Transformer Sizing & Protection

Pick a transformer size, get the maximum primary and secondary overcurrent protection, and size the conductors, bonding jumper, and grounding electrode conductor. Every result shows the NEC section it comes from.

NEC 2023 / 450.3 / 240.4 / 240.6 / 240.21(C) / 250.30 / 250.66 / 250.102 / 250.122 / Table 310.16

Transformer

System

Phase

Rating (kVA)

Primary voltage (V)

Secondary voltage

Impedance (%Z)

From the nameplate. Lower %Z means higher fault current.

Conductor material

Terminal rating

Sets the Table 310.16 column, per 110.14(C)(1).

Protection

Protection scheme

Primary device rating

Rounded up to the next standard rating, never above the NEC maximum.

Secondary device rating

Standard ratings from FLA up to the NEC maximum.

Size the transformer from a load

Calculated load (kVA)

Spare capacity (%)

Spare capacity is a design choice, not an NEC requirement. Use the load calculated per Article 220.

One-line

Updates as you change the inputs. Fault current assumes an infinite source, which gives the worst case.

Full-load current

Primary—

Secondary—

Fault current

—

Secondary terminals, infinite source: I_sc = FLA × 100 / %Z

Minimum equipment rating—

110.9, 110.10

Bonding & grounding

Supply-side bonding jumper—

Grounding electrode conductor—

Primary EGC—

250.30(A), Table 250.102(C)(1), Table 250.66, Table 250.122

Overcurrent protection

Device	NEC limit	Maximum	Selected

Conductors

Primary—

Secondary—

Table 310.16, 110.14(C)(1), 240.4(B), 240.4(D), 240.21(C), 310.10(G)

No ambient or fill adjustments are applied. Apply 310.15(B) and (C) for your installation.

Design checks

Reference tool for design checks. Based on NEC 2023 (NFPA 70). Verify every result against the code edition adopted by your jurisdiction, the transformer nameplate, manufacturer data, and your authority having jurisdiction before using it in construction documents.