Electrical Protection Calculators
Breaker rating selection, prospective fault current, fault MVA, fault types and short-circuit withstand calculators, with stated limits.
Protection selection depends on three numbers: the current the device must carry, the fault current it must interrupt, and the time it takes to clear a fault. Breaker rating selection picks a standard rating from the design current. The fault calculators give the prospective fault current for three-phase, line-to-line and line-to-earth faults. Cable short-circuit withstand shows whether a conductor survives until the device clears.
Where to start
- Breaker Rating Selection: Smallest standard breaker rating at or above the design current.
- Transformer Fault Current: Prospective three-phase fault current at a transformer secondary.
- Fault Current by Fault Type: Three-phase, line-to-line and line-to-earth fault currents.
- Cable Short-Circuit Withstand: Smallest conductor that survives the fault until it is cleared.
All 4 calculators in this category
Protection
- Breaker Rating Selection: Smallest standard breaker rating at or above the design current. Open in Elecmetric
Short Circuit
- Transformer Fault Current: Prospective three-phase fault current at a transformer secondary. Open in Elecmetric
- Fault Level (kA to MVA): Fault MVA from fault current and voltage. Open in Elecmetric
- Fault Current by Fault Type: Three-phase, line-to-line and line-to-earth fault currents from positive-sequence impedance and the zero-sequence ratio. Open in Elecmetric
Also useful here
These sit in other categories but apply to the same work.
- Earth Fault Loop Impedance: Check that an earth fault draws enough current to operate the protective device. Open in Elecmetric
- Cable Short-Circuit Withstand: Smallest conductor that survives a fault for a given clearing time. Open in Elecmetric
What these calculators do not cover
Rating selection is on current only. Breaking capacity, trip characteristic, selectivity and arc flash are separate checks. The Feeder Designer reports breaking capacity as not verified until you enter it, and trip characteristics and selectivity as not checked. Fault currents assume a bolted fault with no arc resistance, and the transformer fault calculator assumes an infinite source, so those values are upper bounds.
Cross-check: all 6 calculators listed here are compared with 143 reference calculations written separately from the engine. This confirms the arithmetic and selection rules. It does not confirm that a standard table or convention matches your standard or manufacturer data.
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