Transformer sizing and protection

EKX treats three transformer questions separately:

The transformer kVA is selected from downstream design load and the project transformer margin. When one transformer feeds another, the upstream transformer counts the loads behind the downstream one, on its own Transformer Sizing Basis, and adds its margin once.
The primary feeder cable is selected from transformer kVA and primary voltage.
Transformer protection and secondary conductors are checked against the project's electrical code: NEC 2026 or CEC 2024.

These are separate from the current selection check. A transformer, primary feeder, or secondary conductor can keep a manually selected value and still be checked. The latest sizing result records a historical recommendation; Recheck evaluates the installed transformer rating, feeder conductor, parallel runs, and secondary conditions from current project or scenario facts. A different value is not a failure when it satisfies the checked requirements.

The transformer edit panel includes help icons beside these code choices. Hover or focus an icon for a short explanation, or use the Learn more link to return to this guide.

Transformer kVA auto-sizing selects from the effective Allowed Single-Phase Transformer Ratings or Allowed Three-Phase Transformer Ratings under Settings → Sizing Rules. If no allowed kVA rating can carry the calculated requirement, EKX leaves the transformer unchanged and reports a blocking error.

Single-phase and three-phase transformers

EKX reads a transformer's phase from what its secondary feeds. When every bus or load first reached from the secondary is single-phase (a 1PH2W or 1PH3W bus, or a single-phase load), the transformer is sized as single-phase; otherwise it is sized as three-phase. To pin it, set Phase on the transformer's code rules tab.

The phase decides the rating list and the rated current that protection and conductors size from. For example, a 50 kVA, 480-240 V single-phase transformer has a rated current of 50 / 0.48 = 104.2 A on the primary and 50 / 0.24 = 208.3 A on the secondary. Sized as three-phase, the same kVA would give 60.1 A and 120.3 A.

Load flow, short circuit, and arc flash still model every transformer as a balanced three-phase unit; the phase affects sizing only.

Saving inputs and applying a transformer size

Saving a transformer sizing basis or margin saves that input first. In Suggest mode, EKX then shows the resulting recommendations for review without changing the transformer or its dependent equipment. In Auto-apply mode, EKX applies the recommendations only when the complete group passes the same validation used for reviewed suggestions.

Use Accept on the transformer suggestion, or Re-size system, to apply the transformer, required primary and secondary protection, and affected conductors as one group in either Suggest or Auto-apply mode. If a required device is locked, needs an equipment choice, or cannot support the required setting, EKX names the blocker and leaves the complete group unchanged.

Use Undo to restore the transformer and every dependent value changed by that acceptance. Refreshing sizing evidence or reopening the project does not apply a recommendation.

When both Table 450.5(A) supervision and the industrial 25 ft secondary-conductor rule apply, EKX shows one Qualified-person supervision checkbox. It confirms that qualified people monitor and service the transformer installation and secondary system, recording both supervision confirmations at once. If only one rule applies, EKX shows that rule's specific supervision label instead.

Plain-language glossary

Protection arrangement

Primary only means the transformer has overcurrent protection on its supply side only. It does not automatically mean the secondary conductors are protected; EKX works out the secondary-conductor pathway separately.
Primary + secondary means overcurrent protection is installed on both sides of the transformer. EKX verifies the installed secondary devices against the selected transformer-protection limits.
Supervised location means maintenance and supervision ensure that only qualified people monitor and service the transformer installation. It does not simply mean that the transformer is inside an industrial facility.
Coordinated thermal overload protection means the transformer manufacturer provides thermal-overload protection coordinated with the primary protection. For the applicable supervised-location table option, this can permit separate secondary protection to be omitted.

Secondary conductor rule

By default the rule is Automatic: EKX reads the diagram (a device at the transformer secondary, the devices each secondary run ends at, the transformer vector group, and the primary device setting), the cable length, and the project's installation assumptions, then applies the least restrictive pathway whose conditions hold. The cable's sizing evidence names the pathway used and lists any installation condition it assumed. Pin a pathway on the transformer only to override that choice.

Protected at source means an overcurrent device protects the secondary conductors where they begin, at the transformer secondary terminals. A tap-rule exception is not needed.
Primary-only protection means the primary device also protects the secondary conductors. NEC permits this only for a qualifying single-phase 2-wire secondary or three-phase delta-delta 3-wire secondary.
10 ft rule applies to secondary conductors no longer than 10 ft that meet the required ampacity, enclosure, and raceway conditions.
Industrial 25 ft rule applies only to a supervised industrial installation supplying switchgear or a switchboard within 25 ft. It also requires qualified servicing, grouped overcurrent devices, full secondary-current ampacity, and physical protection.
25 ft rule applies to secondary conductors no longer than 25 ft that meet the reflected-primary protection and ampacity limits, terminate in one overcurrent device, and are physically protected.

