離れたガレージ分電盤の電線サイズサービスガイド
// 掘削や盤工事の前に、30 A から 100 A のガレージフィーダを許容電流、電圧降下、管路充填、接地、中性線分離、負荷例で確認します。 //
離れたガレージのフィーダは住宅負荷、屋外配管、分電盤のボンディング、長距離の電圧降下、将来の作業場負荷が重なります。工具、照明、空調、開閉装置、溶接機、EVSE の負荷を先に整理し、フィーダ、接地線、管路、接地極を別々に確認します。
Detached building
NEC 225 covers outside feeders and branch circuits to a separate structure, while NEC 250.32 decides grounding and bonding at the garage
4-wire feeder
Modern detached-garage subpanels normally need two hots, an insulated neutral when used, and an equipment grounding conductor with neutral isolated
Voltage drop
A 60 A 120/240 V feeder at 150 ft may pass ampacity with 6 AWG copper but often needs 4 AWG copper or 2 AWG aluminum for about 3% drop
要約
- Choose feeder ampacity from calculated garage loads, not from panel size alone
- Use a 4-wire feeder and isolate neutral from ground in the detached subpanel
- Check voltage drop early because 100-200 ft garage runs often drive conductor upsizing
- Run conduit-fill and pull checks before buying UF cable, THWN-2 conductors, or PVC
- Confirm grounding electrodes and disconnect rules with the local AHJ before trenching
主な定義
Detached-garage subpanel
A detached-garage subpanel is a panelboard in a separate building supplied by a feeder from the main service
Feeder
A feeder is a conductor set that carries power from service equipment or another panel to downstream branch-circuit overcurrent devices
Grounding electrode system
A grounding electrode system is the rod, concrete-encased electrode, metal water pipe, or other electrode connection required to bond the separate structure to earth
サイズ決定手順
Step 1 - load list
Add lighting, receptacles, compressor, mini-split, door opener, freezer, welder, EVSE, and spare circuits. A 5 hp compressor or 40 A welder changes the feeder more than ordinary lights
Step 2 - feeder ampacity
Pick 30 A, 60 A, 90 A, or 100 A from calculated load and diversity. Use NEC Table 310.16 with the correct copper or aluminum column and terminal temperature
Step 3 - voltage drop
Enter one-way distance in the voltage-drop calculator. For a 60 A feeder at 150 ft, compare 6 AWG copper, 4 AWG copper, and 2 AWG aluminum before deciding
Step 4 - grounding and bonding
Under NEC 250.32, keep neutral and grounding bars separate in the garage panel and size the equipment grounding conductor from NEC 250.122
Step 5 - raceway and trench
Check Chapter 9 conduit fill, burial depth, wet-location conductor markings such as THWN-2, expansion fittings, and local inspection requirements before excavation
計算例
60 A hobby shop at 120 ft
Loads include 12 A lighting and receptacles, 18 A compressor, and a 24 A dust collector on a 120/240 V feeder
Ampacity can start around 6 AWG copper THWN-2 at 75C terminals, but voltage drop may justify 4 AWG copper for motor starting margin
100 A workshop at 180 ft
Loads include a 48 A EV charger, 30 A welder receptacle, 16 A mini-split, and general outlets with load diversity documented
Ampacity may point to 3 AWG copper or 1 AWG aluminum at 75C, while voltage drop often pushes the practical choice larger
30 A storage garage at 80 ft
Loads include LED lighting, receptacles, door opener, and a small freezer with no welder or EVSE
10 AWG copper