Wire burial rules for equipment close to house
derekr
| Posted in Building Code Questions on
My hvac unit is around 5 feet from my house and the electrical wire for it is just in the air (3-5 inches off the ground) and not buried, so I wanted to know if there’s specific rules that if your a certain distance from the house then is burying wires not required? How much is that distance
im also going to install a car charger on a post 5 feet from my house, does that mean I won’t need to bury the wire for it either the same as the hvac?
also if I did bury it does the burial depth matter being this close to the house?
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The usual way to deal with power wires to air conditioner compressors is to run the wires in either liquidtight (all plastic) or sealtight (corrugated metal with a plastic outer jacket) flexible conduit. My own preference is to use sealtight, which I use almost exclusively. Either material works. Code doesn’t actually require wires to be buried, it just requires them to be physically protected. Without getting into the details of what constitutes to “protection” in all cases, the flexible conduit I mentioned earlier is sufficient to provide the required protection for the power wires in this application. I would advise making the cable run long enough that it can lay on the ground alongside the refrigerant lines though rather than spanning through the air where it’s more of a trip hazard and is also at more risk of getting damaged.
For your car charger on a free standing post, I would run PVC conduit underground, and they will need to be buried at least 18” below grade (as measured from finished grade to the top of the conduit). I would use separate conductors in that conduit (not UF cable), and the usual dual-rated THHN/THWN the box stores sell by the foot will work fine here. I like conduit here instead of direct burial cable (type UF cable) since it’s easier to work with.
Bill
Ok, I already bought the UF cable awhile ago, the allowable amps surpassed what I needed. It’s a 6/2, will a 1 inch conduit allow enough fill rate for that?
Since the charger is so close to my house is the depth of wire that important? It’s just the footings for my foundation are only 12 inches deep so it’s kind of in the way as the UF approaches my foundation
I could go 18 inches deep for like 4 feet but once I get close to the foundation I’ll only be able to dig 6-10 inches deep
You’re not supposed to run UF cable in conduit for the entire run. Trying to do so will, to put it nicely, not be a fun time, either — especially with that large of a cable. You’d probably need 1-1/4” or maybe even 1-1/2” to clear a 6/2 UF cable. You could use some conduit for physical protection where the cable comes up out of the ground.
I doubt anyone will give you any trouble if you’re a bit shallow where you have to clear the footing. You are supposed to be at code depth for the run though, which would be the flat part between endpoints. If you want to provide some additional protection in the shallow spot, bury a treated board a few inches above the cable to act as a barrier. You still need to put the cable at depth though, regardless of the length of the run between endpoints.
Bill
It’s not going to be the entire run only the outside part, once I get in the crawl space it won’t be in any conduit, I only need about 12 feet of conduit to go down my post, under ground, then up the foundation
I would only be using 5 feet more of conduit than if I only used it to protect the outside
It’s not going to overheat the cable or anything in just a 12 foot conduit distance will it?
derekr,
You should talk to your electrical inspector to see what they will accept before doing the work.
The issue isn't so much heating as it is the UF cable really being unsuitable for installation in conduit -- because it is a nightmare to try to pull it into the conduit. Usually conduit on these kinds of runs is only used on the vertical portions of the runs to protect the cable where it comes out of the ground.
I agree with Malcolm, check with your local electrical inspector and ask about this. They'll probably tell you it's OK with such a short run, but warn you that it will not be easy to pull that heavy UF cable into the conduit. If you do decide to run conduit for the entire run, you'll want to use some wire pulling lubricant on that cable to make the pull easier. I recommed Ideal's products for this, probably their clear stuff.
Bill
I get what your saying being hard to pull through, I don’t think it will be too much trouble though with only 12 feet of conduit
I’ll lay both of them flat on the ground then slide it over
It's the bends that really fight you, not so much the straight part. I would be very careful if you glue the pieces together after running the wire in though, because the PVC solvent can affect the jacket on the cable.
Bill
Is the 53% fillrate rule for ease of pulling or for safey?
It's for pulling. Safety due to heating is covered by the conductor derating rules, which is shown in table 310.15(B)(3)(a), which covers the derating required when there are more than three current carrying conductors in the same conduit. If you look at any of the ampacity charts, you'll see that they list the charts as valid "for not more than three current carrying conductors in raceway or cable". If you go over three conductors, the mutual heating of the multiple conductors becomes and issue. This is not an issue due to conduit fill as a percentage of total area, but is rather due to heating because of the multiple wires all carrying current at the same time.
