Detached Workshop: A 'New Service' That Is Not One
A detached 8 m × 12 m workshop is fed from a dwelling's 200 A service by a 100 A four-wire feeder: two 2 AWG Cu ungrounded conductors, a 2 AWG Cu neutral and an 8 AWG Cu equipment grounding conductor, run 28 m underground in Schedule 80 PVC. At the workshop the 100 A panel sits beside the point where a 25 mm copper water service from a private well enters the slab; the well pump is fed from this panel. The electrician drove two 16 mm × 2.4 m copper-bonded rods 3 m apart, ran 6 AWG Cu to each rod and to the metal water pipe, and installed the green bonding screw in the panel's neutral bar. The plumber bonded the black-iron gas piping to the water pipe with 10 AWG Cu and a listed clamp. The slab contains 20 mm rebar on 300 mm centres, tied with wire, with nothing connected to it. The electrician's position: 'It's a detached building, so this is a new service - the neutral gets bonded here and the water pipe is the main electrode.' The dwelling's service-entrance conductors are 2/0 Cu.
1. Identify each defect and rank them by hazard, explaining the physical mechanism behind each.
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Neutral bonded at the subpanel (green screw installed). This is the most serious defect. Return current from the workshop now divides between the neutral and the equipment grounding conductor, the grounding electrode system, the metal water pipe, the gas piping and the earth. Consequences: the EGC sits at a potential above earth during normal operation, every metal enclosure and appliance case is live to touch, the neutral and EGC share current so the EGC is no longer a clean low-impedance fault path, and loose connections in that parallel path arc - a fire mechanism. The grounded conductor is bonded to the enclosure only at the service point, never at a downstream panel.
Concrete-encased electrode not bonded. The rebar qualifies: 20 mm exceeds the 13 mm minimum, 300 mm spacing is under the 600 mm maximum, and the total length exceeds 6 m. Every qualifying electrode present must be bonded, and this one is the best electrode on the site. It is connected to nothing.
Grounding electrode conductor to the water pipe sized from the wrong reference. 6 AWG is evaluated against the 100 A feeder; the water pipe electrode must be sized from the 2/0 Cu service conductors, which calls for 4 AWG Cu.
Gas piping bonding. The 10 AWG is large enough for a 20 A heater circuit, but the clamp must be listed for the pipe material, the connection accessible, and any CSST manufacturer's larger bonding requirement checked.
No labelling of any electrode connection.
2. State the required size for each bonding and grounding conductor on this job, and say what each size is determined by.
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- Equipment grounding conductor in the feeder: 8 AWG Cu, determined by the 100 A overcurrent device. Correct as installed. 2 AWG Cu feeder conductors are good for well over 100 A, so the feeder ampacity is not the issue.
- GEC to the two ground rods: 6 AWG Cu is adequate; a rod electrode never requires larger than 6 AWG Cu, and 6 AWG exceeds the 8 AWG that Table 250.66 would give for 2 AWG Cu feeder conductors. Correct.
- Rod spacing: 3 m apart exceeds the 1.8 m minimum. Correct.
- GEC to the metal water pipe: 4 AWG Cu, determined by the 2/0 Cu service-entrance conductors at the dwelling, not by the workshop feeder. The 6 AWG installed is undersized. The water pipe connection must be within the first 1.5 m of pipe entry and on the street side of any meter or filter, or the electrode connection must be made elsewhere and the pipe bonded.
- Bonding to the concrete-encased electrode: 4 AWG Cu, determined by the same rule that sizes the GEC for a concrete-encased electrode, which permits a maximum of 4 AWG Cu.
- Bonding to gas piping: sized by the equipment grounding conductor of the circuit likely to energise the piping - 12 AWG Cu for a 20 A heater circuit, so 10 AWG is adequate in size; check the manufacturer's instructions and use a clamp listed for the pipe.
- Well casing, if metallic: it is a grounding electrode too and must be bonded.
3. Draft the correction notice and rebut the electrician's 'detached building = new service' argument.
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Rebuttal, stated plainly: a detached building fed by a feeder is not a service. It has no service point, no service disconnect and no separately derived neutral. The neutral-to-ground bond remains at the dwelling only. Installing a second bond creates a parallel return path - exactly the objectionable current the code prohibits - and the earth and the metal water piping become current-carrying during normal operation. The fact that a grounding electrode system is required at the separate building does not make it a service; the electrode system is there for lightning, potential equalisation and fault current return to the source, not as a normal return path.
Correction notice:
- Remove the green bonding screw; separate neutral and grounding buses; neutral bar isolated from the enclosure. Re-inspect before energising the feeder.
- Replace the 6 AWG GEC to the water pipe with 4 AWG Cu sized on the 2/0 Cu service conductors; verify connection within the first 1.5 m of entry.
- Bond the concrete-encased electrode with 4 AWG Cu, connection to the rebar listed for concrete-encased electrodes and accessible.
- Verify gas piping bonding conductor, clamp listing and accessibility; check manufacturer's instructions.
- Label all electrode connections and the panel with the source of supply and the location of the disconnecting means.
Require a re-inspection with the panel dead, then a live verification of neutral-to-ground separation at the workshop.