can you have a sub panel feed a sub panel

[John Frum]

User wants to setup a property with a main building and 2 yurts.
The closest yurt is 300 feet from the main building.
Second yurt is 100 feet past the first yurt.
Power budget for each yurt is ~6000 watts.
Is this an NEC compliant topology?

main_panel->yurt1_sub-panel->yurt2_sub-panel

 

[FilterGuy]

Is this an NEC compliant topology?

As far as I know it is but....

That is a lot of power going up to 400 ft. Be sure to calculate voltage drop when picking wire size. Is it 120V?

 

[John Frum]

As far as I know it is but....

That is a lot of power going up to 400 ft. Be sure to calculate voltage drop when picking wire size. Is it 120V?

That is what the user thinks they want.
I was coaching for split phase 120/240.
This is just academic as I'm stepping away from this can of worms.
This is the thread is you are interested.

Off-grid solar for new development (Campground+Two Yurts), thanks for any help!

Hello everyone, I've asked a few questions on here and have had some very helpful advice and have learned a ton so thank you. As we're starting to finalize our off grid solar design, I wanted to ask for any input as it would be tremendously helpful. We are designing a property that will have two...

diysolarforum.com

 

[upnorthandpersonal]

Here in Europe that would be 3 phase (400V/32A), with a 5x6mm² connection.

 

[time2roll]

I think you need (want?) a panel where the main cable extends to both rather than feeding the second location off a breaker in the first sub panel.
I believe there could be power sharing rules that would allow less than the combined 12,000 watts. Maybe not much with just 2 units.
If this is on you I would confirm at Mike Holt forum.

 

[svsagres]

NEC allows sub panels off of sub panels. Organization I work with has their own private hydroelectric system with 2400v distribution. Technically speaking, our entrance/service panels are sub panels due to our power source being private, rather than a public utility.

Anyhow, in a couple of our buildings, our panels run 3 layers deep. This was acceptable to our inspector, as long as the grounding is done properly (earth connection lifted from neutral in all the sub panels) and the breakers/feeder wire sized appropriately.

 

[FilterGuy]

This was acceptable to our inspector, as long as the grounding is done properly (earth connection lifted from neutral in all the sub panels) and the breakers/feeder wire sized appropriately.

When dealing with separate structures, I believe the NEC requires an earth grounding electrode where the line enters the building. (but no ground-neutral bond). I question the wisdom of this requirement because it creates multiple earth grounding points in the grounding system.... But that is what the requirements say.

 

[Shale MacGregor]

When dealing with separate structures, I believe the NEC requires an earth grounding electrode where the line enters the building. (but no ground-neutral bond). I question the wisdom of this requirement because it creates multiple earth grounding points in the grounding system.... But that is what the requirements say.

You don't need a separate electrode when fed by a single branch circuit with a ground. In terms of your multiple-earth grounding points, even when this is done in the same building they all need tied together. NEC requires two electrodes unless you can get 25 ohms or less from one.

 

[FilterGuy]

You don't need a separate electrode when fed by a single branch circuit with a ground.

Is that a recent change to the code?

 

[Steve_S]

IF you are having it inspected then you have to follow your National & Local electrical codes and requirements. So DO familiarize yourself with what is required for YOUR region.

 

[FilterGuy]

Is that a recent change to the code?

Strike that. I was remembering the terminology wrong.

Correct, a separate electrode is not needed for a branch circuit, but a branch circuit has a very specific definition,

NEC Article 100 "Definitions" defines branch circuit (BC) as "The circuit conductor between the final overcurrent protection device (OCPD) protecting said circuit and the outlet(s)." The same article also defines outlet as "A point on the wiring system at which current is taken to supply utilization equipment."

If the circuit is feeding a sub panel. It is not a branch circuit.

 

[Shale MacGregor]

Strike that. I was remembering the terminology wrong.

Correct, a separate electrode is not needed for a branch circuit, but a branch circuit has a very specific definition,


If the circuit is feeding a sub panel. It is not a branch circuit.

If a branch circuit is feeding a sub-panel with more than one circuit in it, then it is a feeder, otherwise it would still be considered a branch circuit. Probably more often just a disconnect or a junction box rather than a subpanel, but I guess that is dealers choice. I think it is interesting that it doesn't require it for extending a single circuit essentially but if you add OCPD's to the panel it suddenly requires an electrode. It is supposedly for lightning, but why is one ok and not more. Go figure.

