Okay, let's break this down. Your questions touch on transformer protection and load management.

Bayonet Fuses and Transformer Sizing You are correct

Bayonet fuse sizing is primarily determined by the

load the transformer serves, not the transformer's kVA rating. The purpose of the bayonet fuses is to protect the transformer from overcurrent conditions caused by downstream faults or excessive loads. Important Considerations

While the load is the primary factor, you must consider the transformer's capacity in the fuse selection process. You're basically trying to strike a balance:

Fuse too small

Nuisance tripping, especially during motor starting or inrush currents.

Fuse too large

Inadequate protection for the transformer during sustained overloads or internal faults. The fuse might not blow quickly enough to prevent damage to the transformer windings.

So, how do you determine the proper fuse size?1. Calculate the Expected Load

Determine the actual connected load on the transformer. This should include continuous loads, intermittent loads, and any anticipated future load growth.2.

Consider Inrush Currents

Account for inrush currents from motors, capacitor banks, or other equipment that draw a large current surge upon startup. The fuse needs to be able to withstand these inrushes without blowing.3.

Transformer Full Load Amps (FLA)

Calculate the transformer's FLA using the kVA rating and voltage. `FLA = kVA 1000 / (Voltage sqrt(3))` for three-phase transformers.4.

Fuse Sizing Guidelines (General)

As a starting point, many guidelines suggest a fuse rating of 125% to 150% of the transformer's FLA. However, this is a starting point, not a hard rule. Motor loads and other factors can significantly affect this. Consult transformer manufacturer recommendations for fuse sizing. Use time-current curves (TCC) for the fuse you're considering to ensure it provides adequate protection under various fault conditions and avoids nuisance tripping.5.

Coordination

Coordinate the fuse with any downstream protective devices (circuit breakers, fuses) to ensure proper selectivity. This means that the downstream devices should trip before the bayonet fuses, isolating the fault and minimizing the outage area.

Example: T12

You mentioned T12 went from 112.5 kVA to 500 kVA. Old Scenario (112.5 kVA)

The fuses were sized to protect the 112.5 kVA transformer based on the load at the time.

New Scenario (500 kVA)

The existing fuses are likely way undersized. While they might protect the old load, they won't allow you to fully utilize the 500 kVA transformer's capacity, and they'll probably nuisance trip.

You absolutely need to recalculate and resize the bayonet fuses for the 500 kVA transformer based on the current and anticipated load.Loop Load Management Homework Required

With 14 transformers on one loop and 8 on another, it's critical that someone is doing their homework regarding load additions. Simply having 500 kcmil conductors doesn't guarantee you won't overload a section of the loop or exceed the capacity of the source substation.

Load Studies

Ideally, a power system study or load flow study should be performed when significant load additions are planned. This will help determine: Voltage drop along the loop. Loading on each section of cable. Loading on the transformers. Fault current levels (for proper protective device coordination).

Monitoring

Consider installing monitoring equipment (smart meters, power quality meters) to track the actual load on each transformer and the overall loop. This can provide valuable data for identifying potential overloads or imbalances.

Diversity Factors

Keep in mind that diversity factors (the probability that all loads won't be running at their maximum simultaneously) can be used in load calculations, but these should be based on actual load data or industry-accepted values.

Who's Responsible? Clearly define who is responsible for: Approving new load additions. Performing load studies. Monitoring system performance. Upgrading infrastructure when necessary.

In Summary Bayonet fuse sizing depends on the load, but the transformer capacity is also a critical factor. Don't underestimate the importance of proper fuse selection for protecting your transformers. Aggressively load manage your loops. Continuous load monitoring, load flow studies, and strict planning are vital for reliability.* A qualified electrical engineer with power system experience is best equipped to perform these calculations and make recommendations.If you can provide specific load data (kVA, kW, load types) for each transformer, I can offer more specific advice. Good luck with your upgrade! Flag for review