Supply-Side Bonding Jumper Size Calculator — NEC 250.102(C) (Instant & Accurate)

This article provides precise guidelines for NEC 250.102(C) supply-side bonding jumper sizing calculations and examples.

Instant accurate calculator methodology ensures code compliance, conductor selection, and fault current coordination verification testing.

NEC 250.102(C) scope and engineering basis

Section 250.102(C) of the NEC addresses sizing of the supply-side bonding jumper (SSBJ) between grounded service conductors and grounded enclosure or grounding electrode system. Proper sizing must account for thermal withstand during the available fault condition, conductor material, system configuration, and clearing time of the protective device. Engineering practice uses either NEC-prescribed minimums (where applicable) or calculation by the adiabatic short-circuit heating equation when the conductor must withstand a specific bolted fault without damage.

Key functional requirements

• Carry the maximum fault current likely to flow until the protective device clears.
• Provide a low-impedance path to facilitate operation of overcurrent protection.
• Resist thermal damage and maintain continuity after a fault-clearing event.
• Conform to NEC tables and local amendments where they specify minimum sizes.

Calculation method: adiabatic short-circuit heating equation

For precise SSBJ sizing under known fault and clearing time conditions, use the adiabatic equation:

Where:

• A = required conductor cross-sectional area (mm₂).
• I_sc = peak rms symmetrical fault current in amperes (A) available at the location.
• t = duration of the fault in seconds (s) (clearing time of the protective device).
• k = material constant describing allowable temperature rise and conductor characteristics.

Typical k constants used in industry practice (referenced by IEC 60949 / international practice):

• k = 115 for annealed copper (common reference value for short-circuit heating calculations).
• k = 143 for aluminium (common representative value; some sources use 148).

Notes on variables and typical values:

• I_sc: Obtain from utility short-circuit studies, relay coordination study, or local utility fault current tables.
• t: Use the protective device clearing time (relay, breaker trip curve at the expected fault current). If multiple devices operate, use the fastest clearing device that interrupts the fault current that flows through the SSBJ.
• k: Use the value consistent with the calculation standard referenced by the authority having jurisdiction (AHJ) — document the k used in calculations.

When to use tabulated NEC minima vs calculated approach

NEC provides tabulated minimum sizes for grounding/bonding conductors in several places. Use tabulated minima where they apply; use calculated adiabatic sizing when the required short-circuit withstand cannot be assured by tabulated minima or when an unusual available fault current or clearing time exists.

• Use NEC table-based minimums for routine circuits and service equipment when the table explicitly covers the condition.
• Use adiabatic calculation when available fault current and clearing times yield a required cross-sectional area greater than the table minimum, or when engineering verification is required.

Typical material constants and assumptions (engineering practice)

Use these typical assumptions unless otherwise directed by AHJ or project specifications:

• Copper k = 115 (conservative, widely used in international practice).
• Aluminium k = 143 (representative, check manufacturer or standard if different).
• Initial conductor temperature assumed at normal operating temperature; final allowable temperature determined by insulation rated thermal limit for short-term heating (adiabatic assumption: no heat dissipation to surroundings during t).

Standard conductor sizes and common metric equivalents

Extensive calculation table: required mm² for common fault currents and clearing times

The table below applies the adiabatic equation with copper k = 115 and aluminium k = 143. Values are rounded to two decimal places to show engineering decisions for rounding up to the next standard conductor size.

Step-by-step practical methodology for SSBJ sizing

1. Obtain available fault current at the point where the SSBJ connects (I_sc) from the utility or a grid short-circuit study.
2. Determine the protective device that will clear the fault and establish the expected clearing time t at the calculated fault current (from breaker trip curves or relay coordination study).
3. Select conductor material (copper or aluminium) and choose the corresponding k constant. Document the k chosen.
4. Apply the adiabatic equation: A = (I_sc × sqrt(t)) / k.
5. Round up the computed area A to the nearest standard conductor size. Convert mm² to AWG or kcmil if necessary and select the next larger standard size.
6. Verify that the selected conductor meets mechanical and installation requirements and NEC table minima (if the NEC table requires a larger size, follow the NEC table requirement).
7. Document calculations, assumptions, source for I_sc and t, the k constant used, and the selected standard conductor size for approval by the AHJ.

Two real-world examples with complete solutions

Example 1 — Distribution service SSBJ (copper)

Project data: utility short-circuit study reports 10,000 A available at the service disconnect. The upstream breaker that will clear a ground fault is estimated to clear in t = 0.5 s (worst-case based on coordination curves). The design material is annealed copper. Use k = 115.

Step 1 — Apply adiabatic equation:

Compute numeric values:

• I_sc = 10,000 A
• t = 0.5 s → sqrt(t) = 0.70710678
• k = 115 (copper)

Calculation:

A = (10,000 × 0.70710678) / 115 = 7,071.0678 / 115 = 61.48 mm²

Selection: 61.48 mm² rounded up to the next standard conductor size → 70 mm² copper is selected. In AWG equivalents this approximates between 2/0 and 3/0; choose the standard 70 mm² or 2/0 if required by project standards.

Verification: Check NEC minimums and mechanical termination compatibility. Document I_sc source and breaker curve for t = 0.5 s. If the NEC table requires a larger conductor for other reasons (mechanical strength, corrosion, insulation), follow the NEC requirement.

