what they're talking about.

  • The cables used for "pole lines" are factory-made, not field-fabricated... – ThreePhaseEel Jul 16 '18 at 11:47
  • This panel has the ground and neutral separated and you have to use the bonding screw if you want them tied together, I have found the one grounding rod at the house, but not the second. The house is about 20 years old and this county did not require electrical inspections until 5 years ago. All the circuits will have their own breaker in the Rescue Shed, so it would not be a good day when the 60amp DP trips in the house. I am still unsure on how to secure the ground wire when it is also acting as the messenger wire. The purpose of the MW is to keep weight off the the load wires, right? – Jim Edwards Jul 16 '18 at 23:52
  • @ThreePhaseEel Really? I have a pole line with 1 bare, steel messenger wire and 1 hot spiraled around it. That's the whole deal. The messenger ends are tied off with eyelet/clamp or deadends (nobody serves anymore) and the messenger is tapped, then the hot goes onward to the load. I assumed it was field built, because /1 cable. This was installed by a municipal power company but is definitely on my side of the meter. The pole line powers only lights and all are at least 18' above the ground. – Harper Jul 17 '18 at 5:19
  • @Harper -- duplex cables like that are a factory product, too (you can get them in duplex, triplex, and quadplex) – ThreePhaseEel Jul 17 '18 at 11:37
  • @JimEdwards -- the messenger does carry the mechanical loadings placed on the pole line, yes. – ThreePhaseEel Jul 17 '18 at 11:39
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You'll need both the ground rods and the feeder cable ground wire

The ground wire in a feeder cable to an outbuilding performs a very different function to the ground rods at that outbuilding, and you need both functions to work correctly to have a safe electrical system. In particular, the ground wire in the feeder returns man-made electricity back to its source (the utility), while the ground rods are for returning natural electricity (translation: lightning surges) to its source (terra firma). In addition, you will need to pull the bonding screw in the panel, as neutral and ground are only connected at the main panel.

As to the 100A main breaker in the subpanel, you are correct that it is basically a shutoff switch.

As to that overhead cable run

There are two ways you can do this: either using UF for the entire run, or using wires in conduit for the masts with a service drop quadruplex cable for the overhead portion. Either way, the overhead run needs to be at least 12' above ground, and the messenger wire (or bare wire in a service drop cable) needs to be attached to a secure anchor point on the structure using fittings identified for use with overhead service conductors (such as a fork bolt).

The one-cable approach

This approach uses a 6/3 W/G UF (Underground Feeder, "outdoor Romex", not to be confused with NM) cable for the entire run. You'll be shoving it up conduit (RMC or Schedule 80 PVC) masts on the outside of the buildings with a drip loop at the weatherhead, where it exits and transitions into being supported by a galvanized, copper-clad, or stainless steel messenger wire between the two buildings, and is attached to the messenger wire either using cable saddles that clamp onto the messenger, or by using a field-fitted lashing wire wound around the combination of messenger wire and UF cable. I would recommend the cable saddles, as it will be easier to get a clean install with them, and also less problematic for the rather...flat profile of a typical UF cable. You will also need to ground the messenger at the house end, using a Burndy KSU or equivalent split bolt tied to either an 8AWG bare copper ground wire (in a PVC conduit) or an 8AWG bare jumper going off to a Burndy GC15A or equvalent universal pipe grounding clamp.

Going aluminum

If you want to use aluminum for this, the best route would be to use an overhead quadruplex cable for the feeder with individual wires in RMC or Schedule 80 PVC for the feeder masts and no messenger wire as the quadruplex cable has a bare wire in it already that will act as the messenger. If you are using RMC, then a 1" conduit suffices for 3 4AWG Al XHHW-2 conductors (or 3 6AWG Cu conductors, either THWN or XHHW-2). If you are using PVC in Schedule 80, then I would go with the same options for insulated conductors as before, but with a bare 8AWG copper ground wire in the conduit (to keep conduit fill manageable, especially with aluminum XHHW-2s). As to sizing the quadruplex itself (which provides its own messenger), a 6-6-6-6 (codename: Chola) Al service drop quadruplex is amply sized for a 60A overhead feeder (think about it: a wire in open air is going to be cooled by every passing breeze, something a wire in a cable in a wall or encased in conduit doesn't get). You will also need splicing supplies to join the cable to the mast wiring at each end, namely Cu7Al insulated two-port splice kits such as the Burndy AGS-2, Cu7Al universal split bolts such as the Burndy KSU series for grounding the messenger, and universal pipe clamps such as the Burndy GC15A for grounding the messenger to the mast if you're using a RMC mast, as well as 8AWG bare copper grounding jumpers to connect the split bolts to the pipe clamps in that case. Don't forget to leave a drip loop in the overhead cable connections, just as if you were wiring a service connection.

TORQUE ALL CONNECTIONS TO SPEC

In addition to being a new Code addition in 2017 (NEC 110.14(D)), it is simply good practice to get a calibrated inch-pound torque wrench and use it on all panelboard and breaker lugs and mechanical splices, torquing them to the manufacturer's specifications. That way, your connections won't go off and leave you looking like Greg Biffle's loose lug nuts.

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    Thanks. I have 2" electrical conduit in place. It extends about 2 ' above each roof, tied to the building with a 2x6 PT with carriage bolts through the wood and through the metal building with a 2" x 3" x 1/4" (?) for each of the carriage bolts. From your advice, I will use 6/6/6/6. Was not sure how the wire got supported from building to building. Having the wire being it's own support did not seem right. I will have to look up messenger wire and cable saddles. Thanks for the first clear explanation I have ever had about the differnce between the 2 grounds. – Jim Edwards Jul 15 '18 at 23:10
  • @JimEdwards -- overhead multiplex cables can be run without a messenger wire (they have a bare wire in them that does double duty as ground (for you) or neutral (in a service situation) and messenger) – ThreePhaseEel Jul 16 '18 at 0:11
  • Okay, so 6/4 it is. The messenger wire, does that get loosely wrap around the 6/4, then secured at both ends to the roofs, to take all of the tension off the 6/4? or do you run the MW first, secure it to both structures then use cable ties (?) to secure the wires together? Should anti-ox be used on all AL connections or just on copper to AL? Jim – Jim Edwards Jul 16 '18 at 0:51
  • @JimEdwards -- all Al joints need anti-oxidant, and if you're using the 6-6-6-6 quadplex, you do not need a messenger wire -- you only need it if you're using the UF cable – ThreePhaseEel Jul 16 '18 at 1:15
  • Thank you for clarifying that. I will run the 6/4. Thanks for the anti-ox for all al connections – Jim Edwards Jul 16 '18 at 23:57

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