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NEMA 14-50, 50 feet from panel, is 8 gauge enough?

The electrician from my solar installation company got approval from my city to run 8 gauge wire 50 feet from my panel to the garage NEMA 14-50 outlet. I recall that all the electrical threads here recommend 6 gauge wire. Will 8 be enough or should I insist on 6? It is all run through conduit so pulling the 6 gauge wire later should not be a problem. The electrician is finishing up today and I am getting my car today as well!

If the city approves, you're covered. Some locals would probably recommend 6 awg. This would just add a safety margin and reduce your voltage drop.

I'm using #8 wire inside steel conduit at least 70 ft, and that was approved. If you're only using NEMA 14-50, it should be no problem. But you probably still need to get the permit and inspection, depending on your city/state.

The city approved the permit with 8 gauge. I was wondering if the resistive losses versus 6 gauge is going to increase my electric bill significantly?

Where I am (SF Bay Area) the code required 6 ga.

I would go on the side of safety and use the #6

I had 8guage and NEMA 11-50 since July in both home and office - No issues

I'm a non practicing electrical engineering graduate, so I hope I can remember my entry level EE formulas. Here
they are: V=I*R, P=I*I*R Where V is the voltage in volts, I the current in amps, R the resistance in ohms, and P the power in watts. A number 6 wire has a resistance of .395 ohms /1000 ft, and a number 8 wire has a resistance of .63 ohms/1000 ft. Stay with me, I know you're beginning to glaze over. A 50 ft cable has a resistance of 0.02 ohms if #6 and 0.0315 ohms if #8. But you have to account for the fact that the return wire also has the same resistance since it has to travel 50 ft to the junction box, so we have to double the ohmage to 0.04 for #6 and 0.063 for #8. The voltage drop will be the 40 amps charging current times the resistance, which will be 1.6 volts for #6 and 2.5 volts for #8, not a major drop in either case given the 240 volt start at the panel box. Where I see more of a problem, is in the resistive heating power dissipated by the cable. At 40 amps, I squared times R equals 64 Watts for #6 and 101 Watts for #8. I myself have a 50 ft run to the panel box but I prevailed on the electrician to use a #6 gauge. Even so, I'm wasting 64 Watthours for every hour that I charge at 40 amps. I grant you that we're charging at a rate of 10KWhrs, still we're losing 0.6% of the power during charging, and you'd be losing 1% if you went to the #8. Add this to the vampire losses and over the course of a year it all adds up. Frankly if I were you, I'd go with the #6. The cable supplied by Tesla must be no better than #8 or maybe even a #10 since it gets noticeably warm during charging...

Also, gauge isn't everything. Wire material and its quality (and quantity) is just as important. Several years back, the UL began to notice an abundance of Chinese manufactured extension cords failing their rated load tests. It was later revealed that the amount of conductive material was far below an acceptable level despite the proper gauge of the wire. Basically, the wires used much less copper to save costs, and would melt easily under load.

My electrician also said #8 wire, but I told him that the circut can be under full load for up to 12 hrs and to go one higher to #6... he agreed.

For the difference in cost, I would go with AWG 6. I only had 3 feet to run and used AWG 6.

I had a 20' run and went overkill with 6.

Kleist mentioned the crucial factor: duration. 10-20 hrs is far different than 1-2 hrs.

(Disclaimer: I am not an electrician. Always consult with a reputable and licensed electrician.)

Congrats on the new MS, and welcome to the club! As long as your electrician did not pull the 8 AWG through a ½” conduit AND run the conduit through the attic, or it is exposed to direct summer sun, you are probably ok (marginally). If 8 AWG is indeed routed in a hot environment you should reduce the Tesla draw on the Charge Touch Screen to 39A. If you only charge after midnight, like most of us, you are ok as well.

The Tesla Website “240 Volt Outlet for Mobile Connector” recommends 6 AWG, most likely to cover hot environments or long runs. Hopefully your electrician used ¾” conduit, as the ½” conduit would need to be replaced as well. I made the following info table from Google’g the National Electrical Code NEC and the American Wire Gauge AWG standards for ambient and elevated environmental temperatures. (Also, I’ve been through this the hard way: 125ft of 2 AWG through the attic for a HPWC.)

From the table you can see that you need 6 AWG to draw 40amps at elevated temperatures over 50 feet with a comfortable margin. Note the significant reduction in line power loss from 103W to only 65W due to upgrading to 6 AWG. If you need to change out the ½” conduit, put in 1” instead, as that will allow you to easily upgrade to a HPWC in the future.

