198V to 208V Buck-Boost Transformers

198V to 208V technical overview

198 volts is a measured number, not a nominal one. Nothing is distributed at 198. It is what a 208 volt system reads after a long feeder, an undersized conductor or a heavily loaded service transformer has taken its share. A 198V to 208V buck-boost transformer puts back the missing 5 percent at the point where the load actually connects. This pair belongs to the voltage-drop problem rather than the wrong-equipment problem, and that changes both where the transformer goes and how it is selected. Single-phase and three-phase, 10 through 60 amps.

Where 198V to 208V correction is used

Every 198 volt reading has a length of wire behind it. The pair belongs to installations where the load sits a long way from the source and the conductor was sized for ampacity rather than for voltage drop.

Situations that produce it:

• Distribution centers and warehouses where a homerun crosses several hundred feet of building
• Detached shops, barns and outbuildings fed from a house or main panel
• Agricultural pump houses and grain handling equipment at the end of a farm yard run
• Parking structures, site lighting and gate operators at the far corner of a property
• Rooftop equipment fed from a main distribution panel several floors below
• Car wash bays, self-storage and modular buildings on extended feeders
• Temporary power and rental equipment on long feeder and cord runs

The tell is that the voltage is acceptable at the service and low at the equipment. Measure at the main and it reads 206 or 207. Measure at the machine while it is working and it reads 198. That difference is the conductor, and no amount of discussion with the utility will change it.

Why this 198V to 208V voltage pair matters

198 volts is 4.8 percent below a 208 volt nominal, which is almost exactly the total voltage drop budget the NEC's informational notes describe. NEC 210.19(A) suggests holding branch-circuit drop near 3 percent and combined feeder plus branch-circuit drop near 5 percent for reasonable efficiency of operation. Those notes are advisory rather than enforceable, but they are the target the trade designs to, and a 198 volt reading means the budget is already spent.

What follows is predictable. Any conductor running past that point takes another few percent, so a motor at the end of the branch circuit sees the low 190s, roughly 8 percent under nominal. It draws more current for the same work, heats its windings, and trips its overload on a warm afternoon. Contactor coils sit close to dropout. Drives fault on undervoltage during other equipment's starting dips. And because the loss is resistive, it deepens exactly when the load is heaviest, which is when the equipment can least afford it.

Sizing guidance

Measure under load, at the load. A 198 volt reading taken at the panel at 6 a.m. is a different number by mid-shift, and the number that matters is the one at the equipment terminals while the equipment is working. Take it line to line with the load running, and take it more than once.

Then decide what you are correcting. If the drop occurs in the feeder, a transformer at the subpanel fixes everything downstream of it. If the drop occurs in the last 200 feet of branch circuit, correcting back at the panel does nothing for the far end and the unit belongs near the load.

Select amperage from load current, not breaker size. Total the full-load amps of everything running at once through the transformer and choose the next rating up from 10, 15, 20, 30, 40, 50 or 60 amps. Phase follows the load. Because the transformer handles only the 5 percent difference between input and output, a small unit covers a large load.

Installation notes

Be clear about what a buck-boost does not fix. Correcting voltage at the load end does not repair an undersized or overloaded conductor. The feeder still delivers the same power, still carries essentially the same current, and still runs at the same temperature. If the conductor is at its ampacity limit, or the drop traces to a loose termination, fix that first. A transformer installed on top of a real fault only hides it.

Location matters more here than on most corrections. Mount the unit downstream of the drop, close to the equipment it serves, in an enclosure suited to the environment with the required working clearances.

The unit is an autotransformer, so the output is not isolated from the input and no neutral is derived. Three-phase corrections normally use two transformers in open delta on a three-wire supply. Size conductors and overcurrent protection for full load current, and follow NEC Article 450 along with 210.9 and 215.11 for autotransformer installations.