2

Thanks to anyone who has the time to help me out. I will explain my situation and then ask follow up questions.

I have property with a 600' driveway leading to a cabin (a small workshop). There will also be a detached garage about 250' into the driveway, and perhaps in the future another cabin or house near the garage.

I had an electrician help design and install the electrical service to the cabin. The meter/main shutoff has a 200 amp breaker. From here, we ran ran 3.5" sch80 Conduit underground, with large pull boxes near all of the future buildings. We have 250kcmil Al (2 hots 1 neutral) and 2 AWG Al ground running all the way to the cabin, with enough coiled in the boxes to make splices.

The cabin has been up and running for about a year now, and I am currently building the garage with small apartment above on my own. The electrician, who is no longer involved, instructed me to use polaris taps in the pull box outside the garage to splice the 250 feeders and ground over to the garage subpanel. So far so good, right?

Now, I was planning on basically mimicking the panel work that the electrician completed at the cabin, but after doing a lot of research, I am wondering if I need to take a different approach and perhaps have a problem I now need to address at the cabin.

Inside the cabin, the electrician pulled the 250 feeders and ground into a trough below the panel, and used polaris taps to reduce the 250 Al to what I am guessing is 2/0 Copper (there are no markings on the wire, but that's the largest size that fits into the Square D 100 amp panel).

Now for my concerns and questions:

  1. Is it ok to reduce the feeders like this to fit into the 100 amp panel? I'm trying to figure out if there was a specific reason he did this besides getting it to fit into the 100 amp board. It was my understanding that each feeder was carrying the 200 amps and the wire ampacity must be able to handle this. I've seen some people argue this short length of wire is protected by the 100 amp subpanel, but that doesn't necessarily compute for me.

  2. It was suggested to me that if I install a 200 amp subpanel in the garage (although this is way bigger than needed), the panel should accommodate the larger feeders, and then I do not have to worry about reducing or any of the issues that may come with that. That seems simple enough for me, but do I have a big problem on my hands at the already installed and live cabin sub panel?

  3. As stated, each subpanel is fed by 4 wires and grounded all the way back to the meter/main grounding rod. I see nothing on the cabin subpanel in terms of a grounding electrode. Is this an issue as well? Before pouring the footer for the garage, I asked the electrician if I should be running anything for grounding (this was when he was still involved; he has since moved). He said something along the lines of "no, it will be taken care of later at the meter". Is this another thing I should be worried about?

I like to have all the information before calling anyone out, as I know different circumstances lead to different methods of install and whatnot.

Most of all, I don't want to be cheating off this electrician's paper if he was taking any shortcuts.

I appreciate any advice.

Thanks!

2 Answers 2

2

The 2/0 is protected by the panel breaker not allowing excessive current to be drawn.

There are "tap rules" in the NEC 240.21 that allows specific reductions in wire size for short distances to fuses/breakers when the tap is sized to carry enough current to trip the breaker it feeds. You should be good to go for the 10' tap rule and the probably 25' depending on interpretation by your inspector of physical protection (providing the panel has a 100A breaker and not just a rare disconnect).

Although not allowed 2/0 copper is rated for nearly 200A anyway so no real concern anyway.

You need grounding electrodes at every building, and the NEC requires it in 250.32.

You can read the actual NEC tap rules (and the rest of the code) by signing up for a free membership at https://www.nfpa.org/codes-and-standards/all-codes-and-standards/list-of-codes-and-standards/detail?code=70 . The National Fire Protection Association is the author of the NEC.

1

Markings

A wire with no markings is "mystery meat" that has NO rating, type or ampacity. Often this is obtained from shucking the sheath off a multi-conductor cable, and if it's a short run like a pigtail, that is often tolerated by inspectors. But certainly, it has no thermal rating because we just don't know what that insulation is.

It's probably 75C, most /0 wire and kcmil wire is. But not knowing is a potential inspector flag.

Ampacity

If you have 200A feeder, the general rule is you need 250 kcmil aluminum or 3/0 copper wire, assuming 75C thermal rated insulation, which anything that big ought to be.

However, if the origin electrical service serves a dwelling, a special rule kicks in, 310.15(B)(7). It says (follow carefully):

  • If the entire service is 200A, then the service wires and any feeders to any subpanels never need to be larger than 2/0Cu or 4/0Al.*

(Voltage drop is an entirely separate issue, and USA Code doesn't peg down a specific number, so it is at your discretion. Offer void in Canada, where they require "nanny breakers" to trip the circuit if voltage drop would exceed 3.75%. SMH...)

100A subpanel on 200A feeder

Assuming the wire is in fact 2/0Cu and we disregard the lack of markings... then that, going into a "sub panel with a 100A main breaker" sounds legal. The sub panel might only have 100A or 125A internal busing, but the 100A main breaker will protect those sections.

But I wouldn't do it that way myself. Remember! Voltage drop isn't caused by circuit breakers. It's caused by the actual load in this moment. If you put an ammeter next to a "voltage drop meter", the needles would move together, in lock-sync. The way you contain voltage drop is by not overloading feeders in any given moment, and that's a function of your loads and what your house is capable of.

