Complete guide to wiring a low-voltage transformer

Plan the install before you pull a single conductor

Low-voltage transformers (under 1000V on the secondary, typically 480V to 240/120V or 277V to 120V) live everywhere in commercial and light industrial work. Before you mount anything, confirm the load calculation, the primary voltage available, the secondary voltage required, and whether you need single phase or three phase. Get the kVA right or you will be back to swap iron.

Pull the nameplate data from the spec or the unit itself. Verify primary and secondary FLA, impedance percentage, and the K-factor if you are feeding non-linear loads. NEC 450.3 governs overcurrent protection and NEC 450.13 covers accessibility, both of which dictate where this thing can actually live.

Field tip: if the panel schedule shows a 45 kVA transformer feeding a 100A panel, run the math. 45,000 / (208 x 1.732) = 125A secondary. That 100A panel is undersized for a fully loaded transformer.

Sizing conductors and overcurrent protection

Primary conductors are sized per NEC 450.3(B) and Table 450.3(B). For a transformer with primary current of 9A or more and only primary protection, the OCPD can be set up to 125% of primary FLA. With both primary and secondary protection, the primary OCPD can go up to 250% if the secondary is also protected at 125% or less.

Secondary conductors follow NEC 240.21(C). You have several tap rules available: the 10 foot tap, the 25 foot tap, and the outside tap. Pick the rule that matches your physical layout and document it. Inspectors will ask.

• Primary OCPD: NEC 450.3(B), Table 450.3(B)
• Secondary conductor protection: NEC 240.21(C)
• Conductor ampacity: NEC 310.16, with terminal temperature limits per 110.14(C)
• Equipment grounding conductor: NEC 250.122, sized to the primary OCPD

Do not forget the equipment grounding conductor sized to the primary OCPD per NEC 250.122. The system bonding jumper on the secondary is a separate conversation, covered below.

Mounting, clearances, and ventilation

Dry-type transformers 112.5 kVA and less can sit on combustible surfaces with minimum clearances per NEC 450.21(A). Above 112.5 kVA, you need a fire-resistant transformer room unless the unit is rated Class 155 or higher and separated from combustibles by a fire barrier per 450.21(B).

Working space follows NEC 110.26. For systems 600V and below, that is 36 inches deep minimum, increased based on what is across from it. Headroom is 6.5 feet or the height of the equipment, whichever is greater. Do not block the ventilation openings with conduit, strut, or stored material.

Field tip: a 75 kVA transformer mounted tight against a stud wall will cook itself in summer. Give it the manufacturer's clearance even when code allows tighter, especially in a closet or attic.

Grounding and bonding the separately derived system

A transformer with a secondary that has no direct electrical connection to the primary is a separately derived system, governed by NEC 250.30. This is where most field mistakes happen, and where inspectors fail jobs.

You need four things on the secondary side: a system bonding jumper, a supply-side bonding jumper, a grounding electrode conductor, and a grounding electrode. The system bonding jumper connects the grounded conductor (typically the X0 terminal) to the equipment grounding terminal at the transformer or the first disconnect, but only at one point.

1. Install the system bonding jumper at the transformer X0 or at the first disconnecting means, never both. NEC 250.30(A)(1).
2. Size the supply-side bonding jumper per NEC 250.102(C).
3. Run the grounding electrode conductor per NEC 250.30(A)(5), sized from Table 250.66.
4. Bond to the nearest effective grounding electrode: building steel, a metal water pipe within 5 feet of entry, or a made electrode if neither is available.

If you bond at both the transformer and the downstream panel, you create parallel paths for neutral current on the EGC and conduit. That shows up as nuisance ground-fault trips and burning smell at the lugs.

Termination, energization, and verification

Torque every connection to the manufacturer's spec. NEC 110.14(D) now requires a calibrated torque tool. Loose lugs on a 75 kVA transformer secondary are a fire waiting to happen, especially with aluminum conductors.

Before energizing, megger the windings primary to ground, secondary to ground, and primary to secondary. Look for at least 100 megohms on a dry unit at room temperature. Verify tap settings match the actual primary voltage; most transformers ship on the nominal tap and your service might be running 5% high or low.

• Confirm primary and secondary voltage with a meter before connecting load
• Verify rotation on three-phase units with a phase rotation meter
• Check neutral-to-ground voltage on the secondary, should read near zero
• Document the tap setting, torque values, and megger readings for the closeout package

Energize the primary first with no secondary load, listen for hum and check for heat after 15 minutes. Then close the secondary main and bring loads on in stages. If the unit hums loud or runs hot with no load, you have a wiring or core problem, not a load problem.