No, you cannot simply round up to the next available breaker size after derating a conductor. Here's a breakdown of why and what you need to consider

Understanding the Concepts:
Ampacity

The ampacity of a conductor is the maximum current, in amperes, that a conductor can carry continuously under specified conditions of use without exceeding its temperature rating.

Derating

Derating is reducing the allowable ampacity of a conductor due to factors like:

Ambient Temperature Correction

If the ambient temperature is higher than the temperature rating of the wire's insulation, you must reduce the ampacity.

Conduit Fill (Number of Conductors)

When multiple current-carrying conductors are bundled together in a conduit, they generate heat. This heat reduces the ability of each conductor to dissipate heat, so the ampacity is reduced.

Overcurrent Protection

Overcurrent protection (typically a circuit breaker or fuse) is designed to protect the conductor from overheating and causing a fire.

The Rule

The general rule is that the overcurrent protection device (breaker or fuse) must not exceed the conductor's adjusted ampacity. You derate first, then select an overcurrent device to protect the derated conductor.

Your Example Breakdown


#8 AWG at 90°C Ampacity

As you stated, #8 AWG copper conductor with 90°C insulation has an ampacity of 55A (based on the NEC).2.

Derating for Conduit Fill (50% Derating)

55A 0.50 = 27.5A3.

The Problem

Your calculated derated ampacity is 27.5A.
4.

Applying the Rule

The overcurrent protection (breaker) must not exceed 27.5A.
5.

Conclusion

A 30A breaker is not allowed in this scenario. You'd need to use a breaker that is 25A or lower.

NEC References 240.4(D) Protection of Flexible Cords and Fixture Wires. Flexible cords not larger than 18 AWG, and fixture wires of any size, shall be protected against overcurrent in accordance with their ampacities, as specified in Table 400.5(A) and Table 402.
7. A listed cord set or power supply cord shall be considered to be protected by the overcurrent device that is part of the listed product. 240.4(D)(1) 10 AWG Copper. 10 AWG copper wire shall be protected by a 30-ampere overcurrent device unless otherwise permitted or required in 240.4(E) or (G).

Important Considerations and Alternatives



Standard Breaker Sizes

Circuit breakers come in standard sizes (15A, 20A, 25A, 30A, 35A, 40A, etc.). If 27.5A is the calculated derated ampacity, and a 25A breaker is the next standard size down, that is what you would need to use.

Exceptions in NEC

There are limited exceptions in the NEC that sometimes allow you to round up to the next standard overcurrent device size. However, these are very specific and don't generally apply to general circuit wiring derated for conduit fill.

Use a Larger Conductor

The easiest and most common solution is to use a larger conductor. In your example, if you need a 30A circuit, you could upsize to a #6 AWG wire (75A at 90°C). Even if you derated it by 50%, you'd still have 37.5A which is sufficient for a 30A breaker.

In summary, you CANNOT round up to the next standard breaker size after derating a conductor for conduit fill. You must protect the conductor at its derated* ampacity. The simplest solution is often to upsize the conductor. Flag for review