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Edit: All the breakers are 30A capacity. The main panel has 125A main breaker while the Siemens PL subpanel has built in 100A main breaker and 6 pcs 30A GFCI 2-pole breakers. If I use 3pcs 60A breakers for the feeds from main breaker. The current would divide among the 3 pcs, right? The reason most people use mainly one breaker as main feed is because the are no more spaces in the main panel. But if you have spaces, you can make it redundant by using 3 breakers as main feed, right? From an electrical standpoint, nothing wrong with this, isn't it?

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I'll replace 6 of the 16 2-pole branches with 6 GFCIs so there will be 6 blanks in the main panel. Can I use multiple 2-pole breakers (like 3 pairs) to feed the subpanel? This is because I don't know the brand of the main panel, it has 125 Main Breaker (I bought the house second hand). So I don't know the bus stab current rating so to be sure I'll feed three 2-pole feeds to the subpanel (one one bus stab won't overheat). Will this work? Has anyone seen a multiple feed like this?

Also do you see any problem if I'll just put the subpanel in front of the main panel? Any rule regarding this? I can still access the back.

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    I'll let the pros give the complete answer. But I do know that you can't simply parallel the feeds - i.e., feed 3 * 'x' to get 3x of current, because (among other reasons) if one feed breaker trips then more load goes on the other 2, overloading the wires & breakers very quickly, etc. But if you want to have each of the 3 feeders feed a separate batch of breakers that would be OK (safety-wise) - but I am skeptical that the subpanel would support that type of configuration. – manassehkatz Nov 15 '18 at 20:45
  • The Siemens subpanel already has 100A main breaker and each Gfci breaker is 30A. So the 3 feeders in the main panel can even be just without breakers. So can it work theoretically? – Samzun Nov 15 '18 at 23:25
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    I don't know enough specifics of breakers/panels/etc. to give you a definite answer. But I do know that if the connectors/wires/lugs/etc. are sized large enough to handle 100A then "extras" make no difference, especially on a short run (i.e., no voltage drop issues) and if they are NOT each sized large enough (all components involved) then it isn't safe because you have multiple points of failure instead of the desired redundancy. – manassehkatz Nov 16 '18 at 4:39
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    In the end, this part of things is theoretical for me - I can handle dealing with light switches & outlets and appliance repairs, but when it comes to panels, I leave that to the pros. So if I were in the same situation, I would rely on what Ed Beal or Harper or Three Phase Eel says. And I think Ed Beal has been pretty clear about not tripling things. – manassehkatz Nov 16 '18 at 5:13
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    I don't know. Ed Beal or Harper or Three Phase Eel should know. – manassehkatz Nov 16 '18 at 5:30
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This would not be legal at all you have 3 breakers in parallel. The proper way would be to install 1 breaker to feed the sub.

  • But the 1 breaker may arc since i dont know the bus stab current rating of it. But wont it work.. the current can be distributed in the 3 main breaker feeds?? – Samzun Nov 15 '18 at 23:27
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    As I told you in another post you have a pair of bolts holding the feed from the breaker get a pair of chair lugs , or compression lugs feed the sub I can't tell if it has a main breaker or not so the wire size would need to be the ampacity of the main breaker and the sub panel rating needs to be larger or the ampacity of the main breaker or risk burning everything down. – Ed Beal Nov 16 '18 at 0:39
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    There should be a wire size listed on the lugs. The largest wire size they are listed for would corilate to the capacity on the 90 degree table for the wire type being used. – Ed Beal Nov 16 '18 at 13:19
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    I could not tell if your sub had a main breaker. But since you are tapping the buss I would want the wire size to be 125% for safety. If your sub panel has lugs and the number 2 lug for the tap you could use the 90 degree collum that is 130 amp capacity but if going to a breaker you would be limited to the 75 degree table. At this point it would depend on the actual load if the load is not a continuous load you will be fine as the wire only needs to be rated at 100%. Don't forget you will need conduit from the main panel to the sub. – Ed Beal Nov 19 '18 at 14:45
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    #4 wire to 3/0. Or 000 AWG the breaker states 75 degree wire on the front. – Ed Beal Nov 21 '18 at 13:15
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NO, no no.

no.

It's absolutely no good mounting the subpanel stacked on top of the panel, not a safe working setup.

As mentioned in the comments, you can't parallel with multiple breakers that way.

It may be possible to tap the bus of the main panel or the service wires safely to feed the subpanel, but you'd have to be really careful.

  • But wont the current subdivide among the 3 main breakers so instead of say 100A of one. It can become 33A each of the 3 making it safer? – Samzun Nov 15 '18 at 23:29
  • In the picture, the wires are drawn and put in middle. But in actual, all the wires will be at the sides.. what is the possible safe working setup problem with this idea of putting subpanel in front of it when there is a wooden cabinet holding them all? – Samzun Nov 16 '18 at 0:50
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    From an electrical engineering standpoint, it sounds fine. But picture it this way: If you have a 3-lane highway running smoothly at capacity and one is blocked by an accident. The other 2 lanes are NOT able to take up the excess and in fact are likely to slow down due to the excess traffic. With electricity that means if one feeder trips a breaker (or a wire gets loose or whatever - anything that breaks the circuit) the other 2 are now immediately and dangerously overloaded, overheat, potential fire. Should the other 2 breakers then flip quickly: yes. Can we risk it? no. – manassehkatz Nov 16 '18 at 1:27
  • But the situation is that in normal feed to subpanel. You only use one breaker with say capacity of 100A. So I was using 3 breakers for redundancy just to be sure. Even if one is working, it can be fine.. better if three. What's wrong with this. – Samzun Nov 16 '18 at 2:11
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    Unfortunately, it just doesn't work that way. Logically it seems like it should. But it doesn't. Harper may be able to explain it better. But the bottom line is that most (I can't say "all", but definitely MOST) things in the NEC are there for very good reasons. – manassehkatz Nov 16 '18 at 2:55

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