TeslaMS 90°C Rated Max Current Max Current Power Loss Conduit Fill
Wall Circuit Pulls THHN/THWN Continuous Continuous Watts for Max# of Wires
Mounted Breaker 80% Size AWG @30°C/86°F @60°C/140°F 50Ft @30°C 1/2" 3/4" 1"
NEMA 5-20 20A 16A 12 30 21.3 20.6 10 18 29
NEMA 10-30 30A 24A 10 40 28.4 59.2 6 11 18
NEMA 14-50 50A 40A 8 55 39.1 103.3 3 5 9
NEMA 14-50 50A 40A 6 75 53.3 65.0 1 4 6
NEMA 14-50 50A 40A 4 95 67.5 40.9 1 2 4
Tesla HPWC 100A 80A 3 110 78.1 129.7 1 1 3
Tesla HPWC 100A 80A 2 130 92.3 102.8 1 1 3

NEMA 14-50: 4 Wires; Red-Blk-Wht(Nutral)-Grn(Gnd). Terminals accept 4AWG copper max.
Tesla MS HPWC: 3 Wires; Red-Blk-Grn(Gnd). Terminals accept 2AWG copper max.
http://www.usawire-cable.com/pdfs/NEC%20AMPACITIES.pdf
http://www.calculator.net/voltage-drop-calculator.html

I hope this table helps MS & EV owners plan their charge station wiring, and to quiz their electricians.

I found the news article about Palo Alto, CA requiring Main Panel & Wiring Upgrades for Level II (30A 7.2kW? 40A 9.6kW?) EV Chargers in new construction quite interesting. Does nothing to get a Tesla HPWC installed. If instead they simply required a big enough Main Panel to accommodate a spare 40, 50, or 100 amp drop-in circuit breaker AND finished a 1" Conduit routed to the Garage, then the owner can easily have an electrician drop in a breaker and pull any wire needed, whether for NEMA 10-30, NEMA 14-50, or Tesla HPWC.

All new construction "everywhere" should have a main panel sized to "accommodate" a spare 100A circuit breaker, and finish a 1" conduit to the Garage. This would involve the absolute minimum cost for the new construction, and would address the highest aspects of labor and material costs of doing an upgrade to existing construction. The new homes could be advertised as “EV-100A Rough-In Ready”

Sorry about the Table. All the carefully spaced columns got compressed. If someone knows how to post a Table, let me know, and I'll fix it.

Update: The news article about proposed Palo Alto EV Chargers I read was short, so I found the actual Ordinance Proposal:
http://www.cityofpaloalto.org/civicax/filebank/documents/37930 A 91 page PDF.

I stand corrected for future new single family homes:
Per Page 5 of the PDF, PA EVSE Task Proposal: 240V/50A Circuit Required. Strong recommendation for 100A support.

Per page 7 of the PDF, for single detached homes, the Task Ordinance would install EV Supply Equipment (EVSE) of at least 240V 50A capable. The circuit raceway (panel & conduit) shall be capable of supporting 100A.

Unfortunately, by page 11 of the PDF, the Draft requirement deteriorates to a 208/240V 40Amp wired circuit.

This above PDF Task Force document was released Nov 21, 2013. There was a meeting Dec 12th.
http://www.cityofpaloalto.org/civicax/filebank/documents/38224
It seems there is a move toward installing a 50A wired circuit in a 100A capable raceway.

Therefore a future new home would be NEMA 14-50 Live, and would be easy to upgrade to HPWC.
If the NEMA 14-50 was installed with 4 AWG copper (the max size its terminals will accept), then a HPWC could be installed in place of it with a 90A circuit breaker allowing for a Tesla charge current of 72A; at 30°C/86°F ambient temperature conditions.
If the full 100A is desired, or temperatures and run length are excessive, the wiring in the existing conduit would be pulled out and replaced with 3 AWG or 2 AWG.

Gabe;
The tag <pre> enables fixed font spacing. Not sure if it takes an actual table with borders, etc.

Our house was wired for an electric car charger many years ago when we remodeled and when I had no idea I would ever have an EV. It was wired with 8g over about 75 feet. I installed a NEMA 14-50 outlet and just dial down the amperage to 34. I had been planning on running some 6g romex for another outlet to use 40 amps but have found the 34 amps gives me 24-25 miles per hour which is plenty. If it is already installed with 8g, I would leave it. If not, use 6g (unless you have twin chargers, then maybe 3g)

I have 6 gauge for my NEMA 14-50 and recommend the same. You don't want a fire in the garage like just happened to a tesla owner near me:

http://www.reuters.com/article/2013/12/18/us-autos-tesla-fire-idUSBRE9BH...

