A subpanel is sized from the loads it will supply—not the sum of its branch-circuit breaker ratings. For a typical U.S. 120/240 V single-phase installation, calculate the feeder load first, then coordinate the feeder breaker, conductor ampacity, and panel rating. A higher-rated panel can provide more circuit spaces, but it does not increase the capacity of the feeder supplying it.
Subpanel Sizing Inputs and Selection Checks
| Decision item | Input or reference value | Conditions that matter | What the result determines |
|---|---|---|---|
| Electrical system | Voltage in V; single-phase or three-phase | This guide’s example uses U.S. 120/240 V single-phase, not 120/208 V or three-phase | Which current formula applies |
| Calculated feeder load | Apparent power in VA; current in A | Include the loads supplied by this subpanel and only demand factors permitted by the adopted code | Starting point for feeder sizing |
| Continuous-load portion | Current in A expected to reach maximum for three hours or more | Separate load calculation from conductor and overcurrent-device sizing | Whether the ordinary 125% sizing treatment applies |
| 120 V load distribution | Calculated load on L1 and L2, in A | Dividing total VA by 240 V can conceal unequal line loading | Whether either ungrounded conductor needs more capacity |
| Feeder breaker | Ampere rating | Must coordinate with the calculated load, conductor protection, and equipment ratings | Feeder overcurrent protection |
| Feeder conductors | AWG or kcmil; copper or aluminum | Wiring method, terminal temperature ratings, ambient temperature, and conductor grouping matter | Allowable ampacity |
| Subpanel | Bus rating in A; circuit spaces; permitted breakers | Panel rating and circuit count are separate specifications | Equipment capacity and room for circuits |
| Feeder route | One-way length; indoor, outdoor, or underground conditions | Voltage drop and environmental suitability need separate checks | Whether a larger conductor or different wiring method is appropriate |
The feeder conductor checks are separate: verify the continuous/noncontinuous-load requirement using the applicable terminal temperature limits, and verify ampacity after required adjustment or correction factors. Use the conductor that satisfies both checks; do not simply apply every multiplier to the same load total.
Calculate the Subpanel Load Before Selecting Equipment
A subpanel distributes power already available from the upstream electrical system. Adding one creates circuit spaces and a convenient distribution point; it does not create additional service capacity.
Start with a load schedule for the circuits the subpanel will actually supply. Record equipment voltage, nameplate current or apparent power, expected operating duration, and any special load category. EV charging, motors, welders, heating equipment, and general receptacles do not necessarily use the same calculation rules.
Branch-circuit breaker ratings are not a load schedule. A 20 A breaker describes circuit protection, not a constant 20 A demand. Conversely, an appliance’s average energy use does not establish its maximum feeder load.
NEC Article 220 and the 2026 Article 120 Update
The original Article 220 framework remains relevant when that edition is adopted locally. In the 2026 NEC, NFPA relocated Branch-Circuit, Feeder, and Service Load Calculations from Article 220 to Article 120. This is an edition-specific change, not permission to replace every older section number mechanically.
Two 2026 changes particularly affect residential calculations:
- The dwelling-unit general lighting and receptacle unit load used for feeder/service calculations decreased from 3 VA/ft² to 2 VA/ft².
- Section 120.5(E) removes the 125% continuous-load multiplier from the load-calculation stage. That does not eliminate the separate continuous-load sizing checks for feeder conductors and overcurrent protection.
NFPA also explains that the 2026 dwelling branch-circuit calculation retains a 3 VA/ft² basis. Do not use the lower feeder/service unit load to justify fewer required branch circuits.
Confirm the NEC edition and amendments enforced by the local authority having jurisdiction, or AHJ. An article updated in 2026 does not establish that the 2026 NEC governs a particular permit.
Standard and Optional Calculation Methods
The standard method evaluates load categories and applies their permitted demand treatment separately. A dwelling optional method groups specified loads differently and is available only when its eligibility conditions are met. A garage or workshop subpanel is not automatically eligible for a whole-dwelling optional calculation simply because it is connected to a house.
Use the method applicable to the occupancy and feeder scope. When reviewing an existing installation, distinguish a code-compliant demand calculation from a list of anticipated simultaneous uses; they are not interchangeable.
Follow a Load-to-Equipment Sizing Sequence
- Identify the system and feeder scope. Record voltage, phase, occupancy, and whether the panel is in the same building or a separate structure.
- Establish the calculated load. Use the adopted load-calculation rules, equipment data, and applicable special-load provisions. Preserve the inputs so another person can reproduce the calculation.
- Separate continuous and noncontinuous loads. For the ordinary feeder-sizing case, calculate noncontinuous current plus 125% of continuous current. Listed 100%-rated assemblies have specific exceptions; do not assume a residential panel qualifies.
