While looking for panels, I see various current capacities, and I would like to know what is different between two panels, like a lug fed 50 amp panel that has six slots, and an otherwise identical 100 amp panel.

Without a master breaker, to state the current, what would be different?

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    Would imagine the 100 amp will have thicker(larger) connections/components, which might not be as visible to the eye. More amps usually require larger size wires. – crip659 Jan 22 at 14:33
  • Pennies count in this (and most) business. If 99% of people can make do with a small panel (e.g., only a few circuits for pool or garage, etc.) & don't need more than 60A, then why spend an extra $ 3 on larger lugs, thicker bus, etc. The people who come here will hear Harper harp about "go big" and get the 100A panel anyway to get more spaces. For builders putting in 100 identical houses, that extra few $ adds up and they use the minimum possible. My favorite example is Dell keyboards - look at the progression over 20+ years - each time they cut out more plastic x 'n' = $ on bottom line. – manassehkatz-Moving 2 Codidact Jan 22 at 14:42
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    Me? Harp? :) What ThreePhaseEel and I are saying is mini-panels may be a correct business decision for a builder - but for Joe Citizen-who-DIY-installs-subpanels, a few extra dollars on a larger panel avoids a big problem Joe is likely to have later. People who install subs don't stop buying new things. They also have "the thrift gene" that says spend as little as necessary. So we see the "bought too small a panel" blunder a lot around here. – Harper - Reinstate Monica Jan 22 at 18:36

The panel may look identical to you but the thickness of the buss and the material makes a big difference where a inch of copper can handle 1000 amps per square inch verses aluminum of only 700 amps (NEC 366.23) so the buss material makes a difference it may look the same even be the same thickness but one being solid copper and one being aluminum with a plated copper makes a 30% difference. So a material difference or thickness, even the insulation used can affect the rating but we can’t really tell all of this with a casual observation.

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    Also: A small dimensional change can make a big difference in cross sectional area, hence current carrying capacity. – Sherwood Botsford Jan 22 at 15:32
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    Yes agreed, +1. Different materials make a difference, and when comparing apples-to-apples of the same material, the higher ampacity panel will have the bus bars stamped out of thicker metal than the lower ampacity panel. Additionally, the higher ampacity one will have bigger lugs included, to attach the bigger wires necessary to bring that much current to the panel in the first place. – Nate S. Jan 22 at 17:23

The main breaker doesn't state the current of the busways.

Last year we had a run on questions about 20-space panels with 100A main breakers. The people wanted to add 25-40A of solar, and were getting different answers from solar installers. (With solar, you add main breaker + solar breaker... that can't exceed 120% of bus capacity.) It turned out their panels all had 125A buses, allowing 150A of main+solar. They were all set.

So you simply can't go by main breaker size as a gauge for busway capacity. You have to look at the specs and the label.

It's certainly possible for a panel of almost any size to be built for 125A or even 225A. As Ed Beal says, it's a matter of the busway design. There are a few 125A/30-space panels out there, and they also make 12-space 200A panels.

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