OpenEVSE charging of Roadsters?

[wiztecy]

Strand vs Solid use the same AWG rating, but stranded will "appear" to be about 10%+ thicker in size. Use strand for places where you need flex such as lots of tight turns, solid if you have rather straight runs.

"The AWG tables are for a single, solid, round conductor. The AWG of a stranded wire is determined by the cross-sectional area of the equivalent solid conductor. Because there are also small gaps between the strands, a stranded wire will always have a slightly larger overall diameter than a solid wire with the same AWG."

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Stranded wire AWG sizes
AWG gauges are also used to describe stranded wire. In this case, it describes a wire which is equal in cross-sectional area to the total of all the cross-sectional areas of the individual strands; the gaps between strands are not counted. When made with circular strands, these gaps occupy about 10% of the wire area, thus requiring a wire about 5% thicker than equivalent solid wire.

Stranded wires are specified with three numbers, the overall AWG size, the number of strands, and the AWG size of a strand. The number of strands and the AWG of a strand are separated by a slash. For example, a 22 AWG 7/30 stranded wire is a 22 AWG wire made from seven strands of 30 AWG wire.

American wire gauge - Wikipedia, the free encyclopedia

 

[hcsharp]

The panel is on one side of the house, and the EVSE most of the way across to the other. I'd guess between 35 and 40 feet total run, so not what I would consider close. I'm pretty sure he said that I needed solid wire, but what I'm finding at the big-box store is stranded. Also odd...

Wire that's larger than #8, such as #6 and bigger is very difficult to work with. Even stranded #6 is hard to work with so nobody uses solid wire that big. #8 can usually be found in both solid and stranded versions. The stranded version is usually used when you have to run it through conduit. When he said you needed solid wire he may have been referring to the #6 wire with large but few strands.

 

[PV-EV]

When sizing circuits there are a few things to keep in mind. First is the ampacity of the wire. This is dependent on the size and type of wire (copper or aluminum) and the temperature rating of the conductor which is determined by the type of insulation it has. You also might have to derate the ampacity according to how the conductor is used and the environment it is operated in (temps over 86F and number of conductors over three). For an EVSE circuit the derating is 80%. From NEC table 310.15(B)(16) #6THHN is rated 65 amps. 80%X65=52. #8THHN is rated 50 amps so that is the minimum size for a 40 amp EVSE (80%X50=40). #4THHN is rated 85 amps so with the 80% derating for EVSEs that's only 68 amps! Not good enough for the 70 amp EVSE. This is why #3 COPPER THHN (rated 100amps) IS NEEDED FOR AN 80 (or 70) amp EVSE 80%X100=80. Even though nearly all THHN is dual rated as THHN2 the higher ratings of the wire are not used because nearly all breakers have a temperature limitation of 75C. The higher rating only helps if you have to derate for temperature and/or more conductors.

In general, electricians use #6 for the 14-50 and 40 amp EVSES. If the wire run is all inside you can use 6/3NMB (Romex) which is typical range wire. Run a #10 ground wire with the #6 or use a #8 ground with the #3.

 

[gregd]

Thanks, PV-EV. Very informative.

So, the choices appear to be #6/3NMB+#10 ground if I want to stick with a 50A (40 useful) circuit, or go all the way to #3/3+#8 ground if I want to go any higher. It will probably come down to cost, availability, and ease of manhandling the wire during the install, but I first need to have a chat with the electric company about what I can do with the panel. If I'm stuck with a 125A service (unlikely), that will eliminate the higher charge rates.

One last question - if the installation of a 14-50 outlet always requires 4 wires (two hot, neutral, ground), why isn't there a 4-wire Romex product available? Several #6/3 on the Home Depot website, but nothing with 4 wires except aluminum service entry stuff. Do I just zip-tie the ground lead along with the main feed?

 

[PV-EV]

Thanks, PV-EV. Very informative.

So, the choices appear to be #6/3NMB+#10 ground if I want to stick with a 50A (40 useful) circuit, or go all the way to #3/3+#8 ground if I want to go any higher. It will probably come down to cost, availability, and ease of manhandling the wire during the install, but I first need to have a chat with the electric company about what I can do with the panel. If I'm stuck with a 125A service (unlikely), that will eliminate the higher charge rates.

One last question - if the installation of a 14-50 outlet always requires 4 wires (two hot, neutral, ground), why isn't there a 4-wire Romex product available? Several #6/3 on the Home Depot website, but nothing with 4 wires except aluminum service entry stuff. Do I just zip-tie the ground lead along with the main feed?

Sorry I did not see your post until today. To clarify, the 6/3NMB comes with a #10 inside it. There are 4 wires. I know, it is a little confusing the way they label these cables. With Romex it is always labeled size and number of conductors PLUS ground. With rubber cords it is total number of wires in the cord.

Also, if you decide to go with the higher amperage EVSE you don't really need the neutral wire so you can just run two hots (#3) and a ground (#8). Those would need to be in a conduit.

 

[gregd]

Sorry I did not see your post until today. To clarify, the 6/3NMB comes with a #10 inside it. There are 4 wires. I know, it is a little confusing the way they label these cables. With Romex it is always labeled size and number of conductors PLUS ground. With rubber cords it is total number of wires in the cord.

Also, if you decide to go with the higher amperage EVSE you don't really need the neutral wire so you can just run two hots (#3) and a ground (#8). Those would need to be in a conduit.

Ha, yes confusing. The Home Depot site even lists "# of conductors" as 3, not 4. I guess I needed to go there in person and take a look.

Conduit... I presume this may be a local building code thing, but is conduit needed in the crawl space under the house? The electrician only talked about it in the garage, and to be careful with the staples

Currently waiting for the electric company project engineer to schedule an on-site visit... Will grill him for local requirements then. Thanks for letting me know what questions to ask.

 

[supersnoop]

"# of conductors" as 3, not 4

The ground wire isn't a conductor.

 

[gregd]

The ground wire isn't a conductor.

That'd be news to my college physics professors... Just hope I never have to test that.

 

[PV-EV]

NEC allows 6/3NMB to be stapled to joists in your crawl space. Where subject to physical damage it needs to be protected (usually in conduit). Local codes might have stricter rules.

Supersnoop is correct. Ground wire is not considered a conductor because it does not normally carry any current. It is only there to protect against faults.

 

[coolVariable]

Add me to the list of happy OpenEVSE customers.
Charges the Roadster very nicely at 40A/240V.