New EV Charging/Water Heater SubPanel

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Old 06-12-15, 01:23 AM
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New EV Charging/Water Heater SubPanel

I'm installing a 100A subpanel in the same house as my main panel.

Sub panel will initially have breakers for EV charging (50A, 240V) and Water Heater (30A, 240V)

Distance to the subpanel is 50 feet. My plan is to run #2 copper (#3 would meet code but is not that much cheaper) from the main to the subpanel at least for the two hot wires.

1) Neutral (which won't be used much... maybe just the electronics in the EV charger) and ground will be isolated in the subpanel.

2) What is the minimum size allowable for the neutral and ground wires? My read is that the neutral will have to be the same as the hot (even though it will be lightly used) but I see conflicting info on the ground.

3) Havent been able to find if it is ok to have ground wire go to two set screws. The holes in my ground bar won't fit #2 so if I have to use #2 can I seperate some of the strands and run them under two seperate set screws (or will I have to buy a Lug attachment)?

Thanks
 
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Old 06-12-15, 08:11 AM
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The neutral can be sized to the panel load, which in your case is very low given that 80% of the panel capacity is already dedicated to 240V loads. It is considered good practice to make the neutral the same size or two AWG below the hots.

The ground is sized based on the maximum capacity of the hots (even if you have a smaller breaker installed). In your case this means a #6 copper ground, bare or green.

You need to get lug kits for any wires that don't fit. Modern panels accept up to #4 on the screw terminals, so you probably won't need one.
 
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Old 06-12-15, 11:05 PM
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The neutral can be sized to the panel load, which in your case is very low given that 80% of the panel capacity is already dedicated to 240V loads. It is considered good practice to make the neutral the same size or two AWG below the hots.

The ground is sized based on the maximum capacity of the hots (even if you have a smaller breaker installed). In your case this means a #6 copper ground, bare or green.


Based on this I intend to

1) run the Hots on #2 wires (as sub panel entrance supports 100A and #3 is not any cheaper

2) run neutral on #4 (Overkill but guess I understand the good practice part as someone later could add more 120V circuits)

3) running ground on #6 green stranded.

I am a little confused on the ground comment you made:
"based on the maximum capacity of the hots". The thing that confuses me is that the feed line to the sub panel is 100A ... if these are the hots you are talking about then #6 would be too small. My guess is that you are referring to the hots for the largest single breaker in the subpanel which is 50A so #6 is ok??


Thanks for the help
and fitting #6 in as single hole in my ground bar won't be a problem so that problem is solved.
 

Last edited by pcboss; 06-13-15 at 07:18 AM. Reason: format correction
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Old 06-13-15, 04:34 AM
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Ben made a slight error when he wrote:
The ground is sized based on the maximum capacity of the hots (even if you have a smaller breaker installed). In your case this means a #6 copper ground, bare or green.
That IS a true statement concerning the grounding electrode conductor (GEC) for the service (main) panel but a different rule is used when sizing the equipment grounding conductor (EGC) to a sub-panel. This rule states the conductor size based upon the size of the circuit breaker feeding the panel. Under the EGC rules a 100 ampere circuit breaker requires a #8 copper or #6 aluminum EGC.
 
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Old 06-13-15, 06:07 AM
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That IS a true statement concerning the grounding electrode conductor (GEC) for the service (main) panel but a different rule is used when sizing the equipment grounding conductor (EGC) to a sub-panel. This rule states the conductor size based upon the size of the circuit breaker feeding the panel. Under the EGC rules a 100 ampere circuit breaker requires a #8 copper or #6 aluminum EGC.

Thanks for the update:

1) I see that I will have to use #8 or larger (which is good because I have #8 laying around)

2) Related: I now feel pretty stupid running over 100 feet of #4 to a different subpanel fed by a 60A circuit (I had made all 4 conductors #4). It passed inspection fine but appears to have been complete overkill as could have used #10.


Given that the "The neutral can be sized to the panel load" it would appear that I could use #8 for the neural also (the only 120 I will have is whatever the the EV charger pulls for electronics, the main EV charging is supplied by 240, and the water heater will run off 240 only). Does anyone have a NEC reference for the "The neutral can be sized to the panel load" so I can reference it on the line drawing??
 
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Old 06-13-15, 07:18 AM
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That IS a true statement concerning the grounding electrode conductor (GEC) for the service (main) panel but a different rule is used when sizing the equipment grounding conductor (EGC) to a sub-panel. This rule states the conductor size based upon the size of the circuit breaker feeding the panel. Under the EGC rules a 100 ampere circuit breaker requires a #8 copper or #6 aluminum EGC.
Not sure if this applies to this case, but if the circuit conductors are upsized for any reason, the EGC must increase by the same proportion. This applies regardless of the breaker size. this would apply like where oversized conductors are run to compensate for voltage drop.
 

Last edited by pcboss; 06-13-15 at 10:49 AM.
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Old 06-13-15, 09:29 AM
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Sub panel will initially have breakers for EV charging (50A, 240V) and Water Heater (30A, 240V)
1) Neutral (which won't be used much... maybe just the electronics in the EV charger) and ground will be isolated in the subpanel.
Assuming the charger circuit is indeed a 240 volt circuit, no neutral is required. A neutral is only required if the voltage required is 240/120 volts.
 
