A 120/240 V center-tapped secondary has different fault paths for a 240 V line-to-line fault and a 120 V line-to-neutral fault, so a three-phase line-to-line model does not by itself calculate either split-winding fault correctly. Calculate each fault on the applicable single-phase winding path, represent the upstream three-phase supply by its source impedance, and add feeder impedance to the point being evaluated.
Fault-current quantity and limiting impedance
Available fault current is the symmetrical fault-current component used when evaluating breaker and switchgear interrupting ratings. The first estimate assumes an infinite upstream supply and uses the transformer’s rated full-load current divided by its per-unit impedance. This gives a transformer-limited value; it is not automatically the current at a downstream device when feeder impedance is material.
For the chosen fault path, the governing quantity is total impedance from the source through the transformer and conductors to the fault. In a simplified single-phase loop representation, the relationship is fault voltage divided by total loop impedance. Use impedance values on a consistent base and include the complete return path. The split secondary matters: the 240 V path uses the full winding, while a 120 V fault uses a half-winding path. Do not substitute the full-winding impedance without checking that the transformer data applies to the half winding.
| Quantity | Role | Where to read or derive it |
|---|---|---|
| Transformer kVA and winding voltage | Set the current and per-unit bases for the fault path | Transformer nameplate or manufacturer data |
| Transformer percent impedance | Limits the initial symmetrical fault-current estimate | Nameplate or transformer test/manufacturer data; identify the applicable winding path |
| Source impedance | Reduces fault current below the infinite-source estimate | Utility or upstream system fault data, converted to the chosen base |
| Feeder impedance | Reduces current at downstream switchgear or a fault location | Conductor data and actual circuit length/configuration |
| X/R ratio | Needed to assess the asymmetrical current component | Transformer and upstream-source data |
Model approaches for a split secondary
The proposed three-phase transformation—tripling kVA and impedance and representing the secondary as delta—can be useful for a specific phase-to-phase calculation in a three-phase model. It is not a general equivalence for a single-phase transformer’s phase-to-ground or phase-to-neutral fault. Those faults depend on which portion of the split winding supplies the fault and on the applicable return path.
| Approach | What it represents | Decision |
|---|---|---|
| Single-phase per-unit calculation on the transformer kVA base | Fault on the full 240 V winding or on a 120 V half winding; upstream three-phase source represented by source impedance | Recommended for the requested single-phase secondary fault |
| Three-phase model with scaled kVA/impedance and delta secondary | A modeled three-phase equivalent used for the stated phase-to-phase calculation | Use only for the fault type and model mapping it actually represents; do not treat it as proof of a 120 V split-winding ground-fault result |
| Transformer-only infinite-source estimate | Upper-bound first calculation based on transformer impedance | Use as a screening value; add finite source and feeder impedance when evaluating actual equipment location |
Transformer reactance and X/R data can be difficult to obtain, and the split winding complicates an approximate calculation. If the required half-winding impedance is not available, obtain transformer-specific data or use a calculation method that explicitly models the split winding; do not silently reuse a three-phase model’s impedance.
Fault paths on the 120/240 V winding
Identify the fault voltage and winding section before selecting a base. A line-to-line fault across the two ungrounded conductors is a full-winding, 240 V case. A line-to-neutral fault from one ungrounded conductor to the center tap is a half-winding, 120 V case. The three-phase source upstream does not change the secondary fault into a balanced three-phase fault; represent its contribution as source impedance in the single-phase equivalent.
A phase-to-ground fault is not always interchangeable with a line-to-neutral fault. Confirm that the grounded conductor/center tap is the intended return path and that the fault is actually connected to it. If the grounding arrangement or fault location differs, draw the actual loop and use its voltage and impedances. The fault-current result depends on that circuit, not merely on the nominal 120/240 V label.
Single-phase per-unit calculation procedure
- Define the fault: 240 V across the full winding, 120 V from one line to the center tap, or another explicitly drawn path. Record the fault location and the conductor return path.
- Choose the transformer kVA base and the voltage base for that winding section. Derive the corresponding current base consistently for the single-phase path; do not use a three-phase line-current base by default.
- Obtain the transformer impedance for the applicable winding path. Convert percent impedance to per unit by dividing by 100. For a first estimate with an infinite upstream source, divide the applicable full-load current by the per-unit transformer impedance.
