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I live in a Duplex (build in 1979) with a 200A distribution panel which has 2 meters and 2 100A breakers (one for each unit).The distribution panel is on the outside of the garage and is fed underground by the utility. I live in California and the code currently references NEC 2014.

Each unit has a sub-panel and is about 1200 sqft. There is an aluminum 3 conductor + ground cable going to each unit, I believe it is a #2 AWG Al (but I'm not sure). I assume it was sized correctly in 1979 to be protected with a 100A breaker in the distribution panel.

I wanted to add a sub-panel in the garage (between the distribution panel and the unit sub-panel to charge my EV (14-50R outlet) and add a couple of 20A outlets. In the near future I plan to connect a PV inverter to this panel (20A 240V breaker installed for this future use).

I think based on the tapping rules specified in [240.21(B)(1)] I’m allowed to tap into the cable that runs from the distribution panel (with a 100A breaker) to the sub-panel in the unit. I used a #2 3 conductor, copper, NM-B cable between the junction box where I did the tap back to the distribution panel (<10ft of cable). I used #2 copper conductors between the tapping junction box and the panel below. I did this work without permit, but would like to get it permitted when I get my solar installed.

According to NEC Table 310.15 (b) (7) “These are the permitted ratings for Dwelling Unit service and feeder conductors which carry the total load of the dwelling”. 100A, Copper #4, Aluminum #2. #2 Copper can provide 125A. But is this still relevant now that some of the load of the dwelling is coming from the sub-panel in the garage.

Here is the load calculation according to the optional method of 220.82 For the unit only (excluding the garage):

 1200 sqft @ 3 w/sqft=3600
 3 kitchen 20A appliance circuits: 4500
 Dishwasher NPR: 1188
 garbage disposal NPR: 1224
 microwave NPR: 1350
 washer: 1500
 gas fired dryer:720
 Sauna NPR 6000
 Total: 20082
 1st 10,000W @100%: 10,000
 remaining at 40%: 4032.8
 Total: 14,032 W (58.47 A)

If I add the EVSE in the garage (32A NPR), the total load goes to 71.3A. Do I need to multiply the EVSE rating with 125% (operation >3 hrs) ?

Questions:

  • Am I allowed to run a #2 Cu cable from my tapping junction box back to the main distribution panel? Southwire states it is rated for 95A only, but the main breaker is 100A.

  • Is it OK to use a #8 wire between the 50A breaker at the 14-50R outlet based on the 75C rating of the outlet and the breaker? I read somewhere that you have to use 60C in residential applications

  • If I add a 40A 240V inductive stove to the unit, can I keep my existing aluminum cable? I had an electrician tell me that it needs to be upgraded, but I figured as long as I keep my 100A main breaker, there should not be a problem. I checked the ‘feeder and service load calcuations’ in [220.82] using the optional method and it is within 100A (including the loads on the sub-panel in the garage. I do need to replace the sub-panel in the unit, because it is a Zinsco panel and it is located in a closet.

  • Any other problems you see in the pictures provided?

Overview of garage sub-panel with tapping junction box

garage sub-panel wiring

Tapping junction box

Main Distribution panel for 2 units

Unit Sub-panel

  • 1
    How many square feet is your half of this duplex and can you post a photo of the unit subpanel, including any directory labeling that is present? – ThreePhaseEel Jan 6 at 19:55
  • I've added a picture of the unit sub panel. The unit is 1200 sqft. I did the calculation in accordance to 220.82. For the unit only (excluding the garage): 1200 sqft @ 3 w/sqft=3,600. – Christian K Jan 6 at 22:00
  • I've put the full load calculation in the original post – Christian K Jan 6 at 22:07
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Good news, your article 220 load calculations are correct

Upon reviewing 220.82 and your panel photo -- your article 220 load calculation seems to be spot-on in terms of what it includes. Note that you do not need to use the 125% continuous load multiplier for Article 220 purposes as the applicable Code text explicitly refers to the nameplate rating of the appliance.

As to the tapping...

Your existing feeder conductor is indeed correctly sized for 100A -- the 83% rules for dwelling unit feeders/services in 310.15(B)(7) means that you can get by with a cable that can carry 83A, and 2AWG aluminum SE is allowed to carry 90A when terminated on 75°C lugs as per 338.10(B)(4)(a) (text from 2017 NEC, 334.80 is the section that governs temperature ratings and allowable ampacity of NM cables):

(4) Installation Methods for Branch Circuits and Feeders.

(a) Interior Installations. In addition to the provisions of this article, Type SE service-entrance cable used for interior wiring shall comply with the installation requirements of Part II of Article 334, excluding 334.80.

For Type SE cable with ungrounded conductor sizes 10 AWG and smaller, where installed in thermal insulation, the ampacity shall be in accordance with 60°C (140°F) conductor temperature rating. The maximum conductor temperature rating shall be permitted to be used for ampacity adjustment and correction purposes, if the final derated ampacity does not exceed that for a 60°C (140°F) rated conductor.