Installation conditions

These describe how the secondary conductors are installed, which the diagram cannot show. Set them once for the project under Diagram Settings > Project Defaults > Sizing Rules; the transformer form shows the project value and lets you override it for one transformer.

Qualified industrial supervision means maintenance and supervision ensure that only qualified people service the system. The facility being industrial is not enough by itself.
Terminating devices grouped means all secondary overcurrent devices are installed together in one location.
Physical protection means the conductors are protected from physical damage by an approved raceway or another approved method.
Raceway to termination means the raceway runs from the transformer to the supplied switchboard, switchgear, panelboard, disconnect, or control equipment.
No extension beyond equipment means the secondary conductors stop within the enclosure of the equipment they supply.
Single-phase two-wire secondary confirms a single-voltage 2-wire secondary. Other single-phase arrangements are not protected by the primary device alone.

What physical protection means

For the 25 ft pathways, select Physical protection only after confirming that the installed secondary conductors are protected from likely mechanical damage for their environment. The NEC permits an approved raceway or another method accepted by the authority having jurisdiction (AHJ).

Depending on the location and exposure, examples can include:

Schedule 80 PVC conduit where the raceway is subject to physical damage
rigid metal conduit or intermediate metal conduit in exposed industrial areas
guards, channels, bollards, or barriers protecting a raceway from forklifts and vehicles
a raceway routed behind structural protection or through a controlled electrical space where ordinary activity cannot strike, crush, or abrade it

The cable or raceway name alone is not enough. Cable tray, armored cable, or a light-duty raceway should not be treated as automatically protected without considering the actual exposure. The installation method, supports, bends, entries, accessible sections, and local conditions all matter, and the AHJ makes the final acceptance decision.

This matters because these pathways can permit secondary conductors without an overcurrent device at the transformer terminals. A fault caused by damage before the downstream breaker is on that breaker's line side, so the downstream breaker cannot clear it. The transformer primary protection must operate instead and may allow substantially more fault energy than source protection sized directly for the secondary conductors. The NEC therefore combines the conductor-length limit, ampacity and termination rules, and physical protection to reduce that risk.

In EKX, the checkbox records a confirmed installation condition; EKX does not infer physical protection from the selected cable type or inspect the field route. For the 10 ft pathway, use Raceway to termination instead: that pathway specifically requires the raceway to extend from the transformer to the supplied equipment enclosure.

Transformer protection

Choose whether the installation uses primary-only protection or primary-and-secondary protection. EKX applies NEC 450.5(B) when both windings are 1000 V or less and Table 450.5(A) when a winding is over 1000 V.

For an over-1000-V transformer, enter the transformer impedance and identify whether the installation is supervised. If secondary protection is omitted using coordinated transformer thermal-overload protection, identify that condition as well.

How EKX finds the secondary protection

Unless you pin the arrangement, EKX reads it from the diagram. The transformer has primary-and-secondary protection when every secondary branch starts with a breaker or fuse, and primary-only protection otherwise. EKX recognizes these secondary connections:

A cable from the transformer: EKX uses the first breaker or fuse on every branch beyond the cable.
A breaker or fuse connected directly to the transformer, with or without a cable after it: that device is the secondary protection.
A bus connected directly to the transformer: the breakers and fuses connected directly to that bus are the secondary protection, treated as one group in one location.

A connection drawn without a cable is treated as close-coupled equipment. It does not show that the installed connection needs no conductor protection, and it does not check panelboard protection under NEC 408.36.

Under NEC 2026, up to six breakers or sets of fuses can together be the secondary protection (Tables 450.5(A) and 450.5(B), Note 2). Their ratings must total no more than one device could be rated. When every device has a fixed rating, that includes the next standard rating; if any device has an adjustable setting, the total is held to the calculated maximum. For example, a 1000 kVA transformer with a 480 V secondary allows a 1600 A total, so breakers of 200 A, 70 A and 60 A on its bus pass. The Sizing tab lists the devices it added up.

EKX reports the secondary protection as blocked, with the reason, when:

More than six devices are grouped. Marking the location supervised does not change this. Add one secondary main breaker or fuse between the transformer and the bus.
Their total is above the limit.
A branch from the bus reaches its breaker or fuse only through a cable or another bus. EKX can't treat that device as part of the group at the transformer. Add a breaker or fuse at the bus for that branch, or a secondary main.
The transformer's secondary connects directly to more than one component.

A branch with no breaker or fuse at all makes the arrangement primary-only.

For a medium-voltage circuit breaker, EKX reports two different values:

Equipment frame is the breaker hardware current rating.
Protection setting is the trip-unit or relay setting that protects the transformer.

Those values are intentionally not treated as interchangeable. Autotransformer protection is not auto-sized yet; EKX blocks it and identifies NEC 450.6 instead of applying the two-winding rules.

When a modeled primary breaker has an enabled phase-overcurrent relay pickup, transformer-secondary conductor calculations use that primary-ampere pickup instead of mistaking the breaker frame rating for the protective setting.