Example 2 — Generator SSBJ (aluminium)

Project data: standby generator neutral bond requires a supply-side bonding jumper to ground electrode conductor sized for available fault current of 25,000 A at generator terminals until the breaker upstream clears in 0.2 s. Design requires aluminium due to weight and cost constraints. Use k = 143.

Step 1 — Apply adiabatic equation:

• I_sc = 25,000 A
• t = 0.2 s → sqrt(t) = 0.4472136
• k = 143 (aluminium)

Calculation:

A = (25,000 × 0.4472136) / 143 = 11,180.34 / 143 = 78.17 mm²

Selection: Round up to the next standard aluminium conductor size → 95 mm² aluminium selected. Verify lug and termination compatibility and check for oxidation and anti-oxidation practices for aluminium terminations.

Verification: Confirm protective device clearing characteristics; if the breaker clearing time is actually faster for this fault magnitude, recompute with actual t. If the AHJ requires copper, recompute using copper k and select copper size.

Practical considerations, derating and installation issues

• Temperature and insulation: The adiabatic equation assumes no heat loss during t. Ensure the conductor insulation and terminal ratings withstand the adiabatic temperature reached.
• Parallel conductors: If using parallel parallel SSBJs, follow the rules for paralleling conductors (equal length, same material, equal termination) and verify current sharing; document the approach and AHJ approval.
• Mechanical protection: SSBJs often are exposed; provide mechanical protection and support to prevent damage from physical or thermal stress.
• Terminations: Use appropriately rated lugs; aluminium terminations require anti-oxidation compounds and correct torque values.
• Coordination with other protective devices: Ensure the clearing time t used corresponds to the device that will actually clear the fault current flowing through the SSBJ.

Testing, commissioning and documentation

1. Document the short-circuit study input and results, protective device curves used, and the computed A with k constant specified.
2. Include SSBJ sizes on as-built single line diagrams and equipment grounding plans.
3. During commissioning, confirm protective device settings and trip times; if settings change, recompute SSBJ sizing if needed.
4. Include periodic inspection of SSBJ terminations and corrosion control (especially for aluminium conductors).

Regulatory references and authority links

• NFPA 70, National Electrical Code — official source for Section 250.102(C) and grounding/bonding requirements: https://www.nfpa.org/NEC
• IEC 60949, Calculation of thermally permissible short-circuit currents — reference for adiabatic equation and k constants: https://www.iec.ch
• IEEE and industry standards for short-circuit calculations and grounding practices: https://standards.ieee.org
• OSHA electrical safety and grounding: https://www.osha.gov/electrical
• Manufacturer installation guides for conductors, lugs and terminations (refer to conductor manufacturer datasheets and equipment terminal ratings)

Checklist for using an instant SSBJ calculator

• Input available fault current (I_sc) measured or from study.
• Input the clearing time (t) for the relevant protective device at the computed fault magnitude.
• Select conductor material (copper or aluminium) and the correct k constant.
• Compute A = (I_sc × sqrt(t)) / k and round up to nearest standard conductor size.
• Verify with NEC tables and AHJ rules; apply mechanical and termination checks.
• Document the source data, assumptions, and final selected conductor with supporting calculations.

Advanced topics and edge cases

Multiple fault-clearing devices

When more than one device can clear the fault, the SSBJ sees the fault until the fastest device clears. Use the actual clearing time at the prospective current level for the device that interrupts the majority of the fault current through that jumper. Coordination studies should document which device clears a particular fault scenario.

Series impedance and voltage drop effects

Although the SSBJ is sized primarily for thermal withstand, its resistance contributes to the loop impedance and affects fault current magnitude. In high-impedance grounded systems or long SSBJs, compute the available fault at the equipment point including impedance of conductors to ensure I_sc used in sizing is accurate.

Parallel SSBJ conductors

If the required area exceeds manufacturing sizes or available terminations, paralleling multiple identical conductors may be acceptable. Ensure equal-length runs, identical conductor type and insulation, and symmetrical terminations. Document current-sharing assumptions and AHJ approval.

Summary of best practices

• Always source I_sc from a reliable short-circuit study or utility data; do not guess.
• Use the clearing time of the device that actually clears the fault current through the SSBJ.
• Document k constant and the justification for its selection in the project report.
• Round up computed area to the next standard conductor size and verify NEC table minima.
• Coordinate with mechanical and termination requirements; test and verify during commissioning.

References and normative documents

• NFPA 70 (NEC) — Grounding and bonding requirements including Section 250.102: https://www.nfpa.org/NEC
• IEC 60949 — Calculation of thermal short-circuit current: https://www.iec.ch
• IEEE standards on power system analysis and short-circuit calculation — see IEEE Std resources: https://standards.ieee.org
• OSHA — Electrical standards and grounding guidance: https://www.osha.gov/electrical
• Manufacturer technical data sheets for conductor ratings and termination recommendations (consult specific vendor sites)

For project deliverables, provide the SSBJ sizing worksheet with source documentation (fault current report and protective device curves), the calculation trace using the adiabatic equation, chosen k constant, the standard conductor selected, and verification against NEC table minima as required by the AHJ.