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Old 06-13-15, 11:38 AM
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Assuming the charger circuit is indeed a 240 volt circuit, no neutral is required. A neutral is only required if the voltage required is 240/120 volts.

The plug for the charger is NEMA 14-50. It is four wire (2xHot, Neutral, and Ground). As a result of your question, I did some more looking and found that the charger does not use the Neutral pin. Some people do not wire the Neutral on their NEMA 14-50R.

In this case the whole panel could be 240V (don't know if I would still have to route a neutral to it).

I'm not really comfortable with this approach. When I see a a socket I always assume it is fully wired. And in the future a charger may utilize the neutral pin (I googled and there are some 240V devices that have electronics that use the neutral (run off 120V).

I'm not sure what this means about the sizing of the neutral wire to my panel. The hots to the panel (from the main) are #2 (100 A breaker) but It looks like I will have no 120V draw but a connector NEMA 14-50 that someone could plug something into that used 120V. Do you see any issue with running #8 neutral and ground from the main to the sub and noting on the line drawing that the loads in the sub-panel are 240V only???
 
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Old 06-13-15, 09:58 PM
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A #8 (copper) neutral would allow for a minimum of 40 amperes unbalanced current flow, 50 amperes under some circumstances, and that could mean two 20 ampere, 120 volt circuits on each of the two "hot" leads or a total of four 20 ampere 120 volt circuits. In a garage setting I personally think this would be ample.
 
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Old 06-15-15, 08:05 AM
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What pcboss mentioned is where I was going with the EGC sizing. The upsized hots are capable of handling more than 100A, so the EGC has to be upsized even if the breaker is currently set at 100A. That means a #8 ground is too small, so you've got to go to #6 which is a good for a panel ground up to a 200A panel.
 
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Old 06-15-15, 11:38 AM
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What pcboss mentioned is where I was going with the EGC sizing. The upsized hots are capable of handling more than 100A, so the EGC has to be upsized even if the breaker is currently set at 100A. That means a #8 ground is too small, so you've got to go to #6 which is a good for a panel ground up to a 200A panel.

Thanks. Finally got it (a little slow). If I were to upsize the breaker in the future (which I won't as the subpanel bus bar is only rated to 100A also) then the EGC would have to be upsized also.

As an aside, I'm using a main breaker subpanel (as apposed to a Main Lug subpanel) so the I will be able turn off all circuits locally (vs go 50 ft away and thru 3 doors). So I have a 100A breaker in the main panel, with 2x2 Hot, 8 neutral, 8 ground, feeding a subpanel with 100A breaker (installed by manifacturer, eaton, screwed down). EV 50A circuit fed by 4 number 8 (2xhot, neutral, ground). Water 30A fed by #10 (2xhot, 1xground, no neutral).
 
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Old 06-15-15, 12:09 PM
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No, the size of the ground is based on the size of the hot wires, no matter what size breaker you actually use on the hots.

In your case, #2 copper in conduit is good for up to 115A which we round up to the next standard breaker size of 125A to get final ampacity. That means the ground wire has to be sized for a 125A feeder no matter what size breaker or panel you currently have installed. Using table 250.122, you can use #8 ground for up to a 100A panel; and #6 ground for up to a 200A panel. Thus #6 ground is required for this panel.
 
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Old 06-15-15, 12:25 PM
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Another example would be where you ran say 1/0 to a 60 amp panel. The ground is based on the 1/0, not the 60 amp breaker.
 
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Old 06-15-15, 03:27 PM
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in your case, #2 copper in conduit is good for up to 115A which we round up to the next standard breaker size of 125A to get final ampacity. That means the ground wire has to be sized for a 125A feeder no matter what size breaker or panel you currently have installed. Using table 250.122, you can use #8 ground for up to a 100A panel; and #6 ground for up to a 200A panel. Thus #6 ground is required for this panel.

If I read this correctly, since I'm running THHN and the ampacity is given as:

#4 95A
#3 110A
#2 130A

Then weather I used #3 or #2, both are rated above 100A and since table 250.122 lists

#8 100A
#6 200A

I would have to use #6 in either case (since both #2 and #3 support over 100A)?

If this is so then I'll plan on running: 2x#2 hot, 1x#6 neutral, 1x#6 ground.

P.S. Thanks for citing the NEC table (I want to include NEC references on line drawing)
 
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Old 06-15-15, 04:27 PM
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The plug for the charger is NEMA 14-50. It is four wire (2xHot, Neutral, and Ground). As a result of your question, I did some more looking and found that the charger does not use the Neutral pin. Some people do not wire the Neutral on their NEMA 14-50R.
In this case the whole panel could be 240V
Yes. IF no 120v loads you can wire it as a 240 only panel, no neutral.
 
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Old 06-16-15, 07:05 AM
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Close, but you should use the 75 degree C column in the ampacity table 310.16 as 90 C temp ratings are not used in residential. That means copper ampacities are:

#4: 85A
#3: 100A
#2: 115A
 
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Old 06-16-15, 09:58 AM
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The 90 degree column is used for derating purposes.
 
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