When one breaker or fuse is the transformer's only secondary overcurrent device and also protects a downstream bus, its aggregate sizing treats the permitted secondary rating as a hard maximum. If the transformer is in Auto mode and that device's code minimum does not fit, EKX raises the transformer to the smallest allowed capacity whose secondary protection can accept it, and proposes the transformer, the primary protection setting, the secondary device, its cable, and its bus as one group. The transformer explanation shows the demand basis and the raised capacity separately. If the transformer is Manual or Locked, the device is reported as blocked with both values and the available changes instead. The supervised-location setting matters here: for a secondary at or below 1000 V it raises the permitted secondary rating from 125% to 250% of the secondary full-load current, which usually avoids the larger transformer.

Transformer secondary conductors

EKX infers the NEC 240.21(C) pathway from the diagram, the cable length, and the project's installation assumptions. You do not need to choose a pathway before using Re-size system. Candidates are tried from least to most restrictive. When no pathway fits, EKX reports every candidate's reason. The supported pathways are:

protection at the transformer secondary terminals
primary-only protection for a qualifying single-phase or three-phase delta-delta transformer
the 10 ft rule
the supervised industrial 25 ft rule
the general 25 ft rule

The cable length comes from the cable record. EKX also checks the transformer connection and voltage ratio, calculated load, terminating protection or equipment rating, and the installation conditions for the pathway.

When the secondary cable ends at an auto-sized main breaker or fuse, EKX sizes the two together. The device's demand-based rating is planned first, the cable is sized for that rating, and both appear as one linked suggestion that is accepted or rejected as a unit. A manual or locked device keeps its installed rating as the conductor's requirement; a manual or locked cable keeps its installed ampacity as the device's limit.

For a common secondary feeding several mains in one switchboard, the cable must also carry the switchboard rating under the 10 ft rule. If the switchboard is automatic, EKX uses its proposed rating and applies any required incoming cable increase with the switchboard increase.

Exactly 10 ft and exactly 25 ft are accepted. A missing length blocks a length-limited rule. EKX treats drawn secondary cables as field wiring leaving the transformer enclosure. The 10 ft rule therefore includes the one-tenth reflected primary-device floor under NEC 240.21(C)(2)(4); the general 25 ft rule uses the one-third floor. Both need a usable primary protection setting. An MV breaker without a relay setting leaves those cable pathways waiting; its frame rating cannot replace the setting.

NEC 240.4(B)'s next-higher-size allowance is not permitted for transformer secondary conductors. EKX records that prohibition in the sizing evidence. Outside-secondary-conductor and feeder-tapped-transformer pathways remain blocked until their complete rule models are available.

Separately derived system grounding and bonding under NEC 250.30 is outside this transformer protection/conductor release. EKX does not infer grounding-electrode or bonding conductor sizes from these pathway selections.

CEC 2024 projects

The sections above describe NEC projects. On a CEC 2024 project, the transformer panel's CEC transformer rules section names the CEC rules EKX applies instead:

For a primary winding of 750 V or less, protection follows Rule 26-254 for a dry-type transformer and Rule 26-252 for any other construction. Set Construction on the Equipment tab; picking a cooling class there also sets it. Until it is set, EKX proposes a rating both rules permit, which is usually the dry-type limit, and does not apply the dry-type Primary only pathway below.
For a primary winding over 750 V, primary-only protection follows Rule 26-250(1), and primary-and-secondary protection follows Rule 26-250(4) and Table 50. Table 50 needs a transformer impedance greater than 0% and not more than 10%.
Secondary conductors follow Rule 26-256. The 3 m rule and 7.5 m rule pathways follow Rule 14-100(1)(b) and Rule 14-100(1)(c). The 7.5 m pathway needs the minimum based on one-third of the larger supplying conductor's ampacity; if that value is missing, the pathway waits. A cable protected by a main at the transformer is not subject to that tap length limit. EKX does not model an industrial pathway under CEC 2024.
The Primary only pathway follows Rule 26-254(1). It applies only to a dry-type transformer rated 750 V or less whose primary breaker or fuse is at or below 125% of rated primary current, and the secondary cable must carry at least 125% of rated secondary current. Under those conditions EKX sizes the secondary cable even when there is no breaker or fuse after the transformer, and keeps the auto-sized primary fuse within 125% so that the cable stays protected. Otherwise the cable waits for a secondary breaker or fuse, and the review card says why.
A breaker or fuse connected directly to the transformer counts as its secondary protection. Several breakers or fuses on a bus connected directly to the transformer do not count together under CEC 2024, because EKX has not verified a CEC basis for grouping them. The transformer is sized as primary-only and the Sizing tab says why. Add one secondary main between the transformer and the bus to size it with primary and secondary protection.
The panel hides Supervised location and Coordinated thermal overload, because they are NEC Table 450.5(A) conditions.
Autotransformer protection is not auto-sized; EKX identifies Rule 26-264.