Every house gets a Load Calculation (there's a standard method for doing that, two actually) that will spit out a number like 57.3 amps. It is the canonical number used to decide the minimum service size and other factors incl. voltage drop. I bet a mostly-gas dwelling has a number about like that, and I bet your cabin calculated to that, and that was the basis for the 250 kcmil.

There is no earthly reason to down-breaker the house (or the garage/apt) below 200A. There is no Code requirement to do that. It's a "silly notion" that got into someone's head, because they don't realize the Load Calculation already does that. Maybe they're from Canada LOL.

So no - I would not have done the cabin that way and would not do the garage that way.

  • Right off the bat, you need 3 expensive Polaris splices to step-down from 250 kcmil to 2/0.
  • You need a "Much larger than you'd expect" enclosure for those splices, which means you're stuck in NEC Article 370 hell sizing the box (no place for novices), and have to go to the electrical supply, because big-box stores don't specialize in big boxes.
  • As such, I recommend using subpanels as splice blocks since they provide the lugs and solve the enclosure problem, are cheap(er?), novice-friendly and "come with benefits".
  • The ginormous box adds clutter and consumes wall space, since its cover must remain accessible.

So for Pete's sake... why not just select a 150A or 200A main-breaker sub panel whose lugs can take the 250 kcmil directly? Cleaner installation, cheaper, and it gives you more breaker spaces.

That last is a big deal. If you spend time around sites like this, we have a steady stream of "Help, my panel is out of breaker spaces!" questions. Now when you're buying a panel, more spaces are maybe a buck or two each. But somebody thought themselves a thrifty shopper spending $70 on a panel instead of $85 for spaces they didn't "need". Not thrifty at all.

Voltage drop to the garage

For the garage installation, you are allowed 4/0 wire (since there is a dwelling powered by that service) and you are using 250 kcmil. 200' is just long enough to be worth checking the voltage drop calculator, but no need - you are already doing 1 bump to 250 kcmil. You will have stable voltage there even at 200A usage.

Therefore there is no constructive reason to have less than a 200A breaker at the garage. There's nothing to protect.

How about 200A at the cabin?

For that matter, you could have a 200A breaker at the cabin. Voltage drop would be gravely alarming at full 200A flow, but that isn't going to happen. The breaker does not decide voltage drop! Your actual usage in that moment does. Back to the Load Calculation: That is the realistic number to insert in the voltage drop calculations.

For instance, if you tried installing a 120A tankless water heater at the cabin, that would not work. However, the Load Calculation would tell you that. (at 180A your voltage drop would be 9%, giving you 110/220V, can you imagine having 110V power!?)

Never use breaker trip in a voltage drop calc.

Grounding electrodes

The electrician was wrong (or you misunderstood).

The conversation that was supposed to happen was "Yeah, you will need grounding electrodes at every building. Tell your concrete contractor that you want an Ufer ground cast into the concrete, because that's the best grounding electrode known".

However, that didn't happen (?). If I were king, there'd be a 5% excise tax on every foundation pour that didn't include an Ufer ground. They'd be putting them on sidewalks lol.

As such, you will need 2 8' ground rods on each building including those previously neglected. (legal minimum: 6' apart, actual excellence: opposite corners). Unless your first ground rod passes a complex impedance test that costs more than another ground rod.



* For instance, people often misinterpret 310.15(B)(7) to think they get an 83% favorable derate on any feeder, allowing them to run e.g. #2AL to a 100A sub panel off a 200A main. No, that is not what it says. Worse, when 310.15(B)(7) was a table, people went to that table for all feeders because it's a simpler table than the proper one. 310.15(B)(16).

4
  • Thank you so much for such a detailed response. I will simply splice the 250's into the garage and connect them directly to a 200 amp board. Voila! I definitely asked the electrician about the Ufer when I was pouring the ICF footing and walls, but he said not to worry about it for whatever reason. I'll add two 8' grounding rods to the design no problem.
    – VTcabin
    Oct 30, 2021 at 17:35
  • As for the existing 100 amp panel at the cabin (workshop), do you think there is any immediate danger/risk in not swapping to a 200 amp panel asap? Thanks again your help. I was very apprehensive with how the system was designed but I am feeling much more confident that everything can be brought up to snuff without any major headaches.
    – VTcabin
    Oct 30, 2021 at 17:40
  • @VTcabin full disclosure, I'm the same long-winded iconoclast you're talking to on DIYchatroom. So you're not getting a diversity of opinions here. If the 100A panel is as you describe (2/0 copper) It's permanently fine, as-is. For full fidget-code compliance, replace that mystery wire with 2/0Cu THWN with markings. Planning to exceed 100A is probably a bad idea anyway due to voltage drop. Difference is, I would do that with a load calculation (cottage is designed to be unable), not a nanny breaker (cottage is forbidden). Oct 30, 2021 at 19:18
  • Thanks for the heads up. I appreciate your multi-forum thoroughness. I'm usually just a lurker on these websites, but I sure am glad I ran into you! We seem to be on the same page now and it's all making a lot more sense. I really only got myself down this rabbit hole because I got it stuck in my head I'd be using the 100 amp panel in the garage. I bought a 200amp panel today and will just connect my 250s right to it. I don't know why I would want to make things more complicated. And I feel much better about how the electrician left the cabin/workshop. Thanks again!
    – VTcabin
    Oct 30, 2021 at 20:58

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.