The pad comes first and it is not just concrete
The pad has to carry the weight, stay level, and be larger than the cabinet on every side. A 300 kVA unit is around 3,000 lb and a 2500 kVA unit several times that, all of it on a small footprint. The pad also carries the conduit stubs for primary and secondary, positioned to line up with the compartments, and those positions come off the approval drawing rather than a generic detail. Pouring before the drawing is approved is the single most common way a job ends up breaking concrete.
Clearances are set by the utility, not by you
Every utility publishes its own clearance requirements and they differ. The figures most commonly seen are around 10 feet of clear working space in front of the doors, 3 feet at the sides and rear, and 10 feet from any building opening or combustible wall. Some require bollards where vehicles can reach the unit. Confirm with the serving utility in writing before siting, because a transformer that is one foot too close to a window fails inspection and the fix is moving the pad.
Grounding
A pad mounted transformer needs a grounding electrode system at the pad, bonded to the tank, the cabinet and the secondary neutral. On a grounded wye secondary the neutral to ground bond is made at the transformer, which makes it the origin of the separately derived system. NEC Article 250 governs the electrode and conductor sizing, and the utility will have its own requirement on top. Get both, they are not the same document.
Approval drawings are the long pole
On a made to order unit the manufacturer issues approval drawings after the order, and nothing is built until they come back signed. Four to five weeks is typical, and it runs before the build lead time rather than alongside it. So a 17 week lead time is really 21 or 22 weeks from purchase order. Plan the pad around the approved drawing and the schedule around both.
What actually delays installations
In order, waiting on approval drawings, a pad poured to the wrong stub locations, clearance rejected by the utility inspector, and no crane or route planned for a unit weighing several thousand pounds. None of those are transformer problems and all of them are avoidable.
Common questions
How are pad mounted transformers installed
A reinforced concrete pad is poured to the approved drawing, with conduit stubs positioned for the primary and secondary compartments. The unit is set with a crane or boom truck, primary and secondary cables are terminated, the grounding electrode system is connected and bonded, and the utility inspects before energising.
What clearance is required around a pad mounted transformer
It is set by the serving utility and varies. Commonly around 10 feet of clear working space in front of the doors, 3 feet at sides and rear, and 10 feet from building openings or combustible walls. Always confirm with the utility in writing before siting the pad.
What are the NEC guidelines for grounding a transformer
NEC Article 250 covers the grounding electrode system and conductor sizing. On a grounded wye secondary the neutral to ground bond is made at the transformer, making it the origin of a separately derived system. The serving utility normally has additional requirements beyond the code.
How long does it take to get a pad mounted transformer installed
The transformer itself sets in a day. The schedule is driven by approval drawings, typically four to five weeks after order, then the build lead time, then utility inspection. From purchase order to energised is commonly five to eight months on a made to order unit.
Free to cite with a link to this page. Work out current at any rating with the kVA to amps calculator.