Subpanel Setup Questions


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Old 01-11-24, 11:13 AM
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Exclamation Subpanel Setup Questions

Hi,

I planned this project 2+ years ago - finally underway!

I'm adding a subpanel next to my main 200A panel in my basement. Plywood mounting is up now and I have a plan mostly in mind, but need a little guidance.

The main box is almost full, but has 3 available slots at the bottom that will give me a place for the double-pole breaker for the subpanel.

The subpanel I bought has a main breaker. I remember way back when this started, there was a decision to go with a main breaker or not, but I chose a main (as a disconnect) so that I can shut down the subpanel while not interfering with the main panel and have some redundancy. The new panel is 150A, which I don't need, but thought for a few extra bucks, it's worth the space. Right now, the subpanel will have (1) 20A 120V circuit, (1) 30A 240V circuit and (1) 50A 220V circuit. I might add a few more circuits when I rewire the bedrooms and two bathrooms, but that will effectively take some load away from the main panel.

I plan to put my panel to panel connecting conduit on the lower left corner of the existing box and come in the lower right of the new box which will be to the left of the existing box.

Presently, the existing (main) panel has the main breaker at top. It feels weird to me, but it seems to make sense that the subpanel main be at the bottom where the connecting conduit is, making the connecting cables direct and quite short.

Here's my initial questions?

1. Subpanel upside down next to the main panel that's right side up - weird? Any issues?

2. I have 100A planned for the subpanel already. Should the subpanel breaker in the existing main box be at the 150A subpanel limit so that I have load capacity or how should that be sized?

3. Is there an argument against or an issue having a main in the subpanel since it connects to a dedicated breaker in the main box?

Thanks guys!






 

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02-21-24, 10:53 AM
Zorfdt
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The larger the AIC rating a breaker has, the higher fault current it can handle before literally failing. When a dead-short happens, it's more than just the 20A times 120v going through the wire and breaker, it's instantaneously much higher. But is limited by the size of the relatively small transformer outside at the pole. Those transformers can only put out so much power.

If you were in an industrial environment, or a much larger multi-family residence, the power company might be providing a much larger transformer, capable of providing much higher power to the building. That instantaneous short could be more than 10,000 Amps, which could (in theory) blow apart a breaker rated at 10kA. That's when higher AIC breakers are required.

That's my understanding of it at least.
 
  #2  
Old 01-11-24, 02:07 PM
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You need to install a 2P100A breaker in the bottom left of that main panel.
Your sub panel could have a main breaker but it is not required since there is a 100A
disconnect breaker in the main panel.

You can put the main breaker in the sub panel at the top and still run the wires in from the bottom right.
You must run four wires. Two to the 100A breaker and in your case....two to the neutral ground bar.
You need a neutral and a ground bar in the sub panel.
 
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Old 01-11-24, 08:04 PM
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PJmax thank you.

So that I understand, I will have 100A worth of circuits to start but may add a few more 15-20 110 circuits in the subpanel later. Won't I need more than a 100A breaker to accommodate that?

Four wires: two from the new breaker in the main panel to the main breaker in the subpanel (see subpanel below) and then one to a neutral in the sub and one to a ground in the sub (no bonding screw). Correct?

I also realized with the subpanel upside down, the doors would open into each other so NG.



 

Last edited by PJmax; 01-12-24 at 01:19 PM. Reason: added picture
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Old 01-12-24, 10:16 AM
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you say Quote :So that I understand, I will have 100A worth of circuits to start but may add a few more 15-20 110 circuits in the subpanel later. Won't I need more than a 100A breaker to accommodate that?

You
do not add the breaker amps to determine loads. Look at the main box. If you add all the breakers in it I am sure it is well over 200 amps. A 50 amp breaker will allow you to use up to 50 amps but if the connected device only uses say 40 amps then you have an extra 10 to use out of the box on another breaker. Plus will the 40 amps be used at all times? If not then it can be used by the other breakers.
What your second box will be limited to is 100 amps at the same exact time. Rather that comes from 5 20 amp breakers each using 5 amps or whatever the load may be at the exact same time.
Hopefully this helps you understand and does not confuse you more.
 

Last edited by badeyeben; 01-12-24 at 10:17 AM. Reason: changed quote
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Old 01-12-24, 12:58 PM
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There is no upside down for mounting that panel.
 
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Old 01-12-24, 01:21 PM
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Common sense....put a breaker in the main panel the same size as the breaker in the sub panel.
I can't see what size it is.
The wiring used must match the breaker size.

Mount the sub panel correctly.
Use an 1-1/2" - 2" nipple with bushings to connect the panels.
 
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Old 01-12-24, 08:04 PM
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Read the label for th3 bus stab limits before having breakers across from the new 100.
 
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Old 01-12-24, 08:31 PM
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Thanks for the great support, everyone!

