Adding a subpanel for some exterior 120v plugs around yard, 240v panel?

Discussion in 'Electrical Forum discussion & Blog' started by Tim Fastle, Mar 31, 2021.

  1. Tim Fastle

    Tim Fastle Member

    Joined:
    Feb 10, 2017
    Location:
    New Mexico
    We are on a couple of acres and I want to put some 120v electrical plugs out front for convivence. One to my gate to charge the battery, one for Christmas lights and a couple here and there to plug tools into if and when needed. I will probably run 3 circuits but they will rarely be used and almost never will multiple of them be used at the same time.

    I plan to mount the new subpanel on a pole that is 155' from the main panel. When I did an addition a few years back I ran 1" electrical conduit to the location to use when I added the panel. I also already have enough 8 gauge THHN wire for the runs which I would like to use. My plan is to run 240v - 30 amp service (I would rather go with 40 amp but I think I would have to use 6 gauge wire for that and want to use the wire I have) to the subpanel and then run 120v 15 amp circuits (maybe one of them 20 amp ) out of the panel.
    From my research back when I ran the conduit I believe I will pull 2 hots, a ground and a neutral because I want to have 120v circuits from the subpanel. I will connect the two hot to my main breaker in the sub, and the ground to the ground bus and the neutral to the neutral bus and do not bond them together. Also, it's my understanding that I should not install a grounding rod since there will be a system ground in the sub.

    Am I correct with the approach I have described above or do I need a bit of adjusting? Also, if I get a panel that allows me to connect directly to the hot busbars do I need a main breaker in the sub?

    Thanks for any input!

    T
     
  2. wwhitney

    wwhitney Well-Known Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    So, the most conservative interpretation is that your subpanel on a pole is a separate structure. As such, it will require two ground rods and will require a main breaker (vs the MLO panel you describe). The ground rods connect to the ground bar, and grounds and neutrals are still isolated, with the neutral bar insulated from the case, and any bonding screw removed.

    A minor note, but since you are running conductors in conduit outside, you will need THWN. Almost all THHN is dual labeled as THWN (or THWN-2), but it is important to confirm that by reading the fine print on the conductors.

    Your conductor size may properly be governed by the voltage drop due to the length of the circuits, although that is not an NEC requirement, merely an advisory note. [It might be a requirement of an energy code in your area.] In doing the voltage drop calculations (if you choose to), you would want to use the actual current expected, not the full breaker size of 15/20/30/40 amps.

    #8 copper THWN is allowed to be protected at 50 amps, although given the voltage drop concerns, you are probably just as well off protecting it at 30 amps or 40 amps. Note that if you protect it at 30A, you will need a #8 EGC, and if you protect it at 40A or 50A, a #10 EGC will suffice (yes, that does seem a bit backwards). Note further that you are not supposed to reidentify the color of #6 and smaller conductors, so you are supposed to get white #8 for the neutral conductor, and green #10 or #8 for the EGC.

    If the branch circuits (from the subpanel to the outlets) are also upsized (e.g. #8, or even #12 on a 15A breaker), then you will also need to use an EGC sized to match the circuit conductors.

    Cheers, Wayne
     
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  4. Tim Fastle

    Tim Fastle Member

    Joined:
    Feb 10, 2017
    Location:
    New Mexico
    Well, once again Wayne, excellent and very helpful information!

    Indeed the fine print on the conductors I plan to use does have the THWN designation. I was not aware that I should not reidentify the conductors so I will have to purchase one spool of black - not a big deal though and am glad you pointed that out.

    When you say the "most conservative interpretation is that your subpanel on a pole is a separate structure. As such, it will require two ground rods ...", are you saying a more liberal interpretation could be applied and a single ground rod would be acceptable or that I should install two ground rods? I am guessing the later and will plan on that. Here's a dumb question (or two) but, what size ground conductor should I use from the ground rods to the ground bus of the new sub? I would guess #8. Is that correct? When two ground rods are used do you tie them together with the ground wire and then one ground wire to the ground bus or run them each to the ground bus?

    Again, thanks for the input!
     
  5. wwhitney

    wwhitney Well-Known Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    On the reidentification question, that applies only to white and green, I wasn't clear on that. So you'll need at least 3 colors: white, green, and any other color. The other color you can reidentify as you like to any non-white, non-green color. E.g. if you want to keep track of L1 and L2, it's fine to mark some black conductor with red tape, or vice versa. [Actually, if I recall, the use of orange is restricted, so don't use orange.]

