Sound right? Electrician recommended 40A with UF 8/3 outdoor wire to be buried under my concrete

[FlasherZ]

So a run with 6/3 THHN in conduit will work for a 48A EVSE correct?

There is a technicality here, but I think you'll be ok. Outdoors you must use THWN wire, not THHN. THWN = for wet locations, THHN = dry only. Conduit outdoors is considered wet.

However, most wire made today is dual THHN/THWN rated. Some inspectors will want to see the wiring say outdoor or THWN, though. Be sure the wire you purchase is rated for outdoor/wet/THWN use.

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[FlasherZ]

So you get derated twice, once for the continuos load and again for the cable type, wow. So 4-3 UF for a 50A EVSE, or 6-3 THHN in a conduit. Glad my install is in conduit.

Oh, it gets even more fun than that for some complex stuff. There are times you can end up using one temperature column for one factor while using a second temperature column for a derating factor. It's beyond the scope of discussion here, but it can make your head spin and you get really good at looking at tables.

 

[bewbie]

I don’t understand why 6-3 UF will carry less amps then #6 in conduit if it’s the same size wire

 

[FlasherZ]

I don’t understand why 6-3 UF will carry less amps then #6 in conduit if it’s the same size wire

A lot of people (including me) don't. You can de-rate from 90 degree columns for temperature, but the end result is that ampacity cannot exceed that of the 60 degree column. It was proposed in the 1980's to eliminate that rule and the CMP said "no" and it's been in section 334 / 340 for a very long time (since before NM-B cable even).

 

[bewbie]

A lot of people (including me) don't. You can de-rate from 90 degree columns for temperature, but the end result is that ampacity cannot exceed that of the 60 degree column. It was proposed in the 1980's to eliminate that rule and the CMP said "no" and it's been in section 334 / 340 for a very long time (since before NM-B cable even).

so basically it will work

 

[FlasherZ]

so basically it will work

A lot of things will work - some safely, some unsafely. You could run 48A through #12 wire (it takes 200+A to melt #12 in half), and it will work, but it'll get really hot and if combined with other factors may burn down your house. Or maybe not.

When I wrote the Tesla charging FAQ on TMC years ago, I'd frequently get people who asked me whether something would work, despite being against code... and the answer from an engineering standpoint many times is "yes" - but I recommend against it for two reasons: legal and insurance.

From a legal standpoint, the NEC is generally codified in law. Even if you don't have inspections for the installation now, you may have troubles down the road and may be forced to rip it out and re-do it correctly in the future before you can sell your home. In some states (Oregon is one that comes to mind immediately), violating the NEC is more than just a citable offense and is a misdemeanor - with fines and potential jail time.

In addition, there are insurance implications... let's say your garage burns down and the insurance company inspector comes to assess the loss. If it is noted that you used an illegal wiring method, the insurance company might refuse to pay your loss -- or just simply tie you up in pending litigation hell.

Personally, I'd rather have the conduit anyways, it makes it much easier to maintain and reduces the risk of damage.

 

[Stlww18]

A lot of things will work - some safely, some unsafely. You could run 48A through #12 wire (it takes 200+A to melt #12 in half), and it will work, but it'll get really hot and if combined with other factors may burn down your house. Or maybe not.

When I wrote the Tesla charging FAQ on TMC years ago, I'd frequently get people who asked me whether something would work, despite being against code... and the answer from an engineering standpoint many times is "yes" - but I recommend against it for two reasons: legal and insurance.

From a legal standpoint, the NEC is generally codified in law. Even if you don't have inspections for the installation now, you may have troubles down the road and may be forced to rip it out and re-do it correctly in the future before you can sell your home. In some states (Oregon is one that comes to mind immediately), violating the NEC is more than just a citable offense and is a misdemeanor - with fines and potential jail time.

In addition, there are insurance implications... let's say your garage burns down and the insurance company inspector comes to assess the loss. If it is noted that you used an illegal wiring method, the insurance company might refuse to pay your loss -- or just simply tie you up in pending litigation hell.

Personally, I'd rather have the conduit anyways, it makes it much easier to maintain and reduces the risk of damage.

^This

I was toying around with options for using an L6-30 outlet, garage wall pass throughs, etc. rather than spending $1,300 to have a new 14-50 outlet run.

My deductible costs more than the new outlet, so in the grand scheme it didn't seem worth getting too cute with everything even IF my insurance still covered damages from improper wiring.

It's going to cost maybe a few hundred dollars more to do it right; don't be penny wise and pound foolish.

 

[krusshall]

I don’t understand why 6-3 UF will carry less amps then #6 in conduit if it’s the same size wire

It is the insulation type which limits the current carrying capacity. The insulating materials can only sustain a certain max conductor temperature. Why UF-B is not recognized for its actual specification is likely the result of a glacial and overly conservative bureaucratic organization.

 

[Tony Burgh]

Codes were written for a reason. Just like there are electrical and fire prevention codes, there are mechanical and safety codes. The boards are staffed by knowledgeable and experienced people and backed up by stringent testing facilities.
As was stated, codes can exceed requirements in many specific instances. Code agencies allow engineering calculations verifying safety and performance to construct outside of code requirements. ASME pressure vessel codes come to mind.
Some people have complained about rigid standards that allow no interpretation. For that reason, some safety codes are performance based and the onus is on the engineering and construction contractors. But if a failure occurs, there is no license or code to back you up, just your ”defective“ calculations. Litigation hell is in your future.
I’ll get off my engineering management soapbox since I’m retired and it’s someone else’s problem now. Peace.

 

[tls]

So you get derated twice, once for the continuos load and again for the cable type, wow. So 4-3 UF for a 50A EVSE, or 6-3 THHN in a conduit. Glad my install is in conduit.

Type USE is another option if you must direct bury, but you will have to transition to this cable type outside the building as it is not rated for indoor use (the jacket is flammable, or, at least, is not subject to flame/smoke testing). Most inspectors will pass a few feet of UF indoors to the panel but USE will raise eyebrows and possibly objections.

There are other suitable cable types such as URD but they are typically not UL tested/listed even though they will also conform in theory to NEC specifications such as type RHW. Though where I work in NYC, these are in fact commonly seen indoors because the utility ("poco") puts the demarcation point indoors, it is important to understand that the utility is not subject to the NEC and typically not to any local AHJ and can do things you cannot. You should really consider these cable types off limits unless you are going to have an electrical engineer file full plans and obtain prior approval for their use - in other words, for most of us, these just don't go.

 

[tls]

A potentially cost-saving note here, just looking back over this conversation: if you do intend to direct bury, find it acceptable to limit charging to 40A (not 48A) and are therefore intending to use #6 UF cable, be aware that you don't need 6-3: unless you intend to use the same feeder to serve 120V loads (which in most of the cases we are discussing, it would seem is not the case) you do not need a neutral, so you can use 6/2 UF-B at a somewhat lower price.

 

[jefro]

My question may be what keeps being asked here. I'd think the 48A part is really a 60A circuit question. Correct? So the question becomes distance, condition of install, size or not of conduit and so forth.

My thinking is that people are not starting out with the question of how much will I need to replace each day. They seem to be going straight to the most they can buy.

 

[Pioneer74]

I don’t understand why 6-3 UF will carry less amps then #6 in conduit if it’s the same size wire

Because there is air gap between the conductors is a conduit or raceway.

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