PJmax thanks for the wire drawing. That confirms what I thought.

badeyeben thanks - I kind of understood that but what you're saying makes sense. Essentially, this panel is to power my home workshop. I can see hitting 65-70A at once now and then, but even that's unlikely.

PJmax the subpanel main breaker is a 150. So yeah it would make sense to me that the "branch" breaker in the main panel is a 150. I think that's what you mean by common sense - correct?

pcboss "Read the label for th3 bus stab limits before having breakers across from the new 100". I'm not fully following you. Are you saying that in the lower right of the new subpanel, I can't crowd breakers, or what exactly?
 
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Old 01-12-24, 09:25 PM
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The buss fingers are limited to how much ampacity they can handle. Add up the row(s) across from each other. They need to be less than the limit.
 
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Old 01-14-24, 07:21 AM
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pcboss I was unaware of the bus finger limit - thanks for the heads up.

I will look into that, but that's a new can of worms:

The existing main box was wired (to my knowledge) w/no consideration of a limit. It's mostly 15A and 20A breakers, but there are I believe two 30A dipole breakers. I don't know if anything there is over the limit. Now to install the sub, I need to add a dipole 100A breaker. You're saying that I have to watch row capacity in the new box that will have for now 100A total. In the existing main panel I have 15A and 20A branch breakers opposite of where the 100A will be added to power the sub, so wouldn't the main be at worse risk of passing the limit than the sub?
 
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Old 01-18-24, 10:18 PM
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I talked with Siemens and got some answers.

They have a 125A dual pole breaker to connect the main to the sub and told me there's no bus limit I have to be concerned with. I'll be running #1/0 copper from box to box for the hot and neutrals. I've heard that the ground doesn't have to be that large and have seen #6 used typically.

What is the appropriate ground wire size?

Does it have to be bare copper or can it be shielded?

I prefer copper, but could use aluminum. What size would an aluminum ground need to be?

No ground rod for the sub, just the ground connections through the main panel?

Thanks guys!


 
  #12  
Old 01-19-24, 10:02 AM
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Just one comment/suggestion.

You have a 150A service/panel. I would be hard pressed to install anything bigger than a 60A subpanel.
You can always keep the bigger loads (oven, AC, etc) in the main panel and smaller loads in the sub. Plus, for a one-person workshop, a 30A panel is likely more than you need. Table saw + dust collector + air compressor would be a 'worst case' scenario. How many times do you use more than one tool at a time?

But... of course there's nothing wrong with your plan too. Just a bit excessive IMO.

Sounds like you're on the right track!
 
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Old 01-19-24, 12:09 PM
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#6 Cu is good for the equipment ground up to 200A. The feeder conductors from the 125A breaker can be #1 Cu, which is good up to 130A. 1/0 Cu is up to 150A.
No ground rod is needed for the subpanel since it's by the main panel in the same structure. The equipment ground can be bare or insulated. Aluminum equipment ground will be #4.
 
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Old 01-20-24, 11:58 AM
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Thanks again guys!

The equipment ground can be bare or insulated
I would lean towards stranded, but I see solid too. Is there an advantage of one over the other?
 
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Old 01-20-24, 12:03 PM
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Stranded is easier to work with when using it as an equipment ground. I only use solid when running it outside to a ground rod or water pipe because it is easier to make it look neat.

You can get stranded in insulated or bare. If insulated, the insulation should be green if it is #6 or smaller.
 
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Old 01-22-24, 08:58 PM
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Tolyn Ironhand thank you. I have the #6 stranded installed, but it looks anemic.
 
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Old 02-17-24, 08:55 PM
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Hello!

I am nearly ready for rough inspection on this project and went down a new rabbit hole: 10k vs. 22k AIC rated breakers.

I have all 10k AIC breakers for this. There is a 125A 2 pole breaker in the main panel to feed the subpanel and a 50A 2 pole breaker, 30A 2 pole breaker, and 20A single pole in the subpanel.

Is there any logic argument that the 125A or 50A (for a welder) would be better/safer with 22k AIC breakers?

Thanks guys!
 
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Old 02-18-24, 04:01 AM
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You do not need the 22k rated breakers.
 
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Old 02-18-24, 08:02 AM
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pcboss - thank you

I couldn't find any info. Can you share the logic of when 22k are necessary?
 
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Old 02-21-24, 10:53 AM
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The larger the AIC rating a breaker has, the higher fault current it can handle before literally failing. When a dead-short happens, it's more than just the 20A times 120v going through the wire and breaker, it's instantaneously much higher. But is limited by the size of the relatively small transformer outside at the pole. Those transformers can only put out so much power.

If you were in an industrial environment, or a much larger multi-family residence, the power company might be providing a much larger transformer, capable of providing much higher power to the building. That instantaneous short could be more than 10,000 Amps, which could (in theory) blow apart a breaker rated at 10kA. That's when higher AIC breakers are required.

That's my understanding of it at least.
 
pcboss, Tolyn Ironhand voted this post useful.
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Old 02-29-24, 02:27 PM
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Zorfdt great explanation - thank you!
 