    Every detached structure getting a feeder requires a grounding electrode system. But the definition of structure depends on what year NEC you are subject to. Up through 2014, the definition was "Structure. That which is built or constructed." which fairly clearly applies to a panel on a pole. In the 2017 NEC that was changed to "Structure. That which is built or constructed, other than equipment."

    So without getting into the definition of equipment and all the definitions that references, I think that it's still the case that if you sink a 4x4 post into some concrete and mount a panel on it, that's still a structure. (But I don't think that very strongly, and perhaps I'm being too conservative). If you sink some electrical strut into the concrete and mount the panel to that, then I find it borderline. And if you get a prefab panel/pedestal product that goes directly in the ground or concrete, I think it's clear that is not a structure under the 2017 NEC, and would not require ground rods.

    But if you're uncertain as to the correct answer, it's always allowed to add ground rods connected to the EGC, so there's no harm in doing so. Except possibly increase the risk of lightning strike damage in some cases, I've not thought that part through.

    As for connecting two ground rods, you need one unspliced wire that goes from the EGC bar in the panel to one of the ground rods. The other conductor from the other ground rod can go to another clamp on the first rod (I think), be an extension of the unspliced wire (slide the wire through the first rod's clamp), be split bolted to the first wire, or land on the EGC bar in the panel.

    Oh, on the Grounding Electrode Conductor Size, it's allowed to be #8 Cu when your ungrounded conductors are #2 Cu or smaller. And a GEC that connects only to ground rods never is required to be larger than #6 Cu. However, if you use a #8 you're supposed to protect it with conduit or cable armor. So the typical thing would be to use a #6 bare, run it down your pole and shallowly bury it to your ground rod. [Bare is not required, you can use insulated of any color, it's unregulated.] The ground rod needs to have 8' in contact with the earth, so for an 8' rod, it needs to be fully buried. Which would prevent it being a trip hazard, being hit by mowers, etc. If you use a 10' rod and don't fully bury it, then there is a requirement for protecting the upper end if exposed to damage.

    Cheers, Wayne
     
  6. Tim Fastle

    Tim Fastle Member

    Joined:
    Feb 10, 2017
    Location:
    New Mexico
    I should have asked one more question (another one that feels dumb but I hate to find out later I got the wrong kind of sub panel). I briefly looked at sup panels at one of the big box stores the other day. Sounded simple but was actually a bit confusing. Clearly I want an outdoor panel. I believe what I am looking for is a "Main Lug Load Center" (which I think means no built in breaker) and 6-8 spaces. I assume it does not have to match the amperage of the breaker feeding it (in my main panel) just has to be equal to or greater than - that is, can I use a 60amp sub panel? I am also assuming that all sub panels will accommodate both 120v and 240v breakers or do I need to make sure I get one that "does"?

    For instance, I am thinking one like this would work fine. https://www.lowes.com/pd/Square-D-100-Amp-6-Spaces-12-Circuit-Main-Lug-Load-Center/3129229
     
  7. wwhitney

    wwhitney Well-Known Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    If your pole is not a structure, or if it is a structure and you have no more than 6 breakers (I think) in it (and never plan to exceed that), then you could use a Main Lug panel (MLO, main lug only), which means no main breaker (and the panel bus rating will need to be at least as great the feeder breaker size). But I would advise using a main breaker panel for a convenient way to kill the bus and all the circuits while standing at the pole.

    [You could even get the little rubber boots now required on service panels to cover the always live service conductor connections at the service main breaker, and use them in this panel on the main breaker. Because if you need to work on the panel, it will be very tempting to just kill the main breaker rather than go to the house and kill the feeder, but isn't really safe because of those exposed live terminals on the main breaker.]

    Since the OCPD for the panel feeder is provided by the breaker in the house supplying your feeder, the size of the main breaker in this panel doesn't matter unless it is a lower value. But with a 60A or 100A MB panel it will be higher and can stay that way.

    I've never seen a 120V only panel, any single phase panel has two busses and can provide 240V loads. Bear in the mind that panels are often specified as 8/16 circuits, which means they have 8 spaces (enough for 8 full sized single pole breakers for 120V loads, or 4 full sized double pole breakers for 240V loads), and if you want to get 16 circuits you need to use tandem (two 120V circuits from one space) or quad (two 240V circuits from two spaces) breakers.

