Primary distribution voltages, 4.16 kV to 34.5 kV

Primary distribution voltage chart

Nominal system voltages from the ANSI C84.1 lists, as utilities and state rules publish them. A value written with a Y, such as 12,470Y/7,200, is a four wire grounded wye system with a neutral, and the second number is the phase to ground voltage. A value written alone, such as 4,160 or 13,800, is a three wire system, usually delta or a wye without a distributed neutral. For three wire systems the phase to ground figure is computed for reference only.

Phase to ground computed as phase to phase divided by 1.732. Nominal system names round these, for example 7,620 and 7,970. Insulation class is the equipment class whose rating covers the system voltage.

The voltages you will meet most

Of all the possible voltages between 1 kV and 100 kV, the ones most often used in the United States are 4,160, 12,470, 13,200, 13,800, 24,940 and 34,500 volts for four wire systems and 69,000 volts for three wire systems. Others in service include 2,400, 4,800, 6,900, 8,320, 12,000, 20,780, 22,860, 23,000 and 46,000 volts.

4.16 kV, 6.9 kV and 13.8 kV match standard medium voltage motor ratings, which is why industrial plants favor them for their own distribution. 12.47 kV is the most common 15 kV class utility voltage, and 24.94 kV and 34.5 kV are chosen where feeders are long or loads are spread out.

12.47 kV compared with 13.2 kV and 13.8 kV

All three are 15 kV class systems, so the same switchgear, cable and transformer insulation class covers them, and a 15 kV class pad mount is normally built at 95 kV BIL. The practical differences are the phase to ground voltage, 7,200 against 7,620 and 7,970, which sets the rating of single phase transformers and surge arresters connected line to ground, and the tap range a transformer needs.

A transformer wound for 12,470 V is not automatically right on a 13,200 V system. Order the primary voltage your utility states on its service letter, and ask for taps around it, for example two 2.5 percent taps above and two below. For single phase units on a wye system, order the phase to ground voltage, 7,200 V on a 12.47 kV system or 7,620 V on 13.2 kV.

24.94 kV and 34.5 kV systems

Higher primary voltages carry more power the same distance on the same current. A 600 A feeder, a planning limit some utilities use, carries about 14 MVA at 13.8 kV and about 36 MVA at 34.5 kV. That is why 24.94 kV and 34.5 kV are common for rural feeders that run tens of miles and for wind and solar collector systems, where many MW come back to one substation.

Computed as amps x kV x 1.732. Industry planning guides cite about 4 MVA typical for a 4.16 kV feeder, 12 to 15 MVA at 12.47 or 13.8 kV, about 20 MVA at 22 to 24.9 kV and 30 to 37 MVA at 34.5 kV.

Primary current for common transformer sizes

The primary current of a three phase transformer is small next to its secondary current, which is why primary cable and fuses are modest even on large units. Primary amps equal kVA x 1,000 divided by (1.732 x primary volts).

Computed full load primary current. Fuse sizes follow the manufacturer fuse tables, not these figures alone.

BIL by insulation class

BIL, basic impulse level, is the peak lightning impulse voltage the insulation must withstand in the factory test, using the standard 1.2 x 50 microsecond wave. Each insulation class has a standard BIL and higher optional levels, which are worth specifying for overhead exposure, high altitude above 1,000 m (3,300 ft) and switching heavy sites. Liquid filled distribution units carry higher BILs than dry types of the same class, 95 kV for a 15 kV class pad mount and 125 kV as the ANSI standard at the 25 kV class.

Dry type values per IEEE C57.12.01 and CSA C9 as published by Rex Power Magnetics. Confirm liquid filled BIL against the IEEE C57.12.00 table for the unit you buy.

How a primary voltage reads on a nameplate

12470GrdY/7200 means the winding is for a 12,470 V grounded wye system, with 7,200 V from each phase to ground. The Y, or GrdY, tells you the system has a grounded neutral, and the second number is the phase to ground voltage.

To find your system, read the service letter or the utility design, or the nameplate of the existing transformer you are replacing. The primary voltage, whether it is grounded wye, and the BIL are the three things that must match.

Clearances step up with voltage

Medium voltage equipment is bigger and needs more room than low voltage gear. Design guidance for outdoor medium voltage equipment puts the fence at least 10 feet from live parts or the enclosure, and 15 feet for a nominal 13.8 kV system, with the details in NEC 110.31. Plan the pad and fence early.

What is 12.47 kV phase to ground

7,200 volts. 12,470 divided by 1.732 is 7,200, which is why the system is written 12470Y/7200.

What is 13.2 kV phase to ground

7,620 volts, written 13200Y/7620.

What is 34.5 kV phase to ground

19,920 volts, written 34500Y/19920.

How many volts is 12.47 kV

12,470 volts phase to phase. kV means thousands of volts.

Is 12.47 kV medium voltage

Yes. ANSI C84.1 defines medium voltage as nominal system voltages above 1,000 V and below 100 kV, and 12.47 kV is a 15 kV class system inside that range.

What is the difference between 12.47 kV and 13.8 kV

Both are 15 kV class. 12.47 kV is the usual utility four wire grounded wye voltage at 7,200 V to ground. 13.8 kV is common in large plants and campuses because it matches standard motor and generator voltages, and is 7,970 V to ground when it is a grounded wye.

What primary voltage should I order on a transformer

The nominal voltage your utility or engineer gives you, with grounded wye or delta stated, and the BIL for its class. For a single phase unit on a wye system, order the phase to ground voltage.