Home Datacenter Wiring Reality Check


Old 01-28-19, 09:17 AM
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Home Datacenter Wiring Reality Check

Hello everyone! My first post here, been reading a lot and tons of googling, etc.

So I live in Michigan, in Berrien County. According to the state we adhere to the 2015 version of the NEC.

I want to give some background on my intended project as well as myself. While I consider myself as much of a home-gamer as anyone else, I do have additional experience that others may not. That being said, I want to make sure I'm on the right track.

Add 2 new 240v Circuits with receptacles in unfinished basement.

Some background on my home:
Built: 1959
Current State of Wiring: Mixed new and original wiring. A supposed electrician came through and rewired outlets and switches and the like, however since I've lived here I've been going around and fixing random newbie errors like reversed hot/neutral or reverse neutral/ground. It's... a mixed bag to say the least. I'm still digging up dead bodies left from this guy. Couple that with there are plenty of instances were there is original wiring, but newer fixtures for lights and the like which normally would entail a fire hazard but has been mitigated to some degree (but still needs to be corrected) by the fact that every light I have is a low-current LED smart light, so the thermal danger is mitigated to a degree.

The home is fitted with a separate 100A mains breaker in the garage and a sub-panel in the basement. This is handy as I'm able to completely de-energize the sub-panel to add new circuits and there is room enough in the panel/bus bars for two additional 2-pole breakers.

I have a home datacenter (yes, racks of computers, I'm that nerdy) as I'm a freelance CG artist and I need to reduce my render times.). And I'm going to be converting over from 120v to 240v to increase efficiency and reduce current load on the circuits as well as be able to turn up two new chassis I recently acquired that have... enterprise level power requirements.

The sub-panel in which I will add two new 240v 30A double pole breakers and pull two new runs of 10/2 THHN to terminate to two separately boxed L6-30R receptacles which are what my replacement 240v rack PDUs are fitted with (factory L6-30P, not pigtailed.)

The plan is to run two 3/4" FMT conduits each filled with 10/2 THHN and a separate ground wire as a grounding strap for the racks. This decision was made to avoid having more than 3 current-carrying conductors in a conduit and having to derate. 3/4" might be overkill but I also want to avoid 40% fill and I can't math this early in the morning (started typing this in the wee hours) I'm not looking to "barely get by" with any portion of this configuration as I wouldn't want to potentially screw myself if I decide to sell the house or screw over any potential new owners.

SO! With that, if that planned configuration doesn't look like I'm going to burn my house down immediately or kill myself and be in immeasurable pain the whole time I'm dying... (If it does, let's address my errors before moving onto my further requests.)

  1. I've read about 30,000 different opinions on weather or not FMT can be ran perpendicularly to floor joists mounted on the surface of the joist as this appears to be one of my options. Is there an issue with this fundamentally or code-wise? What is the best practice?
  2. if I choose to run my conduit along the brick wall, what do I do if I have to pass over a water line? There is a surface mounted hot and cold line for a big sink between my washer and dryer running vertically. Is there anything in the code regarding traversing other services? Is there a minimum height requirement from the floor for the conduit or can I run say, 12" off the floor which would allow me to clear all the pipes?
  3. In my particular area, the documentation isn't clear on whether or not I require a permit or to have it inspected. I've read that so long as I'm not executing work on "electrical service equipment" meaning the mains where I can't turn them off, I don't require a permit for residential wiring. Other documentation, however vague, including the electrical permit itself issued by the governing body of the code, in this case, my local township appears to indicate that I'd need a permit and inspection for changing an outlet or switch. Does anyone have any light they can shed on this? Honestly, I'm leaning towards just getting a permit anyway. Though if someone can point me to official documentation clearly stating this demarcation, if any, that could save me a couple of hundo on this project.
  4. If I have to pull a permit for the work and have my work inspected, will I get dinged on wiring that was here prior to my occupancy? Or is the inspection just on the scope of my work? It's my understanding that original wiring only has to adhere to the NEC standard the year it was installed and doesn't require updating. I don't want this project ballooning out into a billion others because now I let the state into my basement and they saw some janky 1950's era circuit.
  5. More to the point of my dinky 100 amp mains breaker. Would this need to be upgraded to support the additional calculated 32.4 amps at full load from my render farm? (For those wanting to nerd out, I have two Dell m1000e Chassis fully populated with 8 machines in each chassis; quad 16-core procs m915's with 1TB RAM each)
    If I have to upgrade that breaker, I understand that I'll absolutely need to hire an electrician and involve the service provider no doubt. But also, if I do need to upgrade, will the mains lines from the transformer be required to be upgraded or were they overbuilt out despite these older houses only having 100A mains? Honestly, I haven't put an ammeter on my mains to see what the rest of my house's current load is on each phase. Also of note: each chassis will have it's own circuit so the total load will be divided between the two circuits. Each chassis has a theoretical maximum current draw of 16.2A.
  6. Lastly.... I haven't cracked open the mains breaker (for obvious reasons.) to see what gauge wire is being used to the sub-box. If I'm required to upgrade the mains breaker, do you think I'd need to upgrade the wiring leading to the sub-box as well? I suppose I could denergize my sub box and look at the wiring in there come to think of it.

