What wire to buy for 30 amp 240v circuit

[timeshifter]

I'm planning to install a 30 amp 240v circuit to charge my Standard Range Model Y. My panel has an unused 30 amp 240v breaker. I've ordering a Bryant 9430FR NEMA 14-30 outlet. Got the Tesla Gen 2 14-30 adapter on order. Only thing left is the wire.

I need 62 feet. At my local Home Depot or Lowes 100' of 10/3 wire is about $250. (50' is $200, they don't have 75')

Southwire 100 ft. 10/3 Solid Romex SIMpull CU NM-B W/G Wire 63948426 - The Home Depot

The Romex Type NM-B Copper Building Wire is thoughtfully designed with a patented SIMpull Cable Jacket, streamlining the process of pulling, stripping, and installation. Widely recognized for its role

www.homedepot.com

https://www.lowes.com/pd/Southwire-Romex-SIMpull-100-ft-10-3-Non-Metallic-Wire-By-the-Roll/3138981

I found 75' on Amazon through a seller called Cable Cabana for $106

Amazon.com

I'm having trouble discerning the differences. The Amazon wire looks like it's flat and the conductors are solid. At first I thought the big box wires were stranded but that's not totally clear. Maybe they're stranded in the sense that the different conducts are twisted in the overall jacket, vs lying flat.

 

[Watts_Up]

Using the following Voltage Drop Calculator:

AWG 10​

​

Voltage drop: 3.88​

Voltage drop percentage: 3.24%​

Voltage at the end: 116.12​

​

AWG 8​

​

Voltage drop: 2.59​

Voltage drop percentage: 2.16%​

Voltage at the end: 117.41​

​

Using this other Voltage Drop Calculator: Max Voltage Drop (3 %)

Voltage Drop: 2.14%​

Minimum Conductor Size: 8 AWG​

​

1 conductor per phase utilizing a 8 AWG copper conductor installed Cable,​

will limit the voltage drop to 2.14% or less when supplying 30 amps for 62 feet​

on a 120 volt single phase system.​

​

​

 

[timeshifter]

Do we want to plug in 30 amps in those calculations even though it will only be actually drawing 24?

Interesting numbers, but how do they translate to my situation?

I was thinking I'd use AWG 10 wire since it's a 30 amp breaker. Are you saying that 8 would be preferred? Is the voltage drop difference that big of a deal?

Initially I was thinking I'd run AWG 6 for this so would I had more capacity I could upgrade the circuit with a 60 amp breaker and outlet or wall charger. But then I'm concerned that the thicker wire wouldn't fit in the breaker correctly. So I'm thinking I'll just install what I need now that best fits what I have now.

Still wondering why the Amazon cable is $100 vs over $200 at Home Depot?

 

[tps5352]

I'm planning to install a 30 amp 240v circuit to charge my Standard Range Model Y...

Not to spend your money unwisely, but is there any chance you might want to upgrade to the popular NEMA 14-50 receptacle some day? The 30a outlet and 30a circuit breaker would of course be relatively easy to replace, but not the wire. Of course, 6 gauge wire would be more expensive. What I did (in 2020) to save some dough was to look for an electrician or homeowner selling surplus wire on Craig's List from a finished job. Got a good deal on an adequate length of #6 Romex-type 3-conductor (plus ground) wire, and also separately on a new-in-the-box Hubbell (NEMA 14-50) commercial-grade receptacle and then a brand-mane GFI 50a circuit breaker (on eBay).

Just a suggestion.

 

[timeshifter]

Believe me, I've thought through all the scenarios, many many times. Not just now but in years passed. This is what's best for me right now. The wire will be easy to replace or leave as a spare circuit in the future. It's all run through an unfinished basement.

Everything else is ready to go, just need to buy the wire, but I'm stuck on that part.

 

[ATPMSD]

You may find this link helpful

Wire Size Calculator | Irrigation & Landscape Lighting

Whether it’s wires, cables, cable assemblies, or electrical accessories for commercial and residential applications, Paige Irrigation and Landscape Lighting offers the best of the best. We build irrigation and landscape lighting systems that are smart, efficient, and reliable.

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You need 8/3 wire.

 

[timeshifter]

You need 8/3 wire. If you go with #6 do not use Romex as it is rated at 55-amps and cannot be used with a wall connector.

