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Wire Size Question for Feeding an External Building (Barn) from Residential Home

Wire Size Question for Feeding an External Building (Barn) from Residential Home

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  #1  
Old 09-11-13, 06:11 PM
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Wire Size Question for Feeding an External Building (Barn) from Residential Home

I am running power to a barn from my primary load panel. The distance is about 250'. The 3 THHN (rated for wet location) wires (2H+N) will be placed in Sched 80 PVC conduit and buried.

Questions:
- What size wire do I need to use for a 50A load?
- For a 60A load?

I am adding a subpanel next to my primary panel. With a 60A double pole breaker feeding it. They will be side by side so the wire run is about 1'.

Question:
- Can I use chassis ratings for the connecting
- What size wires do I need to use?

Thanks in advance,
Kevin
 
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  #2  
Old 09-11-13, 07:09 PM
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The 3 THHN (rated for wet location) wires (2H+N) will be placed in Sched 80 PVC conduit and buried.
Not code compliant. You also need a ground wire. Your subpanel will need a bonded ground bar and an isolated neutral bar. Assuming actual load at any time doesn't exceed 40 amps #4 should be okay. Ground wire (EGC) can be #8.

If your subpanel has space for more than six branch circuits. Best/cheapest just use a 100 amp panel. The 100 amp breaker acts as your disconnect. Also you need one maybe two ground rods connected with #6 (GEC).
 

Last edited by ray2047; 09-12-13 at 05:26 AM.
  #3  
Old 09-11-13, 07:15 PM
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Welcome to the forums! First, your subpanel should be in the garage, not in the house. It needs to be separately grounded and the grounding wire and neutral wire are to be on separate busses, with only the grounded ones tied grounding electrode. You will feed the subpanel with a breaker in your existing panel (50 or 60 amp). For 50 amp use #6 Copper or #4 aluminum. Over 100' it is a good practice to increase one size.

Ray types faster.
 
  #4  
Old 09-11-13, 08:02 PM
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It's not only the load at that distance....it's what are you running. Things like compressors and welders require a large initial inrush of current. #3 THWN is probably the minimum you should go. It's rated for near 100amps of load.
 
  #5  
Old 09-11-13, 08:25 PM
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I may have not been that clear in my questions. There are 2 new panels involved in this install. The first is a new sub panel that is being added adjacent to my primary panel (fed by the utility's SE cable). I need to add this sub panel because the primary is out of circuits. I plan to feed this new sub panel with a 60A (or maybe larger) 2-pole breaker from the primary. I plan to run all 4 wires 2H+N+G to this sub panel. The 2nd set of questions was about what size these 4 wires need to be. The reason for this question is that I thought the short run (< 1') might qualify as "chassis" wiring meaning a given size wire could carry a greater load than when using as a longer circuit. This new sub panel would have a 50A (or 60A) double pole breaker that would supply the outbuilding with power via the cabling described below.

The second panel involved in the install is the new load panel at the outbuilding. This panel would have its own 100A main breaker. It would be fed by the above new sub panel adjacent to the primary on the house. It is my understanding of the code, and I could be wrong, that when wiring outbuildings the panel at the outbuilding is separately grounded. I plan to use #6 with (2) buried 8' ground rods at least 6' apart. Therefore, I thought I only needed to run the 3 conductors (2H+N) in the 250' feet of conduit since the grounding is taking place at the outbuilding. My first set of questions is dealt with what size these wires in the buried PVC conduit need to be given the length (250') and a 50A load and the size for a 60A load.

Thanks,
Kevin
 
  #6  
Old 09-11-13, 09:40 PM
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You need to feed your sub panel next to your main panel with the size wiring dictated by the new breaker size in the main panel. Since you're out of room in your main panel..... you would have to move at least two circuits over to gain the space for your new two pole breaker.

Your remote sub panel has a 100a breaker in it and it acts as a main disconnect means. The breaker in your new sub panel will be providing the circuit protection. By code you do have to run four wires to the outbuilding. The ground does not have to be as large as the hot and neutral wires.

As I spelled out in my post above....the size of the wire is not only figured on static loads but on dynamic loads too. A static load is lights and maybe electric heat. A dynamic load is an air compressor, welder, large power tools.

You're talking about 250'.....at that distance.....what voltage drop is acceptable to you ?
 
  #7  
Old 09-11-13, 10:58 PM
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PJMax thanks for your reply. As you already know you are right about requiring 4 wires to the outbuilding. The older codes (2008 or prior, I think) did allow removing the ground from the feeder or branch circuit feeding the barn. Looking at the code (Table 250.66, NEC 2011) it looks like this ground wire can be #8 as long as my largest conductor is smaller than #2. Am I reading it right or can I use #8 as you suggested?

Now my next question: is it required to have a grounding electrode at the outbuilding? If so, where is that connected if I am running the ground from the primary with the feeder circuit? The las time I did a project like this was under the older code so I am not that familiar with how the ground wire run in my conduit and the grounding electrode (if required) is connected to my remote panel and the building.

To answer your question about voltage drop: I know it is ideal to shoot for 3% or less but somewhere 5% is acceptable. Your other question about inrush currents has already been included in my load calculations.

Thanks for your help.

Kevin
 
  #8  
Old 09-12-13, 02:26 AM
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Looking at the code (Table 250.66, NEC 2011) it looks like this ground wire can be #8 as long as my largest conductor is smaller than #2. Am I reading it right or can I use #8 as you suggested?
You are looking at the table for a grounding electrode conductor. This would be the conductor from the panel in the outbuilding to the grounding electrode (ground rod or rods) and NOT the equipment grounding conductor between the source of the power and the outbuilding. Since ANY grounding electrode conductor less than #6 in size requires additional mechanical protection (conduit) it is common to run #6 for this conductor.

For the equipment grounding conductor you need to size it according to the protection at the source of the power. For a 60 ampere circuit breaker you need #10 copper or #8 aluminum, for a 100 ampere circuit breaker you need #8 copper or #6 aluminum. If you upsize the current-carrying conductors to allow for voltage drop then you MUST also upsize the equipment grounding conductor proportionally.


Is it required to have a grounding electrode at the outbuilding? If so, where is that connected if I am running the ground from the primary with the feeder circuit?
Yes, a grounding electrode conductor and grounding electrode IS required at the outbuilding. The grounding electrode conductor as well as the equipment grounding conductor connects to the equipment grounding bus in the sub-panel. That equipment grounding bus MUST also be "bonded" to the panel as well as any other metallic component. The "neutral" bus in the panel MUST be "isolated" from the enclosure by removing any bonding screw or strap. All branch circuit wiring will have the neutral conductor connected to the neutral bus and the equipment grounding conductor connected to the equipment grounding bus.
 
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