Electrical – Can a sub-panel have a larger main breaker than the feeder breaker in the main panel

Here's the best I am able to put together on the applicability of 30mm wire as subpanel feeder hot off a main breaker.

First, the allowable ampacity of the wire. Let's look at table 310.15b16, with an added column for mm2.

  Gauge      mm2    At 60C   At 75C   At 90C
 3 AWG       26.7      85      100      115
             30.0       ?       ?        ?
 2 AWG       33.6      95      115      130
             38.0       *       *        * 
 1 AWG       42.4     110      130      145  

Assuming values are linear/proportional, and they are linear enough in such a small zone, the ? numbers are 90, 107, and 122, respectively.

The 125A-breakered interpanel feeder

Because we are over 100A, we are not constrained by rule that requires us to use the 60C column for feeder. We can use the 75C or even possibly 90C column if the terminations and wire are rated for it. THHN is.

Now, because this is a feeder taken straight off the main breaker, we get a derate (uprate, really) of 83% (NEC 310.15b7)The planned service is 125A. 83% of that is 104.

104 is less than 107. 30mm2 wire is adequate for this feeder.

The 150A-breakered service conductors

What about the outside feeder, breakered at 150A? Looking again at our modified 310.16 table, you see where the area of 38mm2 wire is dead-nuts halfway between #2 and #1. So, looking at the * numbers, we interpolate exactly halfway between. Giving 102.5, 122.5 and 137.5 respectively.

Assuming we are stuck in the 75C column, we use 122.5. Being a service supply, we "derate" (uprate) 122.5 by 83% giving 147.6 amps.

147.6 amps is less than 150. But: 147.6 amp breakers are not readily available, so we are allowed to round up to the nearest available size. That being 150A.

The 38mm service supply is appropriately breakered by NEC standards.

Other questions

You are always allowed to use larger wire than required. If Code requires #3 and you have a perfect-length scrap of #00 lying around, go for it.

The 150A breaker should rarely trip because the very next stop downline is a 125A breaker. This is not reliable; coordination of breakers is a black art. However, if the 38mm service wires themselves developed a short, the breaker would come into play.

The NEC requirement for protection of the panel is straightforward:

408.36 Overcurrent Protection. In addition to the requirement of 408.30, a panelboard shall be protected by an overcurrent protective device having a rating not greater than that of the panelboard. This overcurrent protective device shall be located within or at any point on the supplyside of the panelboard.

So the panel must be protected by an OCPD at or below it's rating, but that OCPD can be the main or upstream. As long as the upstream OCPD satisfies this rule, the main breaker does not have to; a main breaker larger than the panel rating doesn't satisfy the requirement and doesn't protect the panel bus but it's not a violation to have it there serving as a switch / disconnect.

Keep in mind that the NEC also requires that the panel and the main breaker be listed, and used in accordance with it's listing and the manufacturer's instructions. The combination of a main breaker and panel with a lower rating may violate the manufacturer's instructions. That of course would vary case by case.