I'm confused now. I was going with what I commonly see for wire size based on amps. I recently got a quote from an electrictrician, in fact one who is listed on Tesla's website as a certified installer in my area for the Wall Connector. He didn't measure the run carefully, just eyeballed it, and wrote up the quote for 80'. His proposal was to install a 60 amp service.

"Installation of 60 amp circuit for charger approx. 80 foot through basement to garage.Install and commission homeowner supplied charger."

I asked if he was going to use conduit and he replied "No it will be fished in the wall and will be installed like your other wiring stapled to the floor joists". I asked if it was 6 AWG and he said it would be 6/3.

 

[ATPMSD]

I'm confused now

If you want 60 amps, then you need #4 wire for an 80-foot run, just plug the numbers into the link. Note also that NM-B is not suitable for 60 amps as it only supports 55 amps. You must use the full circuit value of 60 amps and not the 80% value of 48 amps when selecting wires for an EV outlet or connector.

Cerrowire Resources - Ampacity Charts

Ampacity is the maximum current that a conductor can carry continuously under the conditions of use without exceeding its temperature rating. Cerrowire's ampacity chart helps calculate the load requirement for a circuit.

www.cerrowire.com

listed on Tesla's website as a certified installer

Unfortunately this does not mean much.

 

[Rocky_H]

@timeshifter I think you are getting some misleading and unhelpful advice so far. Yes, any form of that 10 gauge wire is fully sufficient for a 30A rated circuit, so either of those selections you found will be fine.

@Watts_Up That's just not useful and is kind of distracting. Voltage drop isn't going to be a factor for a 60 foot run. It's so short it's not going to have an impact, and especially on a lower amp circuit like this. And the car does fine with voltage levels being lower. It doesn't need 8 gauge.

I was thinking I'd use AWG 10 wire since it's a 30 amp breaker.

Yes, that is true. The ampacity table from Cerrowire has been posted in here, and it shows how this is rated. For 10 gauge, the NM-B bundled multi-wire cable is in the 60 degrees column, and that is rated for 30A. Individual wires in conduit would be the 75 degree column, and that's rated for 35A, so either format of installation will be fine with 10 gauge thickness.

You need 8/3 wire.

Definitely not.

His proposal was to install a 60 amp service.

"Installation of 60 amp circuit for charger approx. 80 foot through basement to garage.Install and commission homeowner supplied charger."

I asked if he was going to use conduit and he replied "No it will be fished in the wall and will be installed like your other wiring stapled to the floor joists". I asked if it was 6 AWG and he said it would be 6/3.

Damn it. We see so-called "electricians" make this mistake all the time. From the table I mentioned above, you can see that 6 gauge is right on the border of being acceptable or not acceptable for a 60A circuit. If he were doing wires in conduit, that is rated for 65A and would be fine. But with him saying he would be using 6/3 cable, that's only rated for 55A and is wrong for a 60A circuit. So if it would be Romex cable like that, it would have to go up a size to #4.

But you're not doing a 60A circuit right now, so isn't the main topic. The point is that it is mostly straightforward to double check it against that wire rating table.

 

[ATPMSD]

Definitely not.

Plug in the numbers. This was for his originally stated need, which was to a 14-30 outlet and a 60-foot run. Please explain if you still disagree.

Wire Size Calculator | Irrigation & Landscape Lighting

Whether it’s wires, cables, cable assemblies, or electrical accessories for commercial and residential applications, Paige Irrigation and Landscape Lighting offers the best of the best. We build irrigation and landscape lighting systems that are smart, efficient, and reliable.

www.paigewire.com

 

[Rocky_H]

Plug in the numbers. This was for his originally stated need, which was to a 14-30 outlet and a 60-foot run.

I just did, and it shows 10 gauge. Part of that is because you do the calculation based on the constant current of a continuous load, which is the 24A, not on the full 125% required overcapacity of the circuit, which is 30A.

Voltage Drop - Continuous loads?

So I have a question on how to figure vd on a continuous load. My understanding of the code is that I would do my voltage drop calc and compare that back to the size of wire needed for My continuous load and verify my wire is larger. If that is not explained very well, here is my math. 480v...

forums.mikeholt.com

Please explain if you still disagree.

Also, it's because that calculator you are using has a very aggressive allowed voltage drop of only 3%!!! It doesn't need to be nearly that strict.

The one you linked to is only where you specify the allowed % drop, and it tells you the wire size. Most calculators are switchable, so you can reverse it, and give the wire size you're using and leave the voltage drop % blank and let it calculate that. Using one of those, with the proper 24A and 10 gauge wire, it does show only a 2.6% voltage drop, which is fine. Here is that calculator.

