Which is better? Stranded copper or solid copper of the same gauge.
Stranded copper is easier to pull and flex. Plus the larger sizes only come in stranded. Going with 8 AWG will be less than a 2% drop for the 240V 40A mobile connector with 50 foot run.
The advantage of stranded construction is ease of installation (easier to flex), but it is more expensive than solid construction. NOTE: according to National Electric Code (NEC) minimum wire size for a circuit protected by 50A breaker (40A continuous) is #6 AWG. Do not let your electrician install anything less that #6 AWG
Thanks for the advice. I had to call my electrician back to have him change the #8 to #6.
@tranhv68 - 75C-rated #8 wire can handle 50A, but for residential use you have to use the 60C rating, which requires #6.
Note the above is for copper, if you are using aluminum you need #4.
In either case, you may also have to derate for ambient temperature -- if this will be in a garage that gets hot, you will need larger wire. For example, if you expect to charge at the full rate where the temperature might be above 95F, then you will need #4 (which is good up to 131F).
Jat@jaet.org - Just to clarify, 60deg C ampacity must be used per NEC for all circuits less than 100 A, due to temperature rating of terminations. This applies to industrial, commercial and residential installations.
Derating for higher temperature is not required non continuous (less than 3 hours per NEC) excursions above 86 deg F.
@vrginshpun - charging a car would be considered continuous use though, so wouldn't it still apply?
Resistance is proportionate to square of current density, and current only flows on the wire surface. So most high power wire in weight or cost sensitive use is stranded, to maximize surface area to weight and minimize resistance per pound.
@DTsea - "current only flows on the wire surface" - i thought this was only true at very high (microwave) frequencies - hence the use of wave guide. I thought that at low (50hz) frequencies almost the entire cross section of the wire was active for electron flow.
You guys are such geeks. LOL
A simple stranded is better would have probably been good. :-)
firstname.lastname@example.org - Charging is continuous, but temperature derating is required only if ambient temperature exceeds 30 deg C (86 F) continuously. It is not required if ambient temperature exceeds 30 deg C for a period of time shorter that 3 hours.
Also, since #6AWG has 55A ampacity at 60 deg C conductor temperature and 30 deg C ambient, at a lower ampacity of 50A it can be applied continuously at 35 deg C ( 95 deg F) ambient without exceeding the 60 deg C temperature of the conductor.
@Sundre_, yeah, I pretty much just read the first response and then got confused!
Hi Jat, I was getting ready to install a 35 foot run of #6AWG with a 50 amp breaker. (AFC Cable Systems 50 ft. 6/3 Gauge BX/AC-90 Cable from Home Depot. Copper, already installed in conduit.). However my garage gets very hot (due to 2 solar inverters), often 95-100F. Do I need #4 wire? I am only getting a 40 kWh model, and my longest daily drive is only about 30 miles, so it would not normally be charging at max power for more than an hour or two.
@Barryfinn - I think you should be fine - there is a ton of margin in all this anyway, it is only slightly above that rated temperature, and it probably won't be for 3 hours. If you are worried, you could just dial the current down to 35A or so, and the car remembers what current to use at a given location.
Jat - thanks. Wasn't looking forward to starting to buy all new stuff anyway. Will go with the #6 wire.
And as Jat and others have noted - there is a difference on single wire (THNN?) in a conduit w/ no other circuits vs. the bundled 4-wire type. My electrician installed #8 in the conduit run along a garage beam and #6 from a junction box to the breaker in an enclosed attic space. My understanding is that this set-up it to code and fine for the 14-50 set-up.
@Barryfinn - #6 AWG wire will work with 50A breaker for an ambient temperature of up to 95 deg F (see my earlier post for details).
Just to be clear with margins, though, while it is true that there is a margin when the car charger is connected and monitoring the current, the limiting factor for the design is whether the panelboard breaker (50A) protects the cable and terminations. If ambient temperature is higher than 95 deg F for longer than 3 hours with 50A flowing in the circuit, the breaker will not operate allowing terminations to overheat and potentially causing unsafe conditions. 50A could be flowing in the circuit if there is a low level short in the circuit or in some other abnormal conditions.
That is why, regardless of normal operating current, NEC requires that circuit (including cable and terminations) be protected by the breaker.
I would suggest to measure temperature in your garage at the height of the NEMA 14-50.
If temperatures are higher than 95 deg F i would suggest installing thermostat controlled exhaust fan to keep temperature at 95 Deg F or below. The side benefit of this is that your solar inverters will last longer in lower ambient temperatures.
@jjaeger - If breaker for the NEMA 14-50 outlet is 50A your circuit does not meet NEC requirements, particularly the part of it that uses #8 AWG.
It appears that your electrician is confused about NEC requirements on derating multiple (more than 3) CURRENT CARRYING conductors installed in conduit. The 4-conductor cable used for NEMA 14-50 circuit has two "hot" legs, neutral and ground. According to NEC rules neither neutral nor ground are considered CURRENT CARRYING conductors. The 4-conductor cable for NEMA 14-50 outlet has only two CURRENT CARYING conductors.
Thanks for the info. I actually do have a 1500 cfm exhaust fan installed (with appropriate air intake). Haven't had a whole summer with it yet, but it doesn't seem hugely effective.
Nickjhowe is correct. At 60 cycles, current flows through the cross section of common wire sizes for our purposes.
Stranded is used in lower gagues for flexibility. And in conduit.
Anyone else get a charge out of this thread? I am finding many of the posts to be simply electric. OK, I apologize.
Seriously, it is an education for a layman like myself.
vgrinshpun - I can't quite parse your comments. You note derating for more than 3 current carrying conductors and that a 14-50 has 2, so derating not required?
Bottom line for me - have a 20ft conduit run from a junction box to the 14-50 connector that has 4 individual #8 wires and yes, it's on a 50A breaker. Do you believe this is not to code? Thanks.
jjaeger - yes, derating for 14-50 is not required because only two conductors out of four are considered current carrying.
The common mistake made by electricians when selecting cable is that they look at the rating of the cable itself but do not consider terminations. Majority of cables used in residential applications are rated for maximum conductor temperature (i.e. at max. load) of either 75 deg C or 90 deg C. The standard rating for terminations in circuits less than 100A, however, is 60 deg C. Because of this the limiting factor in sizing the cable for these circuits is not the rating of the cable but rating of the terminations.
The bottom line is that for standard terminations #8 AWG cable (for two hot legs of the circuit) does not satisfy NEC requirements because its rating for maximum conductor temperature of 60 deg C is less than 50A. For two current carrying conductors out of four the minimum required size is #6 AWG (rated 55A at 60 deg C)
This, however, does not apply for two other (neutral and ground) conductors of the circuit.
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