What happens when you pull wires is that they tend to twist, and the wires don't always all pull in perfectly in parallel. Every time a wire twists over another wire, it makes a bulge at that spot, which makes the bundle seem larger at that point. The max fill rule helps to make sure there is enough physical space in the conduit that you'll be able to pull the wire in without so much force that you risk damaging the wire or breaking the conduit.
Bill
So if I’m able to get my 6/2 Ufb through my 1 inch conduit without issue it will be safe from a heating electrical perspective then even if it fills up let’s say 60% of the conduit
Yes, that is correct. You would be exceeding the conduit fill requirements, but not the ampacity or max conductors per conduit rules.
Bill
If you don't want to go down 18" for PVC or 24" for direct burial, you only have to bury rigid metal conduit 6". I don't like the stuff but sometimes it's the best option. Under 2" of concrete you only have to bury PVC 12", and under 4" of concrete the PVC can be directly under the concrete.
How wide does the concrete need to be
It has to extend past the conduit six inches in all directions.
Note that "rigid metal conduit" is type RMC, which looks a lot like galvanized water pipe with threaded ends (but it's NOT exactly the same thing!). This stuff is heavy walled, and much heavier duty than the usual EMT (Electrical Metallic Tubing) that is more commonly seen as conduit.
I make this point because I've seen a lot of people think of "rigid" as meaning "stiff", and then they think EMT is "rigid metal conduit", because it's stiff (rigid), it's metal, and it's conduit -- but it's NOT the code's definition of "rigid metal conduit", since the code means type RMC conduit when you read "rigid metal conduit". They are two very different things, and it's worth making sure everyone is aware of the distinction here.
Bill
Pour the concrete. Cheap easy and even legal...
I know wire has gotten expensive, but less than 50 feet of wire should not be a big deal[~1.50 a foot], don't mess with UF in a conduit.
RE: car charger
They come with a pretty long cord, do you really need it to be 5 feet from the house when it would be much easier to wall mount it?
I put a 20 amp exterior outlet on a GFCI circuit on my garage wall. I 'think' if you hard wire it , you don't need a GFCI , as mine does nuisance trip on occasion
If it's hard wired, you don't need a GFCI with current code, although that may change in the future. It's actually pretty dumb if code tries to require a GFCI for a hardwired charger, because the EV "charger" isn't really a charger -- it's just a little brain that tells the car how much power is available, and a GFCI to protect the cord assembly and the user. The car "charger" is mostly a GFCI device already! The actual charging electronics are in the vehicle itself.
If you have a plug-in charger, then you need a GFCI to protect the outlet, because the outlet and cord assembly is what is being protected. The GFCI functionality of the charger itself only protects the cord going to the vehicle.
Bill
Yeah, that was my understanding, my charger is the kind with 6 feet of cord, charger, wire to vehicle. If it was the box you mount on the wall you wouldn't need a GFCI. I was concerned the GFCI would argue with the charger, but it hasn't been the case. Occasional nuisance trips unrelated, I think, to the charger.
So if that 6 feet of cord gets damaged, passerby are protected from shock hazard, and I have to listen for the clink clank clunk when I plug the car in to make sure the breaker has not tripped.
If the GFCI had argued with the charger, I probably would have moved the wiring inside and cut a hole in the wall. I don't think I would have been happy with that 6 feet of SJOish wire unprotected.
There won't be any "arguments". GFCIs are actually pretty simple devices: they watch to make sure all the power flowing out on one wire returns on the other (in the case of a two-wire circuit). If they detect an imbalance, they assume that the difference is going off to ground somewhere, so they trip assuming there is a ground fault. The implementation of this is simple too: both wires pass through a current transformer internal to the GFCI device. When the currents are equal, each wire imparts opposite magnetic fields on the core (since the current is flowing in opposite directions in the two wires), which makes the magnetic field in the core cancel, and there is no output from the current transformer. If the currents are NOT in balance, the DIFFERENCE in those currents results in an output from the current transformer, which is what the electronics of the GFCI detect, and if that exceeds a set level, the GFCI trips.
You can chain multiple GFCIs together without incident, although since they are all watching the same thing, you can't predict which will trip first if you have more than one in the circuit. All that means is that sometimes you might have to reset one GFCI, other times you might have to reset a different one. They will all still function normally.
Bill
I understand how GFCIs work, when I looked into it several years I think there was some chatter about someone having problems so it was in my head during the install, but as I said it has not been a problem particularly in my install.