Since you used mechanical setscrew-type (Polaris™) splices for your tap, you should be fine with the copper-to-aluminum connections as those connectors are Cu7Al or Cu9Al rated in their common incarnations. Furthermore, the 83% rule for the run wires from the meter-main to your unit still holds thanks to 310.15(B)(7) point 3:

(3) In no case shall a feeder for an individual dwelling unit be required to have an ampacity greater than that specified in 310.15(B)(7)(1) or (2).

Finally, the formal feeder tap rules do not apply here at all since the conductors from the junction box to the new subpanel are of a size which is adequately protected by the feeder breaker (#2 copper wires in conduit or a SE cable can handle 115A) without further consideration. (In other words, they are not tap conductors in the Code sense of the term -- they would need to be too small to handle 100A in order to be considered as such.)

...and the subpanel in the garage

The installation of the receptacles and subpanel in the garage is close to correct, but needs rectification on a couple of points. The good news is that these are not hard fixes, and should be manageable in a day -- nothing major was screwed up here, as you appear to have your neutrals and grounds sorted, and even though this box is full, that's an acceptable concession to make for something special-purpose.

The first issue is that the 50A breaker is a bit large for 8AWG wire going to a receptacle -- it would be fine if that 8AWG was going to say subpanel lugs via a conduit, as you could use the 75°C column in that case, but most receptacles are restricted to 60°C service, so that limits you to using the 60°C column in the ampacity table for circuits feeding receptacles, no matter the type of occupancy. Swapping the 50A breaker for a MP240 or QP240, presuming that's the correct type for what appears from the interior layout to be a Siemens/Murray spa panel, will correct that, while the NEMA 14-50 receptacle can stay as NEC Table 210.21(B)(3) permits 50A receptacles on 40A branch circuits. (It also permits 40A receptacles, but there are no 40A NEMA receptacle configurations.)

The other thing I noticed in your picture is a panoply of utterly redundant grounding wires, at least if the NEC grounding rules govern unchanged in your jurisdiction -- the RMC nipples are an adequate grounding path from the tap box to the subpanel and from the subpanel to the junction boxes in this case (i.e. not a service, and less than 250V to ground), meaning you simply need to use appropriate gauge ground pigtails from the receptacle to the box to finish the grounding path, given that you are using "bump out" covers on your junction boxes. Furthermore, the bonding bushing where the feeder enters the subpanel shouldn't be necessary, either -- that gets rid of the wire from the bonding bushing to the ground bar. In the end upshot, your ground bar will be rather empty, but that is fine given that you are using metal boxes and metal conduit.

Adding the stove is fine too, once you send all the Zinsco hardware to the smelter

It turns out that adding the stove only adds 16A of factored load, maximum (40% of 40A), to your 220.82 calculation, which puts you at 87.3A of computed service load, which is still within what a 100A service can handle -- the electrician who says you absolutely need a larger feeder to handle the additional load is incorrect. However, you might as well get a new meter-main fitted anyway (with the cooperation of the other stakeholders), as you have a serious problem: not only is your existing subpanel a Zinsco, your existing meter main uses Zinsco breakers as well! This renders this entire electrical installation hazardously unserviceable, as a documented failure mode for Zinsco breakers is them not turning the power off when you place the handle in the OFF position.

At this point, what you do is up to you and your fellow stakeholders. A two-socket, 200A unit of whatever specification your utility accepts is not a bad choice for the replacement meter-main as it's one less piece of hardware to swap if future expansion is called for, while I would fit a 42 space minimum, 200 or 225A main lug panel with ground bars to replace the existing subpanel pyromaniac monster lurking in your closet.

  • Thanks for your super detailed and carefully researched answer. A few additional questions, and clarifications. – Christian K Jan 7 at 1:39
  • I didn't realize I had a Zinsco meter main as well. I would prefer to upgrade the service to each unit to 125A, but I'm guessing that is a problem with the #2 Aluminum that is running to both units. I supposed for my unit I could have a 125A main breaker and change my tapping box and put a 100A breaker on the #2 Aluminum feed to the unit, which would give me a bit more headroom for loads in the garage. Does the 83% rule apply to the feeder from the garage to the sub-panel in the unit, since it does not carry all the loads of the dwelling unit? – Christian K Jan 7 at 1:50
  • About the grounds in the sub-panel. I know I have a lot of grounds, but I wanted to not have any reason for an inspector to flag anything. Do I have to remove grounds to make it code compliant? If I want to remove grounds, are you suggesting that in the sub-panel I only need one ground coming in from the tapping box going to the grounding strip in that panel, and nothing else? I would add pigtails in the receptacle boxes from the receptacle to the grounded box. – Christian K Jan 7 at 1:56
  • @ChristianK -- my understanding is that the 83% rule no longer applies at that point, but since it's 2AWG copper from that point onward, you're still good for 100A, just not 125A – ThreePhaseEel Jan 7 at 1:57
  • 1
    Yes, Mike Holt made a comment about this back in 2009. [link] (forums.mikeholt.com/showthread.php?t=115061) In his case he was talking about removing a load from the main panel and moving it to the service entrance, and now needing a bigger feed for a smaller load. I'll post it as a separate question – Christian K Jan 7 at 4:42

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