 

[FilterGuy]

If a branch circuit is feeding a sub-panel with more than one circuit in it, then it is a feeder, otherwise it would still be considered a branch circuit. Probably more often just a disconnect or a junction box rather than a subpanel, but I guess that is dealers choice. I think it is interesting that it doesn't require it for extending a single circuit essentially but if you add OCPD's to the panel it suddenly requires an electrode. It is supposedly for lightning, but why is one ok and not more. Go figure.

Like I said in an earlier post.... I am not convinced the requirement for electrodes on feeder circuits to separate structures is a good idea.
I have tried to reason out why they require it and can't really come up with a good reason. On the flip side, having the 2nd earthing electrode creates the possibility of having a huge power spike go through the grounding system in the event of a nearby lightning strike. If anyone can explain the purpose of the 2nd electrode on the feeder circuit I would love to hear it.

One thing that surprised me when I first started studying this stuff, is that the understanding and subsequent rules around grounding are *not* well understood. I would have thought that basic grounding would have been well understood and settled by the mid-1900s but the reality is, people are still learning and the code around grounding is still changing.

 

[Shale MacGregor]

Like I said in an earlier post.... I am not convinced the requirement for electrodes on feeder circuits to separate structures is a good idea.
I have tried to reason out why they require it and can't really come up with a good reason. On the flip side, having the 2nd earthing electrode creates the possibility of having a huge power spike go through the grounding system in the event of a nearby lightning strike. If anyone can explain the purpose of the 2nd electrode on the feeder circuit I would love to hear it.

One thing that surprised me when I first started studying this stuff, is that the understanding and subsequent rules around grounding are *not* well understood. I would have thought that basic grounding would have been well understood and settled by the mid-1900s but the reality is, people are still learning and the code around grounding is still changing.

I do not understand any of this super well, but my general understanding is that a lot of the code is there to keep the chance of losing a ground to a minimum. It is fairly common to bury electrical out to another structure, be it a garage, pool house, etc. These are often neglected to be marked during digging work and are at an increased risk of being damaged. A partial damage would result in the second structure not having a ground or having a very poor ground.

Why would it create a lightning strike issue? The neutral isn't bonded to the new ground in the sub-panel, it would travel back to the main before hitting ground.

 

[FilterGuy]

Why would it create a lightning strike issue?

When lightning strikes nearby, it can cause a huge voltage gradient across the ground. The two electrodes and connecting wires become a current path that can carry a huge current spike.



The problem is that the large spike can arc to other parts of the circuit and can also create spikes on the neutral and hot due to inductive and/or capacitive coupling.

I do not understand any of this super well, but my general understanding is that a lot of the code is there to keep the chance of losing a ground to a minimum.

I am trying to understand failure modes on this and I am not following the argument.

The primary purpose of tieing a circuit to earth ground is to prevent a large potential between the circuit and earth. Very early on they figured out that letting circuits float and develop these potentials was hard on the equipment.... so they started tieing circuits to the ground.

If only the ground wire is broken between the buildings, the hot and neutral are still tied to earth back at the main building. Having a grounding electrode tied to the disconnected part of the grounding circuit does not change anything.

The big problem with disconnects is if either Ground or Neutral is disconnected but Hot is not. In this case, the circuit no longer 'sees' the NG bond so a short to exposed metal will not clear the fault. Adding a grounding electrode at the outbuilding does not fix this. (The earth ground resistance is too high to clear a fault.)

I keep thinking the reason for the 2nd electrode has something to do with the inductance of the ground wire back to the main building. If the inductance is high, it might prevent a high-frequency pulse from passing through to the main building.. but the Hot and neutral lines would have the same issue. I can't figure out how the 2nd electrode fixes anything.

Almost without exception, if I do not understand the purpose of an NEC requirement I just assume there is a case I don't understand. In this instance, I have done a deep dive on it and simply can't find an explanation for the requirement.

 

[FilterGuy]

I think it is interesting that it doesn't require it for extending a single circuit essentially but if you add OCPD's to the panel it suddenly requires an electrode. It is supposedly for lightning, but why is one ok and not more.

This is another reason why I question the requirement for adding the electrode to a feeder circuit? If it is truly needed for a feeder circuit, why isn't it needed for a branch circuit? How does putting a breaker on the hot wire change the need for an earthing electrode? This oddity in the code implies to me that whatever the reason for requiring the 2nd earthing electrode is.... it must be a mighty weak argument.

Having said all that.... I am open to an explanation. I would feel better knowing the purpose and reasoning than thinking the code got something as fundamental as this wrong.

 

[tigerwillow1]

...........rules around grounding are *not* well understood.

When my PV system was installed many years ago, the contractor did the grounding differently based on which inspector was assigned to the job.