Voltage Drop Calculator | Southwire

www.southwire.com

 

[timeshifter]

Yes, any form of that 10 gauge wire is fully sufficient for a 30A rated circuit, so either of those selections you found will be fine.

To clarify, you said either of those selections would be fine. Is it correct to say any of those selections? The two from the big box stores Lowes and Home Depot are roughly the same. The one on Amazon was a lot less. The big box store wires seem to be in a round bundle or jacket, while the less expensive wire on Amazon is flat.

Which brings me back to the question is why the huge price discrepancy?

 

[Rocky_H]

To clarify, you said either of those selections would be fine. Is it correct to say any of those selections? The two from the big box stores Lowes and Home Depot are roughly the same. The one on Amazon was a lot less. The big box store wires seem to be in a round bundle or jacket, while the less expensive wire on Amazon is flat.

Oh, right. I hadn't noticed there were three. I pulled up the links for them, and they are all forms of Romex (NM-B), so the electrical specs are the same. So yes, they're all about the same. The flat form I guess might just be preferred by some, depending on where it needs to be installed or what is convenient.

Which brings me back to the question is why the huge price discrepancy?

I couldn't even guess. You might as well wonder why Overstock.com has such cheaper prices than the same items from other retailers. Sometimes inventory quantity or age makes places want to get rid of stuff that isn't selling well.

 

[Watts_Up]

Do we want to plug in 30 amps in those calculations even though it will only be actually drawing 24?

If you want to design a plug to handle only 24 A, it is your problem, but it's a code violation.

Maybe one day you will sale you home and someone will use the plug as a 30 A plug.

 

[timeshifter]

If you want to design a plug to handle only 24 A, it is your problem, but it's a code violation.

Maybe one day you will sale you home and someone will use the plug as a 30 A plug.

Not trying to design anything special or out of the ordinary or that doesn't meet code. Just trying to understand the choices. I came into this thread thinking that 10 AWG was the correct size. I'm learning that that its not a 100% consensus. I could easily go to #8 and it's only a couple dollars more.

Amazon.com

I don't want to change the breaker or else I'd go ahead and go 50 or 60 amp. Was even thinking about going ahead and running a much heavier wire that would accommodate a 60 amp breaker in the future but understood that a heavy guage wire may not fit the 30 amp breaker. So I was trying to do just what is needed for a proper 30 amp today and worry about the future and upgrades down the road.

 

[ATPMSD]

Part of that is because you do the calculation based on the constant current of a continuous load, which is the 24A, not on the full 125% required overcapacity of the circuit, which is 30A.

Going to challenge you on this one @Rocky_H, wiring needs to be at the fully rated capacity, so if we are going to install a 30A circuit the wire (and breaker and outlet) needs to be rated 30A. Recall the reason we cannot use #6 Romex with a wall connector on a 60A circuit is because it is rated at only 55A; we cannot justify the use even though the wall connector “only” pulls 48A. Seems like the same argument?

 

[pb2000]

10/3 NM is the correct answer. The tesla adapter automatically limits the current to 24A, which is required for an circuit with a sustained full load. Voltage drop is something to consider, but the loss is so minimal that it would take as much as a decade to recoup the cost of upgrading to 8awg (depending on local power prices and how much you drive)

NM cable is rated such that it can safely be used in attics or other hot spaces without the insulation prematurely breaking down. If the cable run was entirely within a conditioned space, you could (BUT DON'T) run 50A through 10awg without any serious issues.

 

[Watts_Up]

10/3 NM is the correct answer. The tesla adapter automatically limits the current to 24A,

But the circuit breaker and the plug are rated 30A, so the wires should be able to handle 30A, not just 24A.

 

[father_of_6]

@timeshifter, the breaker is such an easy thing to replace, but the wiring... costly and laborious. This is *really* the right time to upsize the wire. Forget about a 60A circuit for 48A charging... just get 50A rated wiring for 40A charging and use it on your existing 30A breaker.

This way, in the future you can just swap the breaker and almost double your charging speed. The increased cost at this point will be nominal.

The wiring is much cheaper on AMZ than at big box stores. Three of these at $46 each and you're in business: https://www.amazon.com/THWN-2-GAUGE-STRANDED-COPPER-BUILDING/dp/B07S47H64J/. Note: this wiring requires conduit.

 

[terranx]

To answer the question actually asked, 10 awg is sufficient for a 30A circuit.