Okay, let's break down the situation and address your questions.

Understanding the Situation

You have a critical continuous load of 800A. Your original design used a 100% rated 800A circuit breaker, and your feeder conductors are sized accordingly (3 sets of 300 kcmil). The equipment vendor is now suggesting using an 80% rated 1000A circuit breaker, but setting the trip point to 800A. They claim testing shows it handles 800A without exceeding thermal limits.

Question 1: Code Compliance (NEC 215.2 & 215.3) NEC 215.2 Minimum Rating and Size

This section generally requires feeder conductors to have an ampacity not less than 125% of the continuous load, plus the noncontinuous load. Since your load is 800A continuous, the minimum required ampacity for the feeder is 800A 1.25 = 1000A. You're already meeting this requirement with (3) sets of 300 kcmil conductors (check the ampacity table in the NEC for your specific conductor type and insulation temperature rating - typically around 310 amps 3 sets = 930A total). You might need to use the parallel conductor adjustment factors in 310.15(C) to get the derated ampacity.

NEC 215.3 Overcurrent Protection

This section requires the overcurrent protection device to protect the feeder conductors from overcurrent. Specifically, it says the overcurrent protection device must be sized to "carry continuously the load it supplies," and must be rated or set so as not to exceed the ampacity of the feeder conductors.

With the proposed change

The 1000A circuit breaker (even with an 800A trip setting) can certainly carry the 800A continuous load (assuming the vendor's thermal testing is legitimate - see below).

However

The critical question is, does the 1000A breaker, even tripped at 800A, adequately protect the feeder conductors? If your feeder conductors have an ampacity of 930A (or more after derating), a 1000A breaker is too large to protect the conductors.

Therefore, the vendor's proposal is NOT code-compliant.Question 2: 100% Rated CB Testing and Application of 1000A/80% rated CB. 100% Rated Circuit Breaker Testing

100% rated circuit breakers are specifically designed and tested to carry 100% of their rated current continuously (under specific conditions, including ambient temperature). They must pass rigorous thermal testing to ensure they do not overheat and create a hazard. The standards for 100% rated breakers are different from those for 80% rated breakers.

Vendor's Claim

The vendor's claim that the 1000A breaker (80% rated) passed thermal testing at 800A doesn't magically make it a "100% rated 800A" breaker. It simply means this particular breaker, under their specific test conditions, didn't overheat at 800A. It doesn't change the inherent design and certification of the breaker as an 80% rated device. You have no guarantee that every 1000A/80% rated breaker of that model would perform the same way, or perform reliably over its expected lifespan at that loading.

Concerns



UL Listing and Standards

The breaker's UL listing (or equivalent certification) is based on its rated current, not on an aftermarket test at a reduced current. Modifying the trip setting of a standard breaker does not change its underlying listing.

In summary, even if the vendor's testing is valid, you cannot treat an 80% rated breaker (even with a lowered trip setting) as a 100% rated breaker at the reduced current. It does not change the listing of the breaker.Recommendations1. Insist on a 100% Rated Breaker

The original design was sound. Stick with a 100% rated 800A circuit breaker. This is the cleanest and safest solution. If the vendor is having trouble sourcing one, explore alternative suppliers.2.

Reject the 1000A/80% Proposal

The vendor's proposal is technically non-compliant to NEC 215.3 and is very likely non-compliant as the breaker is not properly protecting the feeder.

Important Considerations* Consult a Qualified Electrical Engineer: Ultimately, the decision should be made in consultation with a licensed electrical engineer who can review the specific equipment, applicable codes, and potential hazards.Let me know if you have any further questions! Flag for review