The electrician ran 3/4 inch conduit from my panel in the back yard through an attic (which gets quite hot in the summer) to the front of the house and into the garage. All the conduit not in the attic is run outside under the eaves of the house. In light of today's events (garage fire very close to me), I think I will ask the electrician to replace the 8 gauge with 6 gauge wiring. Can 6 gauge be run in 3/4 inch conduit?

amptsup, (Ref my Table above.)
Yes, per NEC and AWG, #6 AWG Blk-Red-Wht-Gnd (4 wires) will fit in a 3/4" conduit.
#4 AWG, the biggest a NEMA 14-50R receptacle will accept, requires a 1" conduit.
If you upgrade to a HPWC, and since you have to go through a summer-hot attic, I'd use #2 AWG Blk-Red-Gnd (3 wires), and that will also fit in 1" conduit.
And from what we have all read recently, careful attention to good workmanship and torquing terminals per spec is critical.
If you are concerned while you wait for the re-wiring, set the charge control (on the touchscreen) to draw only 30A instead of the automatic 40A.

@GabrielB - Thanks for all the info. Really helpful. I am going to ask to pull the 8 gauge out and put in 6 gauge when my solar panels are installed. I think that is the best that can be done with the 3/4 inch conduit.

Note: Copper #8 almost as good as aluminum #6.

The wire material is part of the decision making process. I assume that it is copper if in a conduit.
The less the voltage drops due to wire resistance, the less you pay to heat the wire, and the higher the efficiency.

The confusion here is that there are two ways to run copper wire to your NEMA 14-50 outlet.

Put simply, the wire can either go inside your walls or outside your walls.

You need six gauge wire inside your walls and eight gauge wire outside your walls.

Why? Different types of wire are used for each application. Inside your walls, one thick Romex NM cable containing three conductors and a ground is used. Outside your walls, individual THHN wires are pulled through conduit for each conductor and the ground.

Your electrician (and your city) use the National Electrical Code to decide what wire to use.

What will do a lot will do a little. Never hurts to increase the load safety margin. The small cost difference is not worth the savings.

My electrician ran a #6 to the detached garage 50 feet away and installed a 60 AMP subpanel from which the 50 AMP is then connected.
However for a 30 foot run to the other end of the house he used #8.
Both are copper and under full load the temperature of the wire is about 80 degrees. The voltage drop to the short run is virtually nothing as I get 242. The other has not yet been tested.

You might as well wire it for future support of a HPWC.

I believe HPWC calls for 4ga wire, though the terminal block can handle up to 2ga wire.

We used 1.25" conduit, 2ga wire for the load and 4ga for ground.

If you look at the Ampacity table, THHN copper wire carries 10 more amps than THHW or THWN... but then you have to take into account the installation, how many wires in the conduit (more wires - more heat) which "de-rates" the current capacity by a percentage.

Check out this PDF to help you understand the NEC and its complexities...
http://www.mjobee.com/projects&news/NEC%20Art%20310.15.pdf

Schmo is spot on....

Here is my Romex installation notes:

I spent this day before christmas installing my 14-50r receptacle. I used Romex #6/3 wire (Lowes sell it by the foot) My panel is in my basement and about 45' from the garage plug.

I never see this posted often, but you need to use a very well anchored 2 gang wide wall box that is designed for electric ranges/ovens. The one I bought at Home Depot has a special knock out for #6 wire with a clamp type strain relief. DO NOT try to put this plug into a single with box. It will fit, but there is not enough room for the wires and this can cause several issues. Just don't do it. You will need a special cover that bolts to the receptacle in the center of the box and cover the rest. I chose a stainless steel on because you will use a little force to push the plug into the receptacle.

When you run Romex make sure you drill a 3/4" hole in the center of any studs or rafters that you want to run the cable through. Also use the proper non-metalic straps to hold the cable in the center of the studs/rafters. Make sure they are for #6 wire and do not pinch the cable too much. They are commonly called service straps while the smaller ones are called staples. Keeping the cable in the center is important. It makes it less likely that someone will drive a nail into it without realizing it might be in the wall at some point in the future.

In my area, you have to put a steel plate on the studs where you went through the center to meet code. Whether it is code or not, it is a good idea and only cost a few bucks for the plates. This way you can't drive a nail into the cable.

Make sure all of your connectors are stripped to the proper length and torqued to the proper torque...

When you put the receptacle in the box after you have terminated all of the connections. Carefully coil the excess wires in the same direction around the back of the box. Do not let them get all crossed up as this makes them far more likely to come lose.

Anyway, these are my notes on my installation....

Wire is cheap, go with the thickest wire for the least resistance.


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