So yeah, if you made it this far, I commend you and thank you so very very much for reading through all of my trite details. I also thank anyone in advance for any advice they may be able give or guidance they can provide.
Old 01-28-19, 11:11 AM
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Some comments:
* I personally wouldn't bother with FMT. I would use EMT or PVC, or ENT (smurf tube) if you need it flexible. I tend to lean towards PVC, but whatever works for you. Also, just because you're running 240v, doesn't mean you shouldn't just use 10/3 NM-B.

* Conduit can be run perpendicular to joists. Cabling (NM) needs a running board.

* Going over other services is however you need to. You just don't want dis-similar metals touching. No code-required spacing or anything

* Permits are local specific. Some areas require a permit to replace a receptacle, others are more flexible. Best answer is to call your local building department and let them know your plans (keep it simple - two 240v receptacles), and find out what they require. Many locales have 'quick permit' processes for the more simple jobs.

Sounds like a pretty easy plan!
Old 01-29-19, 01:38 PM
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I would also use EMT or PVC conduit. To go over other piping on the same wall you use a saddle bend to kick the conduit off the wall a bit and bridge over the other stuff. The big box stores have pre-made conduit bends for either EMT or PVC. 1/2" conduit would be plenty for what you need. The other options is to run NM-B cable up in the joist spaces like any other circuit and use a short conduit sleeve where you drop down the wall.

We can't know if your existing service is adequate because we don't know what other electrical loads you have in the house. The formal process is a "demand load calculation". You can google to find online calculators. For a "back of the napkin" approach, if your major appliances use oil/gas (furnace, water heat, stove/oven, cloths dryer) then your 100A service is probably fine for this additional load. If you have electric appliances or major electrical loads like a car charger, hot tub or pool equipment you should go through the full calculation. However don't forget that you're also going to have a big jump in cooling need in a few months. Make sure your existing AC system can handle the additional 7.5kW heat load or budget (money and service amperage) for an upgrade to the air conditioning system.

Ignore anything you've read about isolated grounds. This only applies to specific commercial applications. In a residential building all circuit grounds are bonded together and to metal equipment like your rack.

The vast majority of the time the inspector only looks at the new/modified work listed in the permit. I suppose if they saw something crazy in the existing work they could make a deal out of it, but that would probably only happen if they saw something really dangerous. Sometimes there are conditions on pulling permits, but that would be a local thing. For example one of the cities near me requires modernization of the smoke and CO detectors in a house if an electrical permit is pulled for any reason, but that kind of stuff is pretty rare. Check with local building office to see what local rules apply to you.
Old 01-29-19, 02:07 PM
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That was super helpful, thank you! I'm not concerned about thermal load. My current racks are rear isolated and have a vent through a large basement window instead of back into the ambient environment. Believe me, I've been in large scale datacenters when the AC units failed. 140F ambient is not something I'm interesting in having in my house!
Old 01-29-19, 02:10 PM
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I have read elsewhere or possibly even here that NM-B has no business in conduit which I was somewhat surprised to hear, I've pulled NM-B through conduit before. Is there any rationale behind that opinion or is it just an obstinate electrician that hates the oblong shape?
Old 01-29-19, 05:09 PM
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NM-b is allowed in a conduit. It is just not allowed when used outside or wet location.
Downside of using NM-b in a conduit is it is harder to pull and takes more space. It general, it is best not to pull NM-b in a conduit unless it is a short length and you will be using it just to protect cable.

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