-
Want to remove ads? Register an account and login to see fewer ads, and become a Supporting Member to remove almost all ads.
-
On today's TMC Podcast, we ask the question "What is the Tesla Cybercab?". Join us on YouTube live at 1PM and participate in the chat.
What wire to buy for 30 amp 240v circuit
- Thread starter timeshifter
- Start date
ATPMSD
Active Member
Pretty sure #8 copper is the smallest wire you can use to carry 40A. Yes, #10 THHN is rated for 40A, but this assumes 90C wire, which is not used in homes.
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
I'm with all the others that suggest future-proofing the wiring and planning for a 60amp circuit pulling 48amps continuous. It's very little more money and no more effort than putting in the thinner wires. Even if only running 30amps over it, it still works fine, even better. And it's trivial to pop in a 50amp (14-50) or 60amp (Tesla wall charger) breaker.
ucmndd
Well-Known Member
#10 conductors on a 30 amp circuit at a 60 foot throw is perfectly fine.
There is zero distance- or voltage drop-based reason to upsize the conductors. Ignore that nonsense.
Now personally I generally agree with others in this thread when they say “run the wire once” and if I was doing this I’d personally probably spend the extra buck or two per foot to do it right.
10/3 romex is about $2.50 a foot, give or take. An important note here, you’re running an extra conductor (the neutral) over a longish distance, that you’ll NEVER USE for EV charging. And you’re limiting yourself to a max 30 amp circuit forever.
On the contrary, 6/2 MC cable is about $3.25 a foot online. Not wasting money on an extra needless conductor, good for 65 amps, a better solution all around. Terminate it with a NEMA 6-50 and leave enough extra wire to eventually put in a wall connector or other hardwired EVSE.
Swapping out the breaker is like 20 bucks.
For less than $100 in materials all-in, you can do this right the first time.
There is zero distance- or voltage drop-based reason to upsize the conductors. Ignore that nonsense.
Now personally I generally agree with others in this thread when they say “run the wire once” and if I was doing this I’d personally probably spend the extra buck or two per foot to do it right.
10/3 romex is about $2.50 a foot, give or take. An important note here, you’re running an extra conductor (the neutral) over a longish distance, that you’ll NEVER USE for EV charging. And you’re limiting yourself to a max 30 amp circuit forever.
On the contrary, 6/2 MC cable is about $3.25 a foot online. Not wasting money on an extra needless conductor, good for 65 amps, a better solution all around. Terminate it with a NEMA 6-50 and leave enough extra wire to eventually put in a wall connector or other hardwired EVSE.
Swapping out the breaker is like 20 bucks.
For less than $100 in materials all-in, you can do this right the first time.
It's a slippery slope. First, my service is 100 amp and all my breaker slots are full. I have a quad that includes two 30s linked on the inside and two 15s on the outside. That's a 30 amp 240 volt circuit that I'm not currently using. In theory I could replace that quad with one that has 60 amp and 15 amp, but so far I can't find one. My panel is made by ITE installed in 1985 (1984 date code) and uses Siemens breakers.
If I could find a breaker it might make sense to go ahead and do a 60.
So, how about running a wire that will support a 60a service like a #4 or #6 to future proof my 30a? My understanding is that the thicker wires won't connect well to the 30a breaker as they're too big.
I'd love to upgrade my panel and service right now but got other big spending projects going on too and it's just not a good time for that.
My car is a SR so it won't benefit beyond a 40a circuit anyway right now. Yes in the future that will likely be different.
Finally, running the wire is pretty easy. The entire run is in an unfinished basement, only have to go through the wall once to get to the garage and land right where the outlet is.
If and when I upgrade my panel and service it will be easy to run another wire for a bigger circuit or maybe put a sub panel in the garage.
I've been charging on 100v 15a for three years. Just got an LFP battery put in and need to get to 100% at least once a week. And I already had a close call with an outlet on the current circuit that went bad - it only made the circuit stop working well, but could have been worse.
Going from 120V 15A to 240V 30A will be bliss. If the cable run was hard, I'd agree with upsizing the cable for future considerations, but no point if it's an hour of work to fix if you ever need to.