- Check each line conductor. On a 120/240 V system, assign the 120 V loads to L1 or L2 and add the contribution from 240 V loads to both lines.
- Select a candidate breaker and conductors. Verify conductor protection, terminal ratings, and both ampacity checks before accepting the combination.
- Select the panel configuration. Confirm bus rating, circuit spaces, compatible breakers, neutral and ground provisions, and the required disconnect arrangement.
- Review the feeder route and upstream capacity. Check voltage drop, installation conditions, and the additional demand imposed on the existing service.
This sequence prevents a common reversal: choosing a panel or breaker first, then trying to make the load calculation fit it.
Compare Subpanel and Feeder Ratings
Panel bus rating, feeder breaker rating, and conductor ampacity describe different limits. A panel with a larger bus rating can be supplied by a smaller protected feeder when the equipment listing and installation requirements permit it.
The following are hypothetical equipment combinations, not recommended sizes for every garage or workshop.
| Feeder breaker | Panel bus rating | Relationship | Practical implication |
|---|---|---|---|
| 60 A | 100 A | Panel rating exceeds feeder protection | The feeder remains a 60 A supply |
| 60 A | 125 A | Additional panel rating does not increase feeder capacity | More circuit spaces may be useful without a larger feeder |
| 100 A | 100 A | Panel rating equals feeder protection | Conductors and calculated load still require independent verification |
| 100 A | 125 A | Panel has a higher bus rating | A later feeder increase requires a new load and conductor review |
Do not add a subpanel’s circuit-breaker ratings to determine its feeder size. Likewise, spare spaces or a higher bus rating do not prove that the upstream service can accept additional demand.
Main-Lug-Only and Main-Breaker Panels
A main-lug-only panel has no main overcurrent device inside the panel. A main-breaker panel includes a breaker that can provide a single disconnect for its branch circuits. Eaton describes both configurations and their different incoming-connection arrangements.
| Configuration | Main distinction | Selection consideration |
|---|---|---|
| Main-lug-only | No main breaker within the panel | Verify upstream protection and the required disconnect arrangement |
| Main-breaker | Includes a panel main breaker | Useful for local isolation; does not increase upstream feeder capacity |
A separate building introduces building-disconnect requirements. The panel configuration must satisfy the adopted rules for disconnect rating, location, and any applicable emergency-disconnect provisions—not merely provide a convenient switch. The 2026 NEC revises these requirements in Section 225.31.
Feeder Conductor Reference Values
A wire-size reference is useful only when its temperature column and installation assumptions are visible.
The following values come from Cerrowire’s published ampacity chart. The manufacturer explicitly identifies its chart as based on the 2017 NEC and excludes required temperature corrections and ampacity adjustments. These are reference values, not a table newly issued for 2026 or a substitute for the locally adopted NEC.
| Conductor size | Copper at 60°C | Copper at 75°C | Aluminum at 75°C |
|---|---|---|---|
| 8 AWG | 40 A | 50 A | 40 A |
| 6 AWG | 55 A | 65 A | 50 A |
| 4 AWG | 70 A | 85 A | 65 A |
| 3 AWG | 85 A | 100 A | 75 A |
| 2 AWG | 95 A | 115 A | 90 A |
All values above are from the manufacturer’s chart. Verify the applicable ampacity table, wiring method, and equipment instructions for the actual installation.
The temperature column can change the selection substantially. For example, 6 AWG copper appears as 55 A at 60°C and 65 A at 75°C. A conductor’s 90°C insulation marking does not by itself authorize sizing the completed feeder from the 90°C column; equipment termination limits still apply.
Do not automatically use the dwelling service-and-feeder allowance in Section 310.12 for a garage subpanel. Its qualifying feeder provision concerns conductors supplying the entire load associated with an individual dwelling unit, rather than an ordinary feeder supplying only selected garage circuits.
Worked Example: A Garage Subpanel With EV Charging
This hypothetical example demonstrates feeder-sizing arithmetic. Its load allowances are stipulated inputs, not official garage minimums or a complete permit-ready calculation.
Assume:
- A U.S. 120/240 V single-phase supply.
- A 240 V EV charging load of 32 A, treated as continuous.
- A previously established 480 VA lighting allowance and 3,360 VA receptacle allowance, both noncontinuous.
- Equal distribution of the 120 V loads between L1 and L2.
- No additional motors, welders, heating loads, or permitted demand reductions.
- Ordinary continuous-load sizing, rather than a listed 100%-rated assembly.
