GND-5 — Feeders, Separate Buildings, Separately Derived Systems, and Special Bonding

Exam checkpoints

Keep feeder neutrals isolated

The service bonding connection joins the grounded service conductor to the service enclosure, equipment grounding paths, and grounding electrode system. On the load side, a feeder carries an equipment grounding conductor or qualifying equipment grounding path in addition to any grounded neutral needed by the load. The downstream neutral bar is insulated from the enclosure; the equipment grounding bar is bonded to it.

If neutral and ground are reconnected downstream, normal neutral current divides between the neutral and conductive metal paths. This is not improved grounding. It is a parallel-current problem that can energize raceways, piping, building steel, and communications bonding conductors. Diagnose by drawing the complete path to the source bonding jumper, not by asking whether a panel merely has a ground rod.

Supply a separate building

Under 250.32(A), a building or structure supplied by a feeder or branch circuit generally has a grounding electrode system installed and connected in accordance with Article 250 Part III. The exception covers a building supplied by only a single branch circuit, including a multiwire branch circuit, that includes an equipment grounding conductor for grounding normally non-current-carrying metal parts.

For the ordinary modern feeder in 250.32(B)(1), an equipment grounding conductor described in 250.118 is run with the supply conductors. It connects to the building disconnecting means and is used to ground and bond equipment, structures, and frames. It is sized under 250.122. Any neutral supplied to the building remains isolated from the EGC and grounding electrodes. The building's grounding electrode conductor is sized and installed under the applicable Part III rules; it performs a different job from the feeder EGC. A rod at the second building cannot clear a line-to-case fault reliably through soil.

The exception permitting a supplied grounded conductor to serve as the fault-return path is a legacy provision for installations made in compliance with previous editions. It requires that no EGC was run with the supply, no continuous metallic path is bonded to the grounding systems at both buildings, and no ground-fault protection of equipment is installed on the supply side. Where used, the grounded conductor must be at least the larger size required by the calculated neutral load or 250.122. Do not design a new three-wire feeder by treating this exception as the normal rule.

Article 225 adds the building disconnect. Sections 225.31 and 225.32 require a disconnecting means that is readily accessible outside or inside nearest the point where the conductors enter. The permitted disconnects are grouped, generally no more than six, and equipment must be suitable for use as service equipment under 225.36 even though the feeder-supplied building disconnect is not the service. That suitability marking does not authorize a neutral-to-enclosure bond.

Bond a separately derived system once

A separately derived system has no direct electrical connection, including a solidly connected grounded conductor, to supply conductors of another system. A typical isolation transformer secondary qualifies. For a grounded ac separately derived system, 250.30(A)(1) locates the unspliced system bonding jumper at a single point from the source through the first system disconnecting means or overcurrent device. It can be installed at the source enclosure or first disconnect under the rule. Except for a narrowly conditioned exception that avoids a parallel grounded-conductor path, do not bond at both. Downstream, the derived neutral is isolated again.

If the source and first disconnect are in separate enclosures, 250.30(A)(2) requires a supply-side bonding jumper with the derived conductors. A wire-type jumper is sized under 250.102(C) from the derived ungrounded conductors. The system bonding jumper is likewise sized by the applicable 250.102(C) rule, while the grounding electrode conductor is sized under 250.66 and remains subject to the electrode-specific provisions. These are three functions, not three names for one conductor.

Example: for 3/0 AWG copper derived ungrounded conductors, Table 250.102(C)(1) gives a 4 AWG copper system or supply-side bonding jumper. Table 250.66 also gives a general 4 AWG copper GEC where its table applies. The matching result is coincidental; change the electrode or conductor routing and the rules can produce different required sizes.

A generator may or may not be separately derived. If transfer equipment leaves the neutral solidly connected to the service neutral, the alternate source is not separately derived; adding a generator neutral-to-frame bond can create a parallel neutral path. If the transfer equipment switches the neutral so no solid connection remains, the generator is generally treated as separately derived and the applicable system bonding and grounding rules are applied. Always trace the actual transfer poles and manufacturer-listed arrangement.

Apply special piping bonds

Section 250.104(A) bonds qualifying metal water piping because it can become energized; this bonding duty is not the same as deciding whether 10 ft of underground pipe qualifies as an electrode. In the service context, the water-pipe bonding jumper is generally sized by Table 250.102(C)(1) from the largest ungrounded service-entrance conductor or equivalent area. At a building supplied by a feeder, 250.104(A)(3) provides the permitted connection points and bases the jumper on the feeder's largest ungrounded conductor.

Other metal piping likely to become energized is bonded under 250.104(B). Its bonding jumper is sized by Table 250.122 using the circuit likely to energize the piping. Metal gas piping is not used as a grounding electrode, but applicable bonding of piping likely to become energized is still required. Thus a service water-pipe bond, a fault-exposed gas appliance branch-circuit bond, a feeder EGC, and a GEC can all have different sizing bases. Correct troubleshooting labels each path, locates the authorized neutral bond, and confirms low-impedance continuity to the source.