Not sure if anyone else has mentioned this, but there are 6-30 adapters for the UMC available from 3rd party vendors, so you could run 10/2 and save a bit of $
Not sure if anyone else has mentioned this, but there are 6-30 adapters for the UMC available from 3rd party vendors, so you could run 10/2 and save a bit of $
ATPMSD
Active Member
Small fly in the ointment, the NEC requires a GFCI breaker. The exception if is the circuit was done when the house was built. If the circuit was added or you changed the wiring after the code was revised in 2017, you need to change the breaker (does not look like you have the room). On the other hand, if you install a Wall Connector then the standard breaker will do. The Wall Connector can be configured to 30A and later changed to 40A, 50A or 60A if you decide to change the wiring and breaker.
So then, if the circuit is pre-2017, you probably should not change the wire. IMHO your best bet is add a wall connector now, then install higher capacity wire at a later date if you want too; this will spread out the cost. Just toss the UMC in the car's trunk so it will be with you if needed.
Just something to think about.
ATPMSD
Active Member
@timeshifter
BTW, one way to get around the GFCI issue, if it applies in your case, is to terminal the circuit at a small sub-panel in the garage. From there you run a circuit to the outlet using a GFCI breaker in the sub-panel.
BTW, one way to get around the GFCI issue, if it applies in your case, is to terminal the circuit at a small sub-panel in the garage. From there you run a circuit to the outlet using a GFCI breaker in the sub-panel.
Challenge what? You're trying to mash together two separate topics.
Yes, absolutely it does. We agree. That topic is about figuring the rating level of the circuit. And I already talked about that, by pointing out that the continuous current is 24A, so it must have wire and breaker rated for 125% of that level, which is 30A. The 10 gauge wire does have a 30A rating. Sorted.
The other topic we were talking about, which is entirely different, is how you calculate voltage drop. And for that, you use the actual current being run, not the oversized rating of the circuit. I gave a reference link for that distinction.
stopcrazypp
Well-Known Member
Yes, you use 30A for code calculation, but as per references posted, 10/3 is rated for 30A outlets under code. The code is primarily concerned about insulation and overheating.
Voltage drop is an entirely different subject. For that, code only has suggestions, no requirements. So for that you can use the actual load, don't have to use the intermittent rating.
Reference below:
voltage drop: 2% or 3% or 5% ?
I read various % voltage drops acceptable, 2,3, or 5 % I'm a little confused. In wire sizing for an underground run to a detached building, which is recommended? My customer asks for 60 amps to a barn, 160 feet away. Thanks !
forums.mikeholt.com
Yes, with higher voltage drop, there will be more heat dissipated, however the code is only concerned if it can be safely dissipated, which it can for 10/3. Basically the longer the wire, the more voltage drop, but you also have more wire to dissipate the heat.
And if you want to bring up the case of "what if I use a 30A intermittent load?" Keep in mind the the only commonly used high power appliance through a 14-30 are electric dryers, which not only typically don't actually draw 30A, but they also can tolerate 208V, 220V, so a bit more voltage drop from 240V won't matter.
davewill
Active Member
After seeing the picture of the OP's panel, I understand exactly where he's coming from. However, unless he's simply going to ignore permitting, he still needs to deal with the GFCI issue. I see two ways to deal with that.
1. Switch to a wall connector (at 30a), then he doesn't need one.
2. Terminate the new 30a circuit in a small load center, where the new GFCI breaker can then go.
If it were my house, and I was planning to stay for the forseeable future, I'd be tempted to bite the bullet and replace that panel with something that has more capacity. Especially if by doing so I could up the capacity. Depending on the size of the conductors coming into the house, he could conceivably go to 125a or 150a.
However, short of spending all that money, I'd probably go with the wall connector. You can always take it with you when you leave, and no worries about outlet quality and GFCI.
1. Switch to a wall connector (at 30a), then he doesn't need one.
2. Terminate the new 30a circuit in a small load center, where the new GFCI breaker can then go.
If it were my house, and I was planning to stay for the forseeable future, I'd be tempted to bite the bullet and replace that panel with something that has more capacity. Especially if by doing so I could up the capacity. Depending on the size of the conductors coming into the house, he could conceivably go to 125a or 150a.
However, short of spending all that money, I'd probably go with the wall connector. You can always take it with you when you leave, and no worries about outlet quality and GFCI.
ATPMSD
Active Member
Half-correct. The UMC’s GFCI protects the connection between the UMC and the Tesla. The GFCI in the breaker box protects the outlet and is required by the electrical code - at least in the United States,
I'm normally a stickler for code, but this one seems dumb. There is nothing in the average garage that is going to be bonded to ground and if the floor is wet, you're going to have shoes on. 120V is only bad if you have a good path to ground and if you get the full 240V without a ground path, the GFCI isn't going to help you.
