Multiple sub panels in series?

[transferguy]

posted by: speedy

Hi all! OK, long time lurker, first time poster, and new member. As a little background, I was a ground radio troop in the AF for 21 years, and have done my share of home electrical stuff, so I do at least know some about what I'm doing. More importantly, I know what it is that I don't know well enough, so I don't get in over my head. So anyway.....

Here's the situation: My shop is about 250 feet from my house, and almost exactly in between, is my shed. I'd like to get power to both of them from the house, and obviously, as much as possible to my shop, as I intend to have a compressor, lift, etc. Is there a safe way to run power to the shed, then from there, run it to the shop? I have done some research on sub panels, and my house currently has a 200 amp service, which means 100 amps to the shed, and 60 to the shop (unless there are other ratings of breakers that I'm unaware of). I've heard of 175 and 150 amp breakers, but I haven't been able to find standard panel breakers in these ratings. Is this a safe way to get service to my shop? Is there a way to get more power? Also, I have an offer for 4/0, 4/0, 2/0 direct bury aluminum wire at a really good price ($1.50/ft), would this be recommended? (I'm very familiar with no-ox, and it's application) I live in Texas, outside of city limits, which means I don't have to be within code for code's sake, but I DO want to do it safely.

Thanks in advance to all, and thanks for this great site!!

[transferguy]

posted by: navyguy

I can't offer any code advice since I am from Canada, however I will offer the following on a design perspective...

All panels will have a maximum branch circuit breaker size rating so that could limit what size of feeder you want to install.

The distance is also a factor... meaning that although 250 feet is not that far, if you are going to have large loads on the feeder you may want to "upsize" your cable based on the maximum breaker rating.

I would put the "larger" sub panel in the larger building then from there go to the smaller building. In lieu of that I would take two separate feeds, one to each building if there are critical loads.

While digging up the ground, don't forget conduits for telephone, data, CATV, etc. If you don't want them now, just leave them empty, you will eventually use them.

Consider (within code) the interconnection of smoke detectors with the ones in the house. It is nice to know that you will be alarmed if there is a problem in your out-building(s).

As far as code specifics, I am sure someone can chime in for you. Of particular concern with this type of installation is the sizing of the cable, proper isolation of the sub-panels and the grounding / bonding requirements of the sub-panels.

Cheers

John

[transferguy]

posted by: suemarkp

There are many standard breaker ratings in addition to what you listed. The 60A and 100A tend to be cheaper, but you can get 70, 80, 90, 110, 125,... Double pole breakers from 15A to 60A tend to cost the same.

The first thing you need to determine is how much power you REALLY need. How many items could be used at once? Of all your loads, what are the motor loads and what is the nameplate Full Load Amps (FLA) on the biggest ones? Remember that a 120/240V feeder has two separate 120V "legs" available. So a 40A feeder can do 80A at 120V (as two separate 40A legs), or 40A at 240V, or 20A at 240V and 40A at 120V at the same time. The amps can go in any combination as long as you don't exceed the amp rating on either of the two hot wires (120V take from one hot, 240V takes from both).

Prior to 2008, you could feed a subpanel in a detached building with 3 wires (hot-hot-neutral) with the neutral being a dual purpose ground and neutral. Post 2008, the NEC wants 4 wires (hot-hot-neutral-ground). Personally, if you don't really have any codes, I'd do it the old way as I never had any trouble with a 3 wire feed. It will save the cost of one wire x 250' (and ground wires should really be copper to ensure their survival when buried).

Once you know the load you want to serve (both shed and shop if they could be used at the same time), you would pick your wire size. Based on the distance, I'd upsize at least one or two wire size if your motors are 240V, and two or three sizes if any large motors are 120V only. If the far building has the higher amp need than the closer one, you'll need the closer panel to be as large as the far one. You can save some cost by using a panel with feed through lugs in this case, as you'll just send wires sized the same as the incoming feeder to go to the next building and not need a breaker to the last building. If the last panel has the lesser needs, then you could put a breaker in the shed panel to feed the shop with a smaller circuit.

Each detached building with a breaker panel requires a ground electrode system (so you'll need at least one ground rod at each, with a #6 or larger copper wire from the panel grounding bar to the rod). If you have a 3 wire feed, you install the green bonding screw in the panel and the neutral and ground bar are one and the same (like your main house service). IF you run 4 wires, then you do not install the green screw in the neutral bar and you install a separate grounding bar (it will be electrically isolated from the neutral bar in that panel but there is a connection way back at the house where neutral and ground are combined).

Your panels in the buildings need a main breaker if they have more than 6 circuits breakers installed (a "main" can actually take up to 6 swipes of the hand so 6 separate breaker would still qualify).

If you choose wire that is larger than what the breaker is designed to terminate, you'll need to splice on some smaller pigtails on the wires. A 100A feeder should be plenty for what you want. You'd normally use #2 copper or 1/0 aluminum for that. Upsizing a few sizes would put you in the 1/0 copper or 4/0 aluminum range, but you'll have to use pigtails on a 100A breaker if you use the 4/0 aluminum.

