Separately derived systems

Delgado Fabrication's expansion needed 120 volt receptacles and lighting in a new machine bay, but the shop's service is 480Y/277 volts. The design called for a 75 kVA dry-type transformer, 480 volt delta primary to 208Y/120 volt wye secondary, feeding a new panelboard. Ray Ochoa handed the grounding to Dana Whitfield as a training exercise, and she discovered what every master candidate eventually learns: a transformer is, for grounding purposes, a new service in the middle of the building. Its secondary has no connection to the utility's grounded conductor, so it needs its own system bonding jumper, its own grounding electrode conductor and its own connection to the building's electrodes. This lesson covers those rules, which are among the most heavily tested in Article 250.

The rules are in 250.30, with sizing from Tables 250.66 and 250.102(C)(1) and bonding of piping and steel in 250.104(D). We explain them in our own words with illustrative values stated as this course's examples. Confirm every requirement and value in your own adopted edition.

What makes a system separately derived

Article 100 defines a separately derived system as an electrical source, other than a service, having no direct connection from its circuit conductors to circuit conductors of other systems, other than through grounding and bonding connections. The test is the circuit conductors, including the grounded conductor. A transformer with separate primary and secondary windings is separately derived, because its secondary circuit conductors are connected to the primary only magnetically. An autotransformer, whose windings share a common conductor, is not. A generator is separately derived only if its grounded conductor is not solidly connected to the normal supply's grounded conductor, which in practice depends on the transfer switch.

Generators cause more confusion than transformers on exams. If the transfer switch switches all the ungrounded conductors but leaves the neutral solidly connected between the generator and the normal service, a three-pole switch on a 208Y/120 volt system for example, the generator's neutral is directly connected to the service neutral. The generator is then not separately derived, and its neutral must not be bonded to its frame at the generator; the system is grounded only at the service, and the generator frame is grounded by the equipment grounding conductor run with the generator conductors. If the transfer switch also switches the neutral, a four-pole switch, the generator is separately derived and needs its own system bonding jumper and grounding electrode connection under 250.30. An informational note in 250.30 reminds you that the manufacturer's bonding arrangement must match the system design, because many portable and standby generators ship with the neutral bonded to the frame.

The system bonding jumper

For a grounded separately derived system, 250.30(A)(1) requires an unspliced system bonding jumper that connects the equipment grounding conductors of the derived system to the grounded conductor. It does exactly what the main bonding jumper does at a service: it completes the effective ground-fault current path back to the source, here the transformer secondary. The connection is made at any single point on the separately derived system from the source to the first system disconnecting means or overcurrent device. In practice it is made either at the transformer, in the transformer enclosure, or at the first disconnecting means, in the panelboard or disconnect fed by the transformer.

The phrase single point is the heart of the rule. If the jumper were installed at both the transformer and the first disconnect, the grounded conductor and the equipment grounding path between them would be connected at both ends, forming parallel paths and objectionable current, just as a second bond in a subpanel does. Section 250.30(A)(1) has a narrow exception allowing a bond at both locations where doing so does not establish a parallel path for the grounded conductor, for example where the derived system's grounded conductor and the bonding path are arranged so no metal raceway or other conductive path connects the two enclosures. For exam purposes, remember the rule as one bond, at the source or at the first disconnect, not both.

The system bonding jumper is sized from Table 250.102(C)(1) by the largest ungrounded conductor of the derived system, with the 12.5% rule above the last row, as 250.30(A)(1) and 250.102(C) direct. It must be copper, aluminum or another corrosion-resistant material, and where it is installed at the first disconnecting means, the grounded conductor between the source and the disconnect must be sized at least as large as the jumper, under 250.30(A)(3).

The supply-side bonding jumper and the grounded conductor

When the transformer and its first disconnecting means are in separate enclosures, the metal enclosures need to be bonded together on the supply side of the secondary overcurrent device. Section 250.30(A)(2) requires a supply-side bonding jumper, installed with the derived circuit conductors, sized from Table 250.102(C)(1) by the derived ungrounded conductors. The jumper may be a wire run in the raceway with the secondary conductors, or it may be the metal raceway itself where it qualifies. It is not required to be larger than the derived ungrounded conductors.

Which conductor plays which role depends on where the system bonding jumper is. If the jumper is at the transformer, the neutral and the supply-side bonding jumper run separately to the panelboard, and the panelboard's neutral bar is isolated from its enclosure. If the jumper is at the panelboard, the neutral from the transformer carries the fault current back to the transformer and must be sized at least as large as the table requires under 250.30(A)(3), and the transformer enclosure is bonded through the supply-side bonding jumper.

