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I am adding a mini split system to my house which currently has a smaller air-conditioning condenser in place. In brief, the house is very old so we opted for a ductless mini-split over trying to increase the size of the current unit to avoid needing to add ductwork. We have a 3/4" EMT conduit that leads outside along the back of the house which had only 2 6AWG wires in THHN insulation.

This allowed us to run 2 more 6AWG wires in the same conduit and stay under the NEC fill requirements. Adding a ground wire would have put us over the conduit fill. The original unit was presumably grounded to the panel via the all metal conduit. We also passed the new wire through the original disconnect (going to the original condenser) to a second disconnect (going to the new unit).

To make sure both units were properly grounded an 8' by 5/8" grounding rod was inserted and attached. I may be able to submit for a tax credit on the unit but I'm not sure if the grounding rod meets building codes. The electrician thought possibly not. This route was chosen to avoid running new conduit out of the house (spacing is very difficult). My questions are, does this (grounding to rod and running wire through a disconnect) meet code? Would it pass a permitting inspection?

ArchonOSX
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Elmo
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  • These are *not* on a roof, right? – ThreePhaseEel Oct 05 '17 at 02:53
  • They are not, outside at the back of the house. – Elmo Oct 05 '17 at 03:17
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    Just a note: The Equipment Grounding Conductor (EGC) is part of a safety system that allows fault current to return to the **source**, so that faults can be cleared (open breakers). Bonding your A/C unit to a chunk of metal stuck in the dirt, is not going to meet that requirement. You need an adequately sized, continuous, low resistance path, from the A/C unit back to the service neutral. This is what an EGC is. Using the earth as a EGC is a terrible idea, since the resistance is way too high. – Tester101 Oct 05 '17 at 17:09

2 Answers2

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Don't mix up your EGCs and your GECs!

The EMT, provided it is made up properly and is not excessively exposed to physical damage, is a perfectly fine ground (equipment grounding conductor) by itself in this application -- 440.9 in the 2017 NEC only applies to runs on roofs, which get tread on repeatedly by clueless folks:

440.9 Grounding and Bonding. Where metal raceway is run exposed on a roof for air-conditioning and refrigeration equipment, an equipment grounding conductor of the wire type must be installed within the outdoor portions of metal raceways using non-threaded fittings.

However, the ground rod needs to go, because it inadvertently violates 250.121 by trying to use the EMT as a grounding electrode conductor that ties a remote ground rod back to the main panel bond atop it serving as an equipment grounding conductor:

250.121 Use of Equipment Grounding Conductors. An equipment grounding conductor shall not be used as a grounding electrode conductor.

Exception: A wire-type equipment grounding conductor installed in compliance with 250.6(A) and the applicable requirements for both the equipment grounding conductor and the grounding electrode conductor in Parts II, III, and VI of this article shall be permitted to serve as both an equipment grounding conductor and a grounding electrode conductor.

So, just rip out the errant remote ground rod and whatever wire was hooking it up to the disconnect(s) or just the wire for that matter as the errant rod will do nothing if it isn't connected to anything, and all will be well when the electrical inspector comes around.

ThreePhaseEel
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    +1 Good catch on some arcane Code sections. He could leave the ground rod and just remove the wire connected to it. Just bury it. Depending on the soil, removing a rod can be a real hassle. – ArchonOSX Oct 05 '17 at 09:31
  • I'm not an electrician so this is probably redundant. If I understand this correctly, the EMT is sufficient on its own. The grounding rod is not acceptable because it is now bonded to the panel via the EMT. Bonding it this way is not ok. I don't understand why though. The ground rod is the same size as what the panel is presumably grounded to. I'm not arguing just curious why this is no good. – Elmo Oct 06 '17 at 01:38
  • Also is using the disconnect as a pass through acceptable? Both disconnect are linked via metal conduit. – Elmo Oct 06 '17 at 01:39
  • @Elmo -- re: the disconnect being used as a pass through -- ask it as its own question, so it gets the attention it deserves :) – ThreePhaseEel Oct 06 '17 at 02:13
  • Done. What's the issue with the grounding rod though? That part I don't quite get. – Elmo Oct 06 '17 at 02:42
  • @Elmo -- I'm actually having a bit of a trouble putting the rationale behind it into words – ThreePhaseEel Oct 06 '17 at 02:45
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EMT conduit is acceptable as a ground path. Also, if the work is old enough, you can retrofit a ground external to the conduit, if it the conduit doesn't have your 100% confidence,

Since you have 2 circuits and 4 conductors, you must also attend to the other conduit fill rules - the one that requires a derate when stuffing too many wires in a conduit. (310.15B3a). At 4 wires, your derate is 80% of the 90 degree C rating - that's 75A, so your derate current limit is 60A. Don't exceed 60A even if another rule allows you to.

Harper - Reinstate Monica
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  • Good point on the deration to 60 amps. – ArchonOSX Oct 05 '17 at 10:00
  • @ArchonOSX - Just a small nit on the use of "deration". That term means "to end the rationing of something such as food, fuel etc". The term that would be be more correct here would be derating which is the noun version of derate which means to "To lower the rating of (a device), especially because of a deterioration in efficiency or quality". – Michael Karas Oct 05 '17 at 10:27
  • @MichaelKaras thanks I will endeavor to use the proper term in the future. – ArchonOSX Oct 05 '17 at 10:32
  • does that mean that I'm in good shape with respect to the derating rules so long as neither circuit pulls more than 60A at a time? By this I mean if both units are on they are pulling 60A for 1 and 50A for the other. – Elmo Oct 06 '17 at 01:32
  • @Elmo yes, both are good for 60A at once, unless some other rule says otherwise. Note that most likely, you are limited to 60A on that wire *for other reasons*, typically the 310.15B3a derate doesn't get in your way, but on anything over 10AWG you must run the numbers to be sure. – Harper - Reinstate Monica Oct 06 '17 at 01:40