Two 1600 A MCC feeders crowd adjacent underground duct banks, creating three separate design questions: cable temperature, the fault-current return path, and local isolation at the second building. Treat them independently. Spacing that satisfies a conduit layout does not automatically establish ampacity, and a grounding electrode at the MCC building does not replace an equipment bonding conductor.
Current and thermal limits
The number that matters is the conductor operating temperature under the expected load and installation conditions. A 1600 A MCC bus rating identifies the equipment capability; it does not by itself establish feeder load, cable ampacity, or the required number of parallel cables. Obtain the calculated demand, protective-device rating, system voltage, conductor material, termination temperature rating, and permitted insulation temperature before accepting the layout.
The proposed installation has two six-conduit banks placed side by side, with five conduits in each bank containing 3c750 mcm Teck90 (XLPE). Resolve from the one-line diagram whether each group of five cables serves one MCC and whether every parallel cable carries the same phase set. Cable sharing depends on equal conductor length, impedance, terminations, and routing.
| Quantity | Why it matters | Where to obtain it |
|---|---|---|
| Actual feeder load | Sets conductor current and heat generation | Load calculation or recorded demand |
| 1600 A rating | Defines an equipment or protective-device limit, depending on the one-line | MCC and switchgear nameplates; one-line diagram |
| System voltage | Required for power and voltage-drop calculations | Transformer and switchgear nameplates |
| Soil thermal resistivity | Controls heat flow away from the duct bank | Project geotechnical or thermal test data |
| Burial geometry | Determines mutual heating between loaded conduits and banks | Dimensioned civil/electrical section |
| Cable temperature limit | Defines the maximum allowable operating temperature | Cable data and termination ratings |
If 1600 A represents three-phase line current, apparent power is kVA = sqrt(3) × V_LL × I_line / 1000. The system voltage is not stated, so calculate kVA only after reading V_LL from the design documents. This calculation identifies load magnitude; it does not prove underground ampacity.
Spacing-rule and thermal-study approaches
No universal bank-to-bank separation can guarantee that further derating is unnecessary. Heat spreads through concrete and soil, so interaction changes continuously with separation, burial depth, loading, soil properties, and surface conditions. A prescriptive code arrangement and an engineering ampacity calculation answer related but different questions.
| Approach | Use | Strength | Limitation |
|---|---|---|---|
| Applicable code ampacity and arrangement tables | Establish the minimum code basis for the adopted jurisdiction and voltage class | Directly reviewable by the authority having jurisdiction | May not represent two adjacent banks or project soil conditions |
| Neher-McGrath thermal calculation | Model the complete geometry and heat-transfer path | Accounts for mutual heating and installation-specific inputs | Accuracy depends on correct cable-loss and thermal data |
| Greater physical separation | Reduce thermal coupling before final calculation | Provides more heat-dissipation area | No single distance proves independence for every installation |
The NEC treatment described for circuits below 1000 V differs from its treatment above 1000 V, but absence of a particular low-voltage duct-bank derating rule is not a thermal finding. Table (B-310-2) addresses listed underground arrangements; confirm its applicability, status, and edition with the authority having jurisdiction. For a Canadian installation, obtain the corresponding CSA interpretation rather than transferring an NEC conclusion into the design.
Recommended thermal design basis
Use the governing code method as the compliance floor and run a project-specific thermal calculation for the two adjacent banks. The banks contain ten occupied conduits around two 1600 A loads, so mutual heating is a design input rather than a detail to dismiss. AmpCalc is one identified tool that applies the Neher-McGrath method and can model conductors from separate circuits within the same thermal geometry.
A useful model includes every occupied and spare conduit, bank dimensions, center-to-center conduit spacing, separation between banks, burial depth, concrete envelope, native and backfill thermal properties, ambient earth temperature, conductor losses, sheath or armour losses, load factor, and the thermal boundary at grade. Enter the actual cable construction instead of representing a three-conductor armoured cable as three unrelated conductors.
- Freeze the civil cross-section and identify the operating circuit in every conduit.
- Confirm the normal and contingency loading assigned to each parallel cable.
- Obtain cable electrical-loss data and permissible conductor temperature from the cable manufacturer.
- Measure or specify the soil and backfill thermal properties used by the model.
- Calculate the hottest conductor temperature for simultaneous operation of both MCC feeders.
- Repeat the calculation for credible unequal-loading and circuit-outage cases.
- Adjust separation, cable count, conductor size, or backfill design until the calculated ampacity meets the design load and termination limits.
- Submit the assumptions, geometry, input data, and results with the code basis for review.
Three-conductor cable bonding physics
Balanced phase currents contained within one three-conductor cable produce magnetic fields that largely cancel outside the cable. That makes a 3c750 mcm armoured cable fundamentally different from three separately armoured single-conductor cables, where each sheath can encircle an uncompensated phase field and develop significant induced voltage or circulating current.
For the proposed three-conductor cable, connect the internal equipment grounding or bonding conductor at both the source switchgear and the MCC. Use the normal grounding connectors to bond the metallic sheath or armour at both ends when those connectors form part of the termination. If the cable-end construction does not require such connectors, the armour treatment may differ, but the internal grounding conductor still connects at both ends.