I don’t have a wall to mount it to on this side of the house
Another question I have when I bury this flex conduit do I need to use the 90 degree fittings underground too or do I just slightly curve it underground
I don't think flex is rated for burial .
It says it is
I am as cheap as the day is long, but PVC conduit is cheap, a few fittings, a few feet of THWN, a couple bags of concrete and you are good to go. I think my local inspector would have a stroke if he saw flex buried, rated or not. 1-1/2 at the despot is 22 bucks for a ten footer. In looking that up it shows liquidtite as for burial, but that dont seem right
Something strange is going on!
Why is the closest part of the AC unit so far from the building? 5 feet is too far it will look weird.
OK so you want to put the electrical wires below grade. What’s happening with the copper refrigeration lines? Are they going below grade also? Putting the copper below grade would be very unusual.
If the copper remains above grade why can’t the electric run next to it?
Walta
I haven’t measured it, it looks about 5 feet from looking at it it could be closer to 4 feet
That’s already installed, I’m talking about installing a car charger
Generaly a car charger would be install on the house and the connector that fits the car would be long enough the reach the car easly.
Walta
So am I supposed to use these 90 degree fittings underground or do I just do wide curves with the flex as I enter and leave the the ground?
You can’t pull wire around those 90 degree fittings, they should not be used underground or anywhere other than right where the conduit enters a box or device.
You really shouldn’t be using flexible conduit for burial regardless of the rating. Use rigid PVC conduit instead, and use the premade 90 degree bends (not connectors) if you don’t have one of the thermal bending rigs to make your own bends. Any flexible conduit is FAR more difficult to pull wire through, especially stiff wire with a less than slippery jacket like UF cable.
If you really want to use the “flex” (probably really liquidtight), then just make sweeping bends when coming out of the ground. I very strongly advise using flexible conduit for this application though.
Bill
Bill --
Would flex have to be protected from damage where it comes out of the ground?
I'm looking at Table 300.5. Is flex "Nonmetallic Raceways Listed for Direct Burial without Concrete Encasement or Other Approved Raceways" or is it "Direct Burial Cable" ? Doesn't direct burial have to be protected when it's 24" below the surface?
My understanding
UF cable directly buried is 24 inches
Flex conduit buried 18 inches
Metal conduit buried 6 inches
The flex doesn’t need protection outside the ground
Not exactly. The table in the code book (which I have open on my lap write now while writing this :-) that DC mentioned gives the details.
"Metal conduit" that can be buried 6 inches is ONLY type RMC ("rigid" metal conduit, the stuff like threaded water pipe), or IMC ("intermediate" metal conduit, which looks like RMC but without the threaded ends). EMT is also metal conduit but has to be buried deeper just like PVC conduit does. I generally advise against running EMT underground though, since it's too likely to rust out. The required depth for the other things depends on what it's under.
BTW, for anyone looking at that table: there are two horizontal lines mentioning "driveways". The first one, which reads "under streets, highways, roads, alleys, driveways..." means for COMMERCIAL PROPERTIES, like office buildings. The SECOND line, which reads "One- and two-family dwelling driveways..." is the one that applies to most residential structures being discussed on GBA. The difference is due to the anticipated amount of trafffic and vehicle weight going over the buried cable, with commercial stuff being more heavily travelled.
At work, where I do mostly commerical work, we typically bury things at 24" depth when open trenching, or 36-40" or so for stuff installed by directional drilling. With directional drilling, going deeper usually isn't a big deal, but if you go much under 40" you start running the risk of hitting buried water lines which are usually around 48" deep here to get under the frost line.
We never run UF cable, for anything. We ALWAYS use AT LEAST PVC conduit, then runn THWN wire inside. For directional drilling stuff, we use HDPE duct suitable for power cable, then run THWN wire inside. For larger wires, I sometimes spec XHHW wire, which is more durable than THWN, but it's also harder to pull (due to a less slippery outer surface compared to the dual rated THHN/THWN stuff), and it's often special order if you want copper instead of aluminum.
Bill
It would be a "nonmetallic raceway". Elsewhere in the code, a "cable" is defined as a factory-made assembly consisting of multiple wires in a jacket (paraphrasing a bit there). It would be treated the same as normal PVC conduit for depth of burial purposes. It would not need seperate protection where it exits the ground, unless it was in an area subject to extreme conditions, which isn't something you would normally encounter in residential applications.
Bill