- Verified 75°C terminations, with no required ampacity adjustment or correction.
| Load | Voltage | Assumed apparent power | Duration treatment | Contribution to each line |
|---|---|---|---|---|
| EV charging | 240 V | 7,680 VA | Continuous | 32 A |
| Lighting, equally divided | 120 V | 480 VA total | Noncontinuous | 2 A |
| Receptacle allowance, equally divided | 120 V | 3,360 VA total | Noncontinuous | 14 A |
1. Calculate Unweighted Load Current
For this balanced single-phase example:
[
I=\frac{S}{V}
]
where (I) is current in amperes, (S) is apparent power in volt-amperes, and (V) is line-to-line voltage.
[
S_{\text{total}}=7{,}680+480+3{,}360=11{,}520\text{ VA}
]
[
I_{\text{load}}=\frac{11{,}520}{240}=48\text{ A}
]
Each line carries 32 A from the EV load plus 16 A from its assigned 120 V loads.
2. Apply the Ordinary Feeder-Sizing Treatment
The ordinary continuous/noncontinuous sizing relationship is:
[
I_{\text{sizing}}=I_{\text{noncontinuous}}+1.25I_{\text{continuous}}
]
[
I_{\text{sizing}}=16+(1.25\times32)=56\text{ A}
]
This is the conductor/overcurrent-sizing check, not a second demand factor applied indiscriminately to the entire load calculation.
3. Review a Candidate Equipment Combination
A 60 A feeder breaker is a candidate above the 56 A sizing requirement. The reference chart lists 6 AWG copper and 4 AWG aluminum at 65 A in the 75°C column, making them candidates under the stated assumptions. Final acceptance requires the adopted code’s protection rules, conductor conditions, and equipment listing checks.
A 100 A or 125 A panel could provide adequate bus rating and useful circuit space while remaining supplied through the 60 A feeder.
The result does not establish upstream service capacity, voltage-drop performance, grounding compliance, or permit approval. Those remain separate checks.
4. Check an Unequal 120 V Load Distribution
The total-VA shortcut depends on the stated balance assumption.
If the same 3,840 VA of 120 V load were assigned entirely to L1, its contribution would be:
[
I_{\text{120 V, L1}}=\frac{3{,}840}{120}=32\text{ A}
]
L1’s sizing requirement would then be:
[
I_{\text{sizing, L1}}=32+(1.25\times32)=72\text{ A}
]
That distribution would fail the example’s 60 A candidate even though total connected VA had not changed. Rebalancing appropriate circuits or redesigning the feeder would require a new review.
The Panel and Service Load Calculator can serve as a calculation cross-check. Compare its displayed voltage, load categories, demand factors, and continuous-load treatment with the assumptions above before accepting a matching result; a whole-dwelling calculation is not automatically equivalent to this garage-feeder example.
Check Grounding, Neutral Separation, and Feeder Conditions
For a conventional downstream subpanel, the neutral remains isolated from the enclosure and equipment grounding system. Eaton identifies an isolated, insulated neutral bar and a dedicated ground bar as required subpanel features. A local main breaker does not turn the panel into service equipment.
A separate building supplied by a feeder also requires review of its grounding electrode system and building disconnect. Grounding electrodes do not replace the feeder’s required equipment-grounding path. The neutral must not be bonded to the remote building’s grounded metal parts in the ordinary arrangement.
A wire-size chart is not a blanket approval for a feeder. Its ampacity depends on the temperature basis and installation conditions; a detached-building grounding electrode is also not a substitute for the feeder equipment-grounding path.
For a long feeder, evaluate voltage drop using actual one-way length, conductor material, expected current, and allowable performance at the loads. Increasing conductor size for voltage drop does not, by itself, authorize a larger breaker or additional service demand.
Verify the Design Against the Governing Documents
Use NFPA’s free code-access portal to select and read the relevant NFPA 70 edition. NFPA provides edition selection and read-only access; local adoption and amendments must still be confirmed with the AHJ.
For the actual panel and feeder equipment, consult the model-specific manufacturer instructions. Verify permitted breakers, conductor material and size range, termination ratings, enclosure suitability, and the specified neutral/ground arrangement.
Subpanel Sizing Review Checklist
- The local NEC edition and amendments have been identified.
- The load schedule covers only the intended feeder scope and includes all applicable loads.
- Any demand reduction has a documented, applicable code basis.
- Continuous loads are identified without counting the sizing multiplier twice.
- L1 and L2 have been checked separately.
- Feeder conductor ampacity satisfies both the terminal-based sizing check and applicable adjustment/correction checks.
- Feeder protection and panel bus rating are coordinated.
- The equipment listing permits the selected conductors and breakers.
- Neutral and equipment-grounding arrangements are correct for a downstream panel.
- Separate-building disconnect and grounding requirements have been reviewed where applicable.
- Voltage drop and upstream service capacity have been evaluated.
- Required permits, inspection, and professional verification are included in the project scope.
These calculations support equipment selection and a reviewable design record. They do not replace the adopted NEC, AHJ approval, manufacturer instructions, or qualified on-site judgment.