    Cheers, Wayne
     
  8. Tim Fastle

    Tim Fastle Member

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    Feb 10, 2017
    Location:
    New Mexico
    I really appreciate all the information, it's very helpful and I think I have a good handle on it now. I hope I don't have anym0re questions but, if I do ... I'll be back!

    Thanks!
     
  9. wwhitney

    wwhitney Well-Known Member

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    Mar 17, 2019
    Location:
    Berkeley, CA
    Don't forget to make all your conductor terminations with a torque screw driver set to the values indicated on the panel labels.

    Cheers, Wayne
     
  10. Reach4

    Reach4 Well-Known Member

    Joined:
    Sep 25, 2013
    Location:
    IL
    Should we infer that your existing big spool is white?

    Your 4o amp subpanel could power 80 amps of 120 volt loads if you run two hots.

    Alternatively, if you just run a black and a white and a ground, you could connect the black to both rails of the subpanel and have 120 volt circuits powered by each breaker position.

    Ground can be green, bare, or green+yellow.
     
  11. wwhitney

    wwhitney Well-Known Member

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    Mar 17, 2019
    Location:
    Berkeley, CA
    Seems like an obvious choice to me to run one more wire for double the power. Plus balancing the loads on the two legs will reduce voltage drop.

    If for some reason you do jumper the phases (edit: properly called legs in single phase, as there is only one phase) in the panel to allow the use of all the spaces for 120V loads on the same leg, bear in mind that the terminals on main lugs or main breakers of that size are not rated for more than one conductor. So you'd need a large wire nut for (3) #8s or a 3 port Polaris type connector.

    Cheers, Wayne
     
    Last edited: Apr 5, 2021
  12. Tim Fastle

    Tim Fastle Member

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    Feb 10, 2017
    Location:
    New Mexico
    Once again, great information and very helpful Wayne. I am digging holes and trenches today and, armed with your input, I hope to put up my pole and pull wires next week.

    Thanks very much for the detailed and thoughtful replies!

    Tim
     
  13. Tim Fastle

    Tim Fastle Member

    Joined:
    Feb 10, 2017
    Location:
    New Mexico
    When I last replied I somehow had missed the two replies above and thought I might reply to them with, of course, a question.

    Reach4, thanks for the input. I have quite a bit of both white and green conductor and enough black for one of my two runs. So, I will need to procure either black or red since I can't/shouldn't reidentify the green or white. Also, your suggestion to run two hots is, what I think, I described in my first post, or at least meant to. My terminology was to "run 240v service" and may not be the way to put it. But, I do plan to run 2 hot (which is what I meant by 240v service), 1 ground and 1 common and I think that gives me what you are suggesting, 40 amps on each leg. Is that correct or are you suggesting something different?

    I also assume that if I run the 4 conductors (2 hot) I would not have any reason to (nor be able to) "jumper the phases in the panel to allow the use of all the spaces for 120V loads on the same leg" and, would only jumper the phases in the panel if I were running just one hot but wanted to be able to use all the spaces for 120v. Is that correct? Just trying to understand why I would ever even consider to "jumper the phases."
     
  14. wwhitney

    wwhitney Well-Known Member

    Joined:
    Mar 17, 2019
    Location:
    Berkeley, CA
    Yes. [The word service should only be used for the first disconnect from the utility and upstream of that point. The word you want is feeder.]

    And yes.

    This project seems like a bit of a stretch for your background, so be careful about getting in over your head and come back with more questions if anything isn't clear.

    Cheers, Wayne
     
  15. Reach4

    Reach4 Well-Known Member

    Joined:
    Sep 25, 2013
    Location:
    IL
    Correct on each, except that you don't want to use the word "phases" in this case. Most will prefer legs or some other word to refer to the two hots or rails that are out of phase with each other with respect to ground. Even my mention of "out of phase with" could bring somebody out, but I would be glad to get into that.
     
  16. Tim Fastle

    Tim Fastle Member

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    Feb 10, 2017
    Location:
    New Mexico
    Thanks to both of you. I think I have a good handle on it, will be careful and if I have any questions ... I will be askin!

    T
     
  17. Reach4

    Reach4 Well-Known Member

    Joined:
    Sep 25, 2013
    Location:
    IL
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