O.K. I ran the appropriate wire from my sub-panel to a sub-panel in my 'out-building'. I stopped by the electrical supply house and spoke to an electrician for the correct wire size for that process...

QUESTION: When I put in the 100 Amp breaker in the 200 Amp Main Panel and run it to the 100 amp breaker in my sub-panel to the 100 amp service in my 'out-building', the wire connections in the breakers are NOT that large. So, what size can I use that would be appropriate....

  • 3
    What size is the wire you are attempting to attach to the 100 amp breaker?
    – Kris
    May 17, 2019 at 17:55
  • 2
    It would help if you told us what you did so far. What size are the conductors? What size is the ground wire? Depth of bury? May 17, 2019 at 20:17

3 Answers 3


I'm with Kris. A 100A breakers lugs are sized for a #4 through a 1/0 which is one size larger than the required #1 aluminum. I guess what you are saying is that you have run an over sized feeder.

Since you have already run the feeder you might need to reduce the conductor to fit the breaker lugs using an insulated butt splice, like the picture, inside the panel.

enter image description here

Good Luck

  • Also called Polaris connectors. On the subpanel end, those will cost more than the price diff for a larger subpanel. On the main panel end, those Polaris connectors will cost more than the cost diff for a larger breaker. May 17, 2019 at 21:38
  • greaves-usa.com/wp-content/uploads/2015/09/…. Have you any experience with the pin adapters that can reduce the wire fir fitting undersized connectors?
    – Kris
    May 17, 2019 at 23:20
  • @Kris -- I wouldn't expect loadcenter/circuit breaker lugs to be listed for use with pin connectors May 19, 2019 at 23:17

Change the subpanel to a much larger panel

The reason the wires won't fit on the subpanel is that you shortchanged yourself and bought a panel that is only 100A. Possibly you thought this was required. If aiming to save money, the Polaris connectors and pigtails will cost more than the price difference to a bigger panel.

The othe problem with 100A panels is they are small in terms of number of spaces. 100A is a lot of power (and you actually have more still), and can power a lot of stuff. Some of those things take 2 breaker spaces. Many of them are not allowed to use "twin/duplex" double-stuff breakers, because almost every 120V circuit these days needs AFCI and/or GFCI, and they don't make those breakers in double-stuff. You really don't want to be backed into a corner of having to cram more circuits into the too-small panel.

So tear that thing off the wall and take it back and get a subpanel with enough spaces, and most likely a 150, 200 or 225A main breaker. The wires will fit just fine on that.

Change the feed breaker to a larger breaker

Look at the wire you bought. Look at Table 310.15(B)16, in the 75C column for your wire... and read the amps listed. Now, round up to the next available breaker size. That is the largest breaker you are allowed to use on this wire.

Buy a larger feed breaker, up to that size, that will fit your wires. If you bought 2/0 wire for instance, that is rated for 135A, round up to 140? 150? - regardless, it should fit just fine on a 125A breaker.

"But wait, I didn't follow your above advice and my subpanel is still 100A" -->> Okay, does the subpanel have a 100A main breaker? Yes, then you're still fine, because that will protect the subpanel from overload.

Don't confuse "bump for voltage drop" with "maximum amps"

The whole thing you do with calculating voltage drop, the 3% and all that jazz, is optional. You can run 5, 6, 7% voltage drop if you're accounting for ALL the voltage drop that will occur from the meter to the ultimate load, and keeping that sane e.g. 8%.

This is why you are freely allowed to breaker a long wire run right up to the max allowed for that wire (e.g. 150A for 2/0). Wire amp rating doesn't care about wire length.

What about the ground? When you upsize wire for voltage drop, you're also required to upsize ground in proportion -- that better not be a #8 ground. As a result, the ground is good for whatever the hots are good for.


Pretend you ran 2/0 wire, and intend to pull 100A actual of load.

  • Because you enlarged the wire, at 100A actual, your voltage drop will be exactly where you want it to be.
  • Because the wire is safe to 150A (albeit at worse drop), you can breaker at 150A.
  • Because you plan for 100A actual of load, you must provision 125% of that, or 125A in both wire capacity and breaker. This requirement is satisfied.
  • You can use a 200A subpanel. Obviously, the 150A feed breaker protects the 200A subpanel with good margin.
  • The 200A main breaker in this panel is unlikely to ever trip, but it serves as a shutoff switch.

I ran the appropriate wire from my sub-panel to a sub-panel in my 'out-building'.

Without telling is what you already ran it's impossible to answer this question. Some think it's "appropriate" to only run a 3 cable feeder to a detached structure for a 240V sub panel and this is highly incorrect.

Did you place triplex in conduit buried 18" deep or direct bury UF @ 24" deep? What gauge wiring did you run and how many conductors?

The feeder for a 100A panel would fit into the main lugs of a 100A breaker or panel, check the writing on the cable and provide the exact wording and number of conductors, the model number of the panel you're using, and the rest of these details so we can ensure you're doing this correctly and safely.

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