So the 4/0 may work for you, but I'd read the jacket of that wire. If it says USE and nothing else, you can't run it inside or on a building (only the riser is allowed and a direct entry into a panel). If it has multiple listings such as "USE or XHHW or RHW", then you can use it in conduit inside or along a building which gives you more options. You can run a separate #6 copper wire along with the 4/0-2/0-4/0 to make a 4 wire feeder if your want, just run the wires bundled together in the same trench and keep them all in the same conduit. If it turns out that 50A would be enough, then you could probably get by with the next more common house feeder cable which is 2-4-2 aluminum. You'd need a #8 copper ground to go with that, or look for Mobile Home Feeder cable as it already is 4 conductors and probably rated for direct burial.

Mark
Kent, WA

[transferguy]

posted by: speedy

Thank you all SO much for such fast replies! This solves several problems.1) I didn't know for sure if there were other breaker sizes, and now I do know. 2), I didn't realize there were service boxes with feed through lugs, so this will help tremendously. I only need two circuits in the shed(first building), for a couple of outlets and a light, so that should be pretty easy. 3) I was also wondering about how to wire the ground/neutral circuits, but I now have that answer as well.
OK, more info from me. The wire is Pirelli Supertuf-XLS, and from what I can tell, it looks like it's UD cable. The guy that has it said he pulled it from a conduit, but who knows if it was supposed to be there or not? I found one website where it was supposedly replaced by "Prysman 600V Supertuf Ruggedized", under a new manufacturer, but again, who knows. The bigger load will definitely be at the farthest location, but if I use a box with feed through lugs, this shouldn't be an issue, (if I understand correctly?). The max load I would expect is from the compressor, which states that it's FLA is 55 Amps@208 Volts. Quick research says most lifts use a 208v, 20A breaker, so let's say 80 amps max draw, as I doubt I'll ever use a power tool in one hand, and run the lift lever with the other, all while the compressor is running I'm flexible in how I do the install, so if I can only run the 4/0 to the box, outside the building, and no conduit, then I will. I also will very likely take your advice, and run it as a 3 wire, with a separate ground for the building. Was planning on doing a ground rod at the shop anyway, so this just makes sense. Curiosity question, though: why would a direct bury cable not be approved for running inside a conduit? I can see a standard cable not being approved for direct burial, for obvious reasons, but the other way around makes no sense to me.

And just to be sure I understand, if I run a 100 amp service to the shop, through a box with feed through lugs, the aluminum 4/0 should be fine?

Again, THANK YOU to you guys for your very quick and thorough answers!!!


PS, here is a pic of the wire, if I can get it to upload.

[transferguy]

posed by: suemarkp

Finding panelboards with feed thru lugs is kind of tough, as most of the home centers don't carry that type. Many times, they are the smaller panels that have them, so if you can find a 2 or 4 or 6 slot panel with them, rated at 100, 125, or 200 amps, that would be what you want for the first panel. A 200A panel will have lugs large enough for that wire which will save some work. A 125A may take 4/0, but I doubt it.

The photo you have of the wire doesn't help. I see no writing on that for a NEC recognized wire type. This looks like URD cable which is used by the power company but not allowed under the NEC. If it has some other listing that the NEC recognizes (e.g. USE, RHW, XHHW, THWN, or a host of others) you can use it. It probably says USE or USE-2 on it somewhere. See if you can any of the other terms written on that cable.

The problem with USE or unrated wire is that it isn't proven to be flameproof. If it is direct buried, you don't care about flammability. However, that cable needs to come out of the ground and into a box which is usually on a building, so then the flammability matters even in a conduit. If it is listed as USE, you can only come directly out of the ground (in a conduit -- USE can't be exposed on a pole or wall) and into a panel, or into an LB that then goes directly into the back of a panel. You can't run it horizontally inside or outside of a building in a conduit (only buried, but the buried portion can be in a conduit if you want). If it has the other listings (RHW or XHHW are the most common), then you can use it inside the building in any direction and for any distance as long as it is in conduit. Most cable (conductors inside an overall jacket) types can be used without conduit (e.g. NM or Romex, UF, SE). USE and individual wires like RHW/XHHW/THWN must be in a conduit (unless rated for direct burial and then only the buried portion can be done without conduit).

That compressor is huge at 55A. I'm also concerned with your nameplate voltages. 208V is common in industrial locations, but not in most houses. 95% of houses are 240V and not 208V. 208V comes from a 3 phase service, so a big apartment building may be 208V as are most industrial plants. A 208V only motor should not be run at 240V or vice-versa. However, some are capable of either voltage (HVAC units are common for this) and their nameplate may say "200V - 230V" or may say "220V". Most others will say 200V if intended for 208V, and 230V if intended for 240V. The overload device for the motor/compressor needs to know the operating voltage as it will be sized differently for 208V -vs- 240V. So your compressor may not work at your house. That 55A nameplate is also a huge burden on your wires and transformer. Many residential transformers won't even be able to startup that compressor, or you'll get a huge light dimming effect in the house when it starts. A 5 ton heat pump compressor is generally about a 30A nameplate, and the power company wants to know when you have a large heat pump. This will be even worse.