- Represent the three-phase source as its source impedance on the same base when source strength is finite. Convert and combine impedances on a consistent base before calculating fault current.
- Add the impedance of conductors between transformer and evaluation point. Recalculate at the switchgear or fault location; this gives the point-specific available current rather than only the transformer-terminal estimate.
- Compare the resulting symmetrical available fault current with the interrupting rating of the protective device and the applicable equipment rating. If the estimate exceeds the desired rating, feeder impedance may reduce the current at the equipment location, but calculate that reduction from actual conductor data rather than assuming it.
For the infinite-source screening calculation, use the transformer’s full-load current corresponding to the same kVA and winding base as the percent impedance. The simple division is a first calculation, not a substitute for adding source impedance or verifying the half-winding data. Record the bases and conversions so another engineer can reproduce the result.
Source and feeder contributions
An infinite source assumes upstream impedance is negligible. A real source has finite impedance, which limits available fault current. Where utility or upstream fault data is available, convert it to impedance on the selected per-unit base and include it in the single-phase equivalent. Then include transformer and feeder impedance to the point under study. Avoid mixing per-unit values from different bases without converting them first.
Calculate at the relevant location. Transformer secondary terminals and a downstream switchgear bus do not have identical available fault current if the connecting feeder has appreciable impedance. If a transformer-only estimate is above the equipment rating, the next decision is whether actual source and feeder impedances bring the calculated current at the equipment down to an acceptable value. The result should be based on documented impedance, not a guessed distance or a generic percentage reduction.
Symmetrical rating and asymmetrical current
Available fault current is the symmetrical component; an asymmetrical current also includes a decaying offset. Its magnitude depends on the X/R ratio and the source contribution. The asymmetrical current may be about twice the symmetrical available current, but that is not a universal conversion factor. Calculate or obtain the asymmetrical value when the application requires it, using transformer X/R and upstream-source data rather than multiplying by two as a design rule.
Keep the rating comparison aligned with the rating basis. Breaker and switchgear ratings are evaluated against available fault current, while equipment withstand must account for the applicable asymmetrical duty. Confirm the manufacturer’s rating basis and applicable installation requirements for the selected equipment; a symmetrical calculation alone does not resolve an asymmetrical withstand question.
Calculation checks before equipment selection
- Check that the stated fault is 120 V half-winding or 240 V full-winding, not an assumed three-phase fault.
- Check that voltage, kVA, full-load current, impedance, and per-unit values all use compatible bases.
- Check that source impedance is included when the upstream source is not treated as infinite, and feeder impedance is included at a downstream evaluation point.
- Check that the impedance data applies to the winding section used. If the half-winding impedance or X/R is unavailable, obtain manufacturer-specific information or use a method explicitly suited to split-winding faults.
- Check the symmetrical available-current result against the protective device interrupting rating and equipment rating; separately resolve asymmetrical duty where required.
Frequently asked questions
How do I calculate a 120 V fault on a 120/240 V transformer?
Model the single-phase loop from one line through the half winding to the center tap and fault return. Use impedance data applicable to that half-winding path, then include source and feeder impedance on a consistent per-unit base.
How do I calculate a 240 V fault on a split-phase transformer?
Treat a line-to-line fault across the two ungrounded conductors as a full-winding, 240 V single-phase fault. Use the transformer’s applicable impedance and current base, then add upstream source and feeder impedances.
Can I model a single-phase transformer as a three-phase transformer?
A scaled three-phase model may represent a particular phase-to-phase calculation, but that does not establish the 120 V half-winding or phase-to-ground fault current. Use a single-phase per-unit calculation for the split-winding fault path.
How do I include utility impedance in the fault-current calculation?
Represent the upstream three-phase source as source impedance, convert it to the same per-unit base as the transformer, and combine it with transformer and feeder impedance in the fault loop. Use utility or upstream system data for the actual source strength.
When should I stop and escalate a transformer fault-current calculation?
Stop equipment selection if the applicable half-winding impedance, source data, or X/R needed for the rating comparison is unavailable or inconsistent. Request transformer-specific data from the manufacturer and source fault data from the utility; have the equipment manufacturer or a qualified power-systems engineer resolve any rating-basis question before specifying the gear.