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Old 02-29-24, 07:17 PM
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I passed rough inspection, but I was told everything in a basement needs to be on a GFCI. The only circuit w/o one was the 50A for the welder. I have one on order now, but after doing a little research, I am unclear about wiring.

I had pulled red, black, and white #8 with no 4th ground since I'm using a NEMA 6-50 3 prong plug. My understanding was that 240V single phase has two hots and a neutral and that the ground would be through the conduit. The inspector checked my conduit fitting screws for tightness saying the conduit is the ground. I'm reading that with 3 wires, there is a ground wire and no neutral wire. The receptacle has a green screw labeled G.

The 30A GFCI I already installed (50A would be the same) shows a neutral connection between the two hots. The pigtail from the GFCI is connected to the neutral bus per instructions. The instructions (see #6) say "Connect neutral load wire if available). So I don't get it anymore and may have moved forward w/a wrong connection.

The 3rd wire (mine is white) going to the receptacle. Is it a neutral or ground?

Is my wiring plan wrong somehow?

Does the GFCI require a separate ground?

Do I not use the neutral terminal on the GFCI and connect my 3rd wire (white) to the ground bar?

All help is appreciated!




 
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Old 03-01-24, 03:04 AM
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The white is a neutral.
 
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Old 03-01-24, 11:11 AM
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Can anyone give me more detailed answers?
 
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Old 03-01-24, 12:09 PM
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You didn't need a white wire. Don't connect it to anything. (The white GFCI pigtail must still be connected because the GFCI's electronics use 120V).
 
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Old 03-01-24, 06:27 PM
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Since you only have a 240 volt only receptacle (as opposed to a 120/240 volt) you do not need a neutral wire connected to the GFCI breaker. The white pigtail of the breaker gets connected to the neutral bus. The neutral wire in the conduit can either be capped off in the panel as well or can be connected to the neutral bus. The receptacle will need a ground wire connected to it. You have a couple of options:

1) Connect a green wire of at least #10 copper from the box to the receptacle.
2) Run a separate ground wire back to the panel and connect it to the ground bus.
 
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Old 03-02-24, 01:08 PM
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This is my 1st 240V project and clearly, I lack some understanding of the theory side, especially w/the GFCIs. Thank you all for helping me along.

So if I understand this now:

1. the GFCI connects to the neutral, but that neutral does not leave the box in any fashion?

2. don't use the neutral terminal on the 240V GFCIs?

3. typically, the receptacle receives the two hot legs and gets its ground from the ground terminal using a pigtail to the box/conduit?

4. I ran a white wire end to end unnecessarily, but since it's there, I can use it on the ground terminal on the receptacle and then connect the panel end to the panel's ground bar?

Here are photos from my dual 120/240 outlet box with a 30A 240 receptacle as it is at the moment. The 50A would follow the same theory. (the ground wire is only connected to the 120V outlet)



5. Here's the panel. So just use my two 240V whites as a ground wire on the (unbonded) neutral?
 
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Old 03-02-24, 02:58 PM
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1. the GFCI connects to the neutral, but that neutral does not leave the box in any fashion?
The white pigtail on the GFCI breaker gets connected to the neutral bar of the panel.

2. don't use the neutral terminal on the 240V GFCIs?
No neutral wire is required to connect to the GFCI breaker on a 240 volt only circuit.

​​​​​​​typically, the receptacle receives the two hot legs and gets its ground from the ground terminal using a pigtail to the box/conduit?
Yes, if the box is properly grounded. Otherwise, a separate ground wire is connected from the panel to the receptacle.

​​​​​​​I ran a white wire end to end unnecessarily, but since it's there, I can use it on the ground terminal on the receptacle and then connect the panel end to the panel's ground bar?
Techinally you can't reidentify a white wire as a ground smaller than a #4 but in this case I would have no issue doing that. Just remark the white wire with green tape. (Side note: You should do the same to the white wire connected to the ground terminal going to your 30 amp 240 volt receptacle.)

​​​​​​​5. Here's the panel. So just use my two 240V whites as a ground wire on the (unbonded) neutral?
No. If it is a ground it needs to go to the ground bar.
 
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Old 03-02-24, 03:30 PM
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There should be no need to use the white as a ground. A ground should already exist in the circuit. White is reserved for neutrals. Grounds are green or bare.
 
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Old 03-02-24, 03:49 PM
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Tolyn Ironhand Thanks very much. I have green tape and will wrap all white ends to "make them green."

On #5 I meant to say ground but somehow it came out neutral.

pcboss I get it now, but the white wire is there, so after making it green, it makes sense to me to use the extra ground connection.
 
  #31  
Old 03-28-24, 11:44 AM
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FINAL NOTE:

I completed the job and passed the inspection.

Thank you everyone for your help!!!
 
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Old 03-28-24, 06:03 PM
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Good deal!
 
 

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