Grounding at separate buildings

Farms, schools, industrial sites, campgrounds and many homes have more than one building supplied from a single service. A feeder runs from the main service equipment to a detached garage, a workshop, a barn or a second commercial building, and at the far end there is a panel serving that building's circuits. How should that second building be grounded and bonded? Should it have its own electrode? Should its neutral be bonded to its enclosure? What conductors does the feeder need?

These questions cause more confusion in the field than almost any other grounding topic, partly because older codes, and other countries' codes, answered them differently. The CE Code's rule on grounding connections for buildings supplied from a single service gives a clear answer, and it follows directly from the single-point principle you have met throughout this course. This lesson works through it using the Hartley Farms installation, then looks at the special concerns of agricultural buildings.

Requirements are paraphrased. Read the rule on two or more buildings supplied from a single service, and the Appendix B figures that illustrate it, in your own 2024 CE Code Part I.

The bin yard and the shop

Hartley Farms' main service is a 200 A, 120/240 V service at the farmhouse yard, with a pole-mounted meter and service equipment. From it, Parkland Valley has run a 100 A feeder to the grain-drying shop, about 60 m away, and a 60 A feeder to the bin yard, where aeration fans and a moisture monitoring system are housed in a small steel building. Each building has its own panel. Denise asks the group to sketch how they would ground and bond the shop.

Tyler's sketch has three conductors in the feeder, two hots and a neutral, with the neutral bonded to the shop panel's enclosure and a ground rod at the shop. It looks like a miniature service, and it is the arrangement many older farm buildings still have. Marcus's sketch has four conductors: two hots, a neutral and a bonding conductor, with the neutral isolated at the shop panel, the bonding conductor connected to the panel enclosure, and a ground rod at the shop connected to the bonding system. Denise says Marcus has drawn what the current Code requires, and she spends the rest of the evening explaining why.

Why not bond the neutral at the second building?

If the shop's neutral is bonded to its enclosure, and the enclosure is connected to a ground rod and to everything else metallic in the shop, then the shop has a second neutral-to-bonding connection in the same system. The service is the first. Normal neutral current from the shop's loads now has two paths back to the service: along the feeder neutral, and through the shop's electrode, the earth, the service electrode and any metal paths that connect the two buildings, such as water lines, fences, communication cables or a metal conveyor. Some of the neutral current flows on those paths, which were never meant to carry it.

On a farm, that is more than an inefficiency. Neutral current flowing through the earth and through metal structures creates small voltages between metal objects and the ground around them. Livestock, especially dairy cattle, are very sensitive to such voltages through their hooves and mouths, and voltages a person would never notice can change their behaviour, their water intake and their milk production. Agricultural stray voltage has been investigated for decades, and improper neutral bonding at outbuildings is one of its classic causes.

What the Code requires, in outline

The CE Code's approach for a building supplied by a feeder from a single service, paraphrased:

• The feeder includes a bonding conductor, run with the circuit conductors, sized from Table 16 using the feeder's overcurrent device rating.
• At the second building, the grounded conductor (neutral) is isolated from the enclosure and from the bonding system. There is no second system bonding jumper.
• A grounding electrode is installed at the second building and connected to the bonding system there, so that the building's metal is tied to the local earth for surge and lightning purposes.
• Metal piping and other systems in the second building are bonded to its bonding system as in any building.

Read the rule carefully for any conditions and exceptions, such as arrangements for a building supplied by its own separately derived system, or for small structures. The principle, though, is constant: one system, one neutral-to-bonding connection, at the service.

Why there is still an electrode at the second building

If the bonding conductor brings the fault path back to the service, why install an electrode at the shop at all? Because the shop is a long way from the service electrode, and lightning, surges and nearby faults on the utility system raise the local earth potential at the shop differently from at the house. A local electrode, bonded to the shop's metal, keeps the shop's metal close to the potential of the earth around it, which protects people and equipment inside during those events. It is the grounding job from lesson 1, done locally, while the bonding job is done by the conductor back to the service.

It also gives the surge protective devices at the shop a short path to earth. A surge protector with a long lead to a distant electrode is much less effective than one with a short connection to a local electrode, because the inductance of the long lead raises the voltage the protected equipment sees during a fast surge.

Neutral current and voltages on the farm

To see why the neutral bond at the outbuilding matters, put numbers on it. The voltage between the neutral at a building and the local earth, often called the neutral-to-earth voltage, depends on the neutral current and the resistance of the neutral path back to the source.

def split(i_total, *paths):
    g = sum(1 / r for r in paths)
    v = i_total / g
    return v, [v / r for r in paths]

v, (neutral, bond, earth) = split(25, 0.10, 0.16, 40)
print(f'voltage across the paths: {v:.2f} V')
print(f'neutral {neutral:5.1f} A   bonding conductor {bond:5.1f} A   earth {earth:5.2f} A')

voltage across the paths: 1.54 V
neutral  15.4 A   bonding conductor   9.6 A   earth  0.04 A

Change the bonding conductor's resistance and see how much normal current it picks up. In the Code-compliant arrangement, the bonding conductor is not a parallel neutral at all, because the neutral is isolated at the shop, and the calculation does not apply. That is the whole point.

Equipotential planes in livestock buildings

Agricultural buildings that house livestock raise one more issue. Even with perfect neutral practice on the farm's own wiring, some neutral-to-earth voltage can come from the utility's distribution system, because the utility's primary neutral is grounded at many points along the line. To protect animals in areas where they stand and drink, the Code has requirements for equipotential planes in certain livestock areas: a wire mesh or other conductive element in the concrete floor, bonded to the building's bonding system and to the metal equipment the animals touch, so that the floor, the waterers, the stanchions and the feeders are all at the same potential. Read the agricultural bonding requirements in your code book; they are a frequent exam topic for candidates in farm provinces.

The idea is the same one that governs bonding everywhere: if everything an animal or person can touch at once is at the same voltage, no current flows through them, however that voltage relates to remote earth. An equipotential plane makes the floor itself part of the bonded system.

Other separate-building situations

The same principles apply well beyond farms. A detached garage with a sub-panel, a pool house, a school portable, a second building on an industrial site, or a campground's washroom building supplied from the main service all follow the same pattern: bonding conductor in the feeder, neutral isolated at the building, local electrode bonded to the building's metal. Where the second building is supplied by its own separately derived system, such as a transformer in that building, lesson 9's rules apply to the derived system instead.

A few situations have their own rules: buildings supplied by their own service from the utility are not supplied from a single service and are grounded as services; mobile and modular structures may have specific requirements; and some supplementary sections modify the approach. On an exam, read the question carefully to see whether the building is supplied by a feeder from another building's service, by its own service, or by a derived system. The answer depends on it.

Upgrading an old outbuilding

Many existing outbuildings were wired under older codes with three-conductor feeders and the neutral bonded at the outbuilding. When such a building is extended or its panel replaced, the upgrade question arises: can the old feeder stay? The answer depends on the extent of the work, the inspector, any provincial direction and the rules on existing installations. In practice, adding a bonding conductor to the feeder, isolating the neutral at the outbuilding and bonding the local electrode to the enclosure brings it into line with current requirements. Denise's view from her inspection days was simple: on a dairy farm, it is worth doing whether or not anyone requires it.