When you size a branch circuit and a feeder for a motor (e.g. compressor), you need to add 25%. So a 55A compressor would be a 68.75A load. A 100A feeder would run that, a 20A lift, and still have 11A left. You'll definitely need the 4/0 wires with a 55A motor to start. The circuit breaker feeding a motor is generally even larger than this (it is a special exception in the code where the breaker is sized larger than the wire feeding it). This is so it won't trip during a startup surge. You may need a 100A, 125A, or maybe even larger breaker to start that compressor. In a residential panelboard, that's a problem as most panels are limited to a 100A or maybe a 125A branch circuit breaker. I'd look for a smaller 240V compressor... If you really want to use it, I'd go with 200A panelboards and talk to your power company. You'll still be limited to whatever size breaker is the largest you can put in you house main panel though unless they make a 200A "subfeed lug" for it and your bus stabs are rated for 200A. Another solution is to replace your meter with a 320/400A version. This would be split into two feeds -- a 200A to your existing house panel, and a 200A service to your shed/shop. The dollar signs are going up...

Mark
Kent, WA

[transferguy]

posted by: speedy

Thanks again! OK, better info. I first read the tag last night, without good lighting. I went out this afternoon in better light, and what I thought was printing on the label were specs of dirt and gunk. It's actually 208~230V, 15 Amps. Sorry to waste your time with bad info, I understand it can be frustrating.

So now I guess by biggest load is about a tie between the lift, the compressor, and my welder, at 20 Amps. I can definitely see my welder and my compressor running at the same time, at least occasionally. At this point, I think I can get by with a 60 amp service, so I'm thinking the 4/0 might be overkill. You've mentioned enough things that cause concern about it, I don't think I'll get it, anyway. Is your original suggestion of mobile home feeder cable or 4-2-4-aluminum still a good choice, now that you have all the correct info?
I went to the 2 big home improvement stores today, and the "electrical guys" at both places said that the only boxes they had that had feed through lugs were mobile home boxes. They're 200A and about 3 feet tall, with 24 (I think) spaces. WAY more than I need or want in that shed. I have since found a Square D at both of their stores' websites, that is 100A, 6 space, outdoor main lug load center (QO612L100RBCP). The specifications tab on one of their sites says it has feed through, but I haven't been back to look at it. Since it's an outdoor box, do I have to mount it outside?

Again, really sorry for the bad info, I didn't mean to waste your time. I know you guys all provide your personal time answering these threads, so I will make sure and not repeat my error.

[transferguy]

posted by: suemarkp

OK, that sounds much better. A 60A feeder is common and at a nice price point. Normally, you'd use #6 copper or #4 aluminum for that, but because of the length you should go bigger. #2 aluminum would be a good choice (2-2-4-6 would be ideal and should be easy as that is a typical mobile home feed). Just make sure you get something that can be direct buried (you'll need to put it 24" deep). If you only want to dig 18" and don't mind running conduit, then you could buy regular THWN wire (#4 copper or #2 aluminum) and pull that through the conduit. I'd use 1.5" conduit for that distance.

I used to have a post-it note with a few small panel numbers with feed through lugs, but I think I tossed it. I just searched some online electrical suppliers to find them at reasonable prices (they should less than $100 for a 12 slot). But the home centers can be hard to beat, and it may be cheaper to put a huge panel in there even though you only need 2 circuits in the shed. An outdoor panel can go outside or inside, but not the other way around. The outside ones can be kind of a pain inside because of how they open (you sometimes can't mount them flush in the wall because of the drip lip around the door or how the door rotates off), but if you don't mind surface mounting the panel it will be fine.

Since you only need 60A, there's not really a need for feed through lugs anymore. I think a 60A breaker will generally take a #2 conductor, so you could use a regular small interior panel and put a 60A double pole in it for the shop (so get a least a 4 slot panel, and 6 or 8 may be better at the shed). That breaker is about $15, so if the feed through panel costs more than $15 more than a non-feed through, then just put in a breaker and be done with it. This will also make it so you only have to walk to the shed if you want to do electrical work in the shop with the power turned off.

Remember to get a main breaker panel for the shop if you have more than 6 breakers (compressor, welder, lift, lights, 120V tools). It sounds like you'll be close, so I'd definitely go larger than smaller. It is OK to buy a panel with a 100A main even though you're feeding it with only 60A. You could compare that cost to a Main Lug Only panel with a 60A backfed breaker and a breaker retaining (hold down) kit.

Mark
Kent, WA

[transferguy]

posted by: navyguy

Posted by Speedy
It's actually 208~230V, 15 Amps.

That still sounds like three phase to me. Better confirm that it is single phase or three phase.

It could be 208 single phase but 230 is a three phase delta configuration and 208 is usually a three phase wye configuration. Don't worry about the jargon; if you can take a pic of the nameplate it would be helpful. If your compressor is three phase there is not much you can do with it.

Cheers

John