You are on the right track with the 125% calculation for continuous loads. Let's break down the code references and clarify the calculations.

Key Concepts & Code References (NEC - National Electrical Code)



Continuous Load

A load where the maximum current is expected to continue for 3 hours or more. (NEC Article 100 - Definitions)

125% Rule

Overcurrent protection (breakers, fuses) and conductor sizing must be rated for at least 125% of the continuous load. (NEC 210.19(A)(1), 215.3, 430.22)

Calculations


Rated Generator Output

18 kW2.

Rated Generator Current (Assuming Single Phase 240V)

I = P / V = 18,000 W / 240 V = 75 Amps3.

Applying the 125% Rule

75 Amps 1.25 = 93.75 Amps

Next steps and considerations



Conductor Ampacity

The conductors (wires) connecting the generator to the load (or transfer switch) must be rated for at least 93.75 amps, as calculated above. You would then choose the appropriate wire gauge based on the applicable ampacity tables in the NEC (Table 310.16 and related tables). It's important to account for temperature correction and adjustment factors as required by the NEC.

Overcurrent Protection

The breaker or fuse protecting the generator output must also be rated for at least 93.75 amps.

Standard Breaker/Fuse Sizes

Standard breaker and fuse sizes don't always match the exact calculated value. You need to select the next standard size up. So, in this case, a 100 Amp breaker would be the correct choice.

Important Considerations and Why You Need to Consult a Qualified Electrician



Voltage

Your initial calculation assumes 240V single phase. If the generator is wired for 120V/240V split phase or three-phase, the current calculations will be different. You must use the correct voltage for your generator configuration.

Temperature Correction/Adjustment

Wire ampacity is affected by ambient temperature and the number of conductors bundled together. You must apply the appropriate correction and adjustment factors from NEC tables to ensure your conductors are properly sized.

Grounding

Proper grounding of the generator is essential for safety. Follow the generator manufacturer's instructions and the NEC requirements for grounding.

Local Codes and Permits

Electrical work is often subject to local codes and requires permits and inspections. Check with your local authorities before starting any work.

Load Diversity

In some cases, you can account for load diversity (not all loads operating at their maximum at the same time). However, this requires careful analysis and documentation.

Neutral Conductor Sizing

Determine if the neutral conductor needs to be larger based on the types of loads connected.*

Generator Nameplate

The most accurate information is ALWAYS on the generator's nameplate. Verify the rated voltage, current, and any specific installation instructions provided by the manufacturer.

In summary

You are correct in applying the 125% rule for continuous loads, and 100A is a good start but, your calculation shows you need a minimum of 93.75 amps.

However, this is a simplified calculation. You MUST consult a qualified electrician to perform a complete load calculation, select the correct wire sizes, grounding, and overcurrent protection, and ensure compliance with all applicable codes. Incorrect wiring can lead to fire hazards, equipment damage, and serious injury or death. Flag for review