This is for my shop - i have 2 units with separate meters - each unit getting 100 amp service. Each unit has its own electric panel. Now what i want to do is to pull two 100 amp circuit from each panel - and joining them parallel to a 200 Amp Disconnect. From the Disconnect it goes to my equipment which needs 175 Amp power. Here is a drawing.

Does this make sense? Is this doable. Basically connecting 2 power source in parallel. We learn back in engineering - voltage remains same but current doubles. Any thoughts or comment.

  • what is the resulting voltage of the combined circuit?
    – jsotola
    May 10, 2018 at 4:27
  • 4
    Why can't you simply get the utility to run you a 200A service to one of your units? May 10, 2018 at 4:30
  • This isn't advisable (and not to code) as the differences cable lengths to each box can cause an offset affecting the peak amplitude of your phases.
    – virtualxtc
    May 10, 2018 at 7:08
  • 1
    Is the load subividable? (bitcoin miner array, grow lights, on-demand water heater etc.) GIGO; the more you tell us about the load/situation the more helpful the answers will be. May 10, 2018 at 15:43
  • no it is not divided.
    – shawn
    May 14, 2018 at 22:58

4 Answers 4


Illegal, and impossible at any sane cost

Paralleling is, for all practical purposes of interest to you, illegal in mains wiring. Even when it is done owing to ampacity needs, it uses special equipment on the line side which renders it useless for anything like this.

The only way to inter-tie two supplies is with some sort of airgapping which assures energy flow is one way always, and provides isolation. Think of it as "magic diodes that work on AC". M-G sets with overrunning clutches, or railway-style substations where you feed an ungrounded transformer then rectify to DC and run your machine off the DC bus. Any of these would be prohibitively expensive, of course.

The path of least resistance (heh) is to have the conversation with the power company. You need to do this anyway, as you cannot simply slap a 175A load on their line without permission. Their supply lines and equipment are simply not factored for a load that large! Most houses take 1kw (4A) on average, and they absolutely rely on residential loads being intermittent.

Expect a lot of things to come out of this conversation, such as 480V - you could run your load off three #6 wires instead of two #0000's..

Also did you remember the 125% derate?

  • Thanks for clarifying that. I was also in the same page as it did not make sense to me in practical life.
    – shawn
    May 14, 2018 at 22:56

Let's forget about the engineering for a minute and just say, NEC 230.2 "A Building or other structure served shall be supplied by only one service, unless permitted in 230.2 (A) through (D) and you don't meet those exceptions. If you want to argue that the services are in different buildings. Then 230.3's title is "One Building or Other Structure Not to Be Supplied Through Another."

In short it isn't allowed by code.

  • Thanks for pointing out the code. This helps. thanks for your answer.
    – shawn
    May 14, 2018 at 22:57

It won't work, you almost surely will get power from 2 different phases of utility 3-ph supply so your current won't 'match' (you put 2 phases of your utility into short-circuit). Assuming you're on TN-C-S split-phase 'american-style' distribution with transformer for each user.
If you're on standard 3-phase distribution, with single block served by a single transformer, it may work but you have to splice phases R-to-R, S-to-S and T-to-T (0V between each pair and full delta voltage between each couple of 'pairs')

My suggestion is, in any case, is to ask a service update to your power company.

  • 4
    Even then, it would be a disaster, because if the PoCo changes anything on their end, e.g. swaps out a delta transformer for wye, KABOOM and then they sue you since the damage will be way north of $10k. May 10, 2018 at 14:36
  • thanks for your answer. Yes i am going to the city to ask to provide me a 200amp line.
    – shawn
    May 14, 2018 at 22:57

Bascially, you would be back feeding one service from the other service if it were disconnected. They don't allow that. The same thing happens with grid tie alternative energy production. That's why they require anti islanding on the devices that put power in to the grid, so that if the service is disconnected it won't still have electricity in it coming from the other side.

Assuming you're lucky enough to have both on the same phase so you can parallel them, maybe there is some kind of device that accomplishes this? Aside from getting it UL listed, it would be fairly simple to make. It would just be a relay on the second service that would connect it to the 1st service to parallel them. The logic would only turn on the relay when it senses Voltage drop on the 1st service which would happen when more current is needed, and then it would ensure that the phases match before turning on the relay. Once the load is reduced (or flows in reverse), it would open the relay again.

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