Guides to transformers, switchgear and generators
Plain answers for people buying and running power equipment, written from the standards and from the units we test. Each guide starts with the answer, and the calculators show their working.
20 guides
Calculators and sizing
Five calculators that show their working: kVA to amps, the standard transformer size for a load, the largest fuse or breaker the NEC allows a transformer, the fault current its impedance lets through, and the generator a load needs.
Calculator9 min read
kVA to amps calculator and chart
A calculator, the formulas, and full-load amps for every standard size at the common voltages.
Calculator9 min read
How to size a transformer
Load to kVA to a standard size, the motor-starting check, and a calculator and sizing chart.
Calculator10 min read
Transformer fuse and breaker sizing
NEC 450.3 in a table and a calculator, a chart for standard sizes, bayonet fuses, and why inrush sets the fuse.
Calculator8 min read
Transformer impedance, explained
What %Z means, the fault current it sets (with a calculator), regulation, and the 7.5% rule for paralleling.
Calculator12 min read
How to size a generator for a building or a plant
Running load, the largest motor, non-linear loads, derating and ratings classes, with a calculator and a kW-to-amps chart.
Transformers
How transformers are built, specified, tested and priced: step-up and step-down, delta and wye, buck-boost, tap changers, the types of dry-type, oil and dissolved-gas analysis, the nameplate, and what a used pad-mount costs.
Guide13 min read
How to test a transformer
The six tests a used transformer gets before it ships, what each one proves, and the limits that pass.
Guide11 min read
What a pad-mount transformer costs
New planning ranges by size, the real asking prices of used units listed here, the 75 kVA question, and lead times.
Guide10 min read
How to read a transformer nameplate
kVA, voltages, taps, %Z, BIL, rise, fluid, weights, vector diagram, polarity: what each field means and what to check.
Guide9 min read
Step-up vs step-down transformers
What the turns ratio does, where each kind is used, running one backwards, and what to specify.
Guide9 min read
Delta-wye transformers
The four connections, why delta primary and wye secondary is the standard, the 30° shift, and reading a nameplate.
Guide9 min read
Buck-boost transformers
Correcting 208 V to 240 V, or the reverse, with a unit a tenth the size of the load: how, when and the sizing formula.
Guide11 min read
Load tap changers
On-load versus de-energized taps, how an LTC switches, the controller, maintenance by operations count, and buying used.
Guide10 min read
Types of dry-type transformers
Open-wound, VPI, VPE and cast coil: how each is built, what it resists, and where it belongs.
Guide12 min read
Transformer oil and dissolved gas analysis
The fluids, the oil screen and its limits, and DGA: which gas means which fault, and what to ask for on a used unit.
Switchgear and motor control
Switchgear against switchboards, metal-clad against metal-enclosed, vacuum circuit breakers, MCC buckets, and the trapped-key interlocks that keep a switching sequence safe.
Guide9 min read
Switchgear vs switchboard
The standards, the breakers, the 30-cycle withstand that sets them apart, and which to buy for a project.
Guide8 min read
Metal-clad vs metal-enclosed switchgear
Draw-out breakers in compartments versus fixed switches and fuses: the standards, the construction, pad-mounted gear, and buying used.
Guide10 min read
Vacuum circuit breakers
Interruption in a vacuum bottle, the ratings on the nameplate, makers and retrofits, the test set, and buying used.
Guide10 min read
Motor control center buckets
The plug-in unit explained: contents, NEMA sizes, space factors, which makers’ buckets fit which lineups, and prices new and used.
Guide8 min read
Kirk key interlocks
Trapped keys that force a safe sequence on breakers, switches and doors: how a scheme works and what to check on used gear.
Selling equipment
Who buys surplus transformers, switchgear and generators, what they pay against new and scrap, and what to send a buyer.