Single-point sheath bonding belongs to a deliberately engineered single-conductor cable system. It is not a reason to leave one end of the grounding conductor in a three-conductor feeder disconnected. Applying the single-conductor sheath practice to the internal equipment grounding conductor removes a defined low-impedance fault path and can leave fault clearing dependent on unintended metal paths.
Ground grid and parallel-path requirements
The MCC building ground grid controls local earth-reference and lightning or surge behavior; it is not an effective substitute for a metallic equipment fault-current path back to the source. Earth impedance is generally too high and variable to provide predictable operation of a feeder overcurrent device. Bond the MCC enclosure, building grounding electrode system, cable grounding conductors, and permitted armour paths into one coordinated grounding and bonding system.
The Delta-Delta transformer connection does not, by itself, define the system grounding method or eliminate equipment bonding. Read the transformer secondary grounding and bonding arrangement from the one-line and field connections. A statement that single-conductor cables need no separate ground wire is valid only when another approved path—such as a qualifying metallic sheath, raceway, or specifically designed bonding conductor—performs that function.
Parallel feeders need special attention because each raceway or cable must provide an acceptable fault-current path. The supplied grounding conductor in an armoured cable may not match the sizing expected for the complete parallel feeder. Verify the internal conductor size, armour qualification, feeder protective-device basis, and applicable parallel-path rule with the cable manufacturer and local authority before cable purchase or pulling.
Separate-building disconnect decision
A 400 ft separation makes remote isolation operationally weak even when the upstream breaker protects the feeder. Under the NEC path identified by Article 225.31, a separate building generally requires a building disconnect, subject to applicable exceptions. The Canadian requirement must be established from the adopted CSA edition and the local inspection authority.
A required disconnect is not automatically a fusible disconnect. The device can be selected only after resolving whether it must provide isolation alone or also supply overcurrent protection. Locate the building disconnect so personnel can de-energize the MCC building without a 400 ft trip to the substation. Provide a lockable isolation method and a clear source identification at the MCC room.
Article 250.32 is the identified NEC grounding and bonding reference for a supplied separate building. Check it together with the feeder and building-disconnect rules rather than treating the ground grid as permission to omit the cable grounding conductors.
Ground-fault protection and coordination
Upstream ground-fault protection does not automatically answer whether a downstream fusible disconnect requires another ground-fault function. Separate the questions: code-required protection at each equipment location, conductor and equipment protection, and selective coordination between devices.
| Question | Required decision input | Verification |
|---|---|---|
| Is downstream ground-fault protection mandated? | System grounding, voltage class, device role, ratings, and adopted code | Code review accepted by the authority having jurisdiction |
| Will the upstream device clear an MCC fault? | Available ground-fault current and complete return-path impedance | Short-circuit and ground-fault study |
| Will upstream and downstream devices coordinate? | Fuse curves, breaker trip settings, and calculated fault levels | Time-current coordination plots |
| Does the local device provide only isolation? | Approved one-line and equipment listing | Nameplate and drawing inspection |
If a downstream protective device is added, plot it against the upstream switchgear protection. Duplicate ground-fault elements without coordination can trip the substation device first, expanding an MCC fault into a wider outage.
Installation and commissioning verification
- Compare the installed bank dimensions, conduit occupancy, cable type, and burial details with the thermal-study drawing.
- Record conductor and grounding-conductor terminations at both ends of every
3c750 mcmcable. - Verify bonding continuity from each MCC enclosure through the intended metallic path to the source equipment.
- Confirm that cable armour and connectors match the approved bonding design.
- Measure phase current in each parallel cable under load. Investigate unequal sharing before accepting thermal performance.
- Check termination temperatures and accessible duct-bank surface trends during representative loading. Compare observations with the study assumptions rather than using one temperature reading as an ampacity test.
- Function-test the building disconnect and verify its lockable isolation arrangement.
- Test protective-device operation and confirm that installed settings match the coordination study.
- Place the accepted thermal study, grounding details, and protective settings in the project turnover package.
Frequently asked questions
Why does duct-bank spacing affect cable ampacity?
Adjacent loaded banks share the same soil heat path. Smaller separation raises the surrounding temperature and can reduce the current that keeps 3c750 mcm Teck90 (XLPE) below its permitted conductor and termination temperatures.
Why does a code table not settle the two-bank heating question?
A table represents defined arrangements and assumptions. Two side-by-side six-conduit banks, soil thermal resistivity, burial depth, and simultaneous loading require either a directly applicable listed configuration or a Neher-McGrath calculation.
Why does the three-conductor cable ground connect at both ends?
The equipment grounding conductor supplies the low-impedance fault-current path back to the source. Balanced currents within a three-conductor cable largely cancel external magnetic fields, so single-conductor sheath-bonding practice does not justify opening this conductor.
Why does the MCC building need a local disconnect?
The source switchgear is 400 ft away, and the NEC path identified by Article 225.31 generally calls for a separate-building disconnect unless an applicable exception is accepted. The adopted CSA rules and local authority decide the Canadian installation.
When should engineering stop and escalate to official support?
Stop cable procurement or pulling when the authority has not accepted the ampacity method, the cable grounding conductor or armour qualification is unresolved, or the disconnect requirement remains disputed. Escalate with the one-line, dimensioned duct-bank section, thermal inputs, cable data, and protection study to the cable manufacturer, equipment manufacturer, and authority having jurisdiction.