The grounding electrode and its conductor

Section 250.30(A)(4) requires the derived system to be connected to the building's grounding electrode system. Indoor transformers use the grounding electrode system of the building or structure they are in. Where the building has accessible structural steel or a metal water pipe near the transformer, the conductor runs to it at the nearest practical point, subject to the limits in 250.68(C): metal water piping only within 5 feet of its entry to the building, except in qualified-maintenance commercial and industrial buildings where exposed piping may be used, and structural steel that is itself connected to the electrode system. Outdoor separately derived systems have their own rule in 250.30(C): they get a grounding electrode at the source location, meeting 250.50.

The grounding electrode conductor of a single separately derived system is sized from Table 250.66 by the largest derived ungrounded conductor, under 250.30(A)(5). It connects to the grounded conductor at the same point where the system bonding jumper is connected. That requirement keeps the bonding point, the electrode connection and the fault return path all at one place, which avoids parallel paths.

Consider what the transformer's primary side needs. The primary feeder is a normal feeder, protected by an overcurrent device at the 480 volt panel. Its equipment grounding conductor is sized from Table 250.122 by that primary overcurrent device, not from the secondary conductors. For Delgado's transformer, a primary current of 75,000 divided by (480 × 1.732), or 90.2 amperes, protected at 125 amperes, would need the equipment grounding conductor from the row that covers 125 amperes, which this course's example gives as 6 AWG copper.

Several transformers sharing one conductor

Large buildings often have many transformers, and running a full-size grounding electrode conductor from each one back to the electrodes would be wasteful. Section 250.30(A)(6) allows a common grounding electrode conductor with taps. The common conductor must be at least 3/0 AWG copper or 250 kcmil aluminum in this course's examples. Each tap from a transformer to the common conductor is sized from Table 250.66 by that transformer's largest derived ungrounded conductor. The taps connect to the common conductor by listed irreversible compression connectors, exothermic welding or a copper busbar of at least the dimensions the section specifies, so the common conductor stays continuous. The common conductor may also be the building's structural steel or interior metal water pipe that meets 250.68(C).

Bonding piping and steel near the derived system

Section 250.104(D) requires metal water piping and exposed structural metal in the area served by a separately derived system to be bonded to the derived system's grounded conductor at the same point the grounding electrode conductor connects. The jumper is sized from Table 250.102(C)(1) by the derived ungrounded conductors. Where the building steel or the water pipe already serves as the grounding electrode for that system, the separate bond is not needed. The goal is the same as at the service: everything metal in the area served by the new system rises and falls together.

Ungrounded separately derived systems

Section 250.30(B) covers derived systems that are permitted to operate ungrounded, such as a 480 volt delta secondary feeding only three-phase motors in a plant with qualified maintenance. There is no system bonding jumper, because no conductor is grounded. The equipment is still bonded and grounded, and a grounding electrode conductor connects the metal enclosures of the derived system to the grounding electrode, sized from Table 250.66 by the derived conductors. Lesson 10 covers the ground detectors such systems need.

A calculator you can edit

The listing below finds the secondary and primary currents of a transformer and sizes the bonding jumper, grounding electrode conductor and primary equipment grounding conductor from this lesson's illustrative rows. Change the kVA and conductor sizes to practice.

import math

def three_phase_amps(kva, volts):
    return kva * 1000 / (math.sqrt(3) * volts)

GEC_ROWS = [(133.1, "6 AWG"), (167.8, "4 AWG"), (350, "2 AWG"), (600, "1/0 AWG")]   # illustrative
EGC_ROWS = [(60, "10 AWG"), (100, "8 AWG"), (200, "6 AWG"), (300, "4 AWG")]        # illustrative

def row(rows, value):
    return next(size for limit, size in rows if value <= limit)

kva = 75
print(f"Secondary current: {three_phase_amps(kva, 208):.1f} A at 208 V")
print(f"Primary current: {three_phase_amps(kva, 480):.1f} A at 480 V")
secondary_kcmil = 250
print("System bonding jumper and GEC:", row(GEC_ROWS, secondary_kcmil), "copper")
print("Primary feeder EGC (125 A device):", row(EGC_ROWS, 125), "copper")
for k in (167.8, 250, 500):
    print(f"Tap for {k} kcmil secondary: {row(GEC_ROWS, k)}")

Secondary current: 208.2 A at 208 V
Primary current: 90.2 A at 480 V
System bonding jumper and GEC: 2 AWG copper
Primary feeder EGC (125 A device): 6 AWG copper
Tap for 167.8 kcmil secondary: 4 AWG
Tap for 250 kcmil secondary: 2 AWG
Tap for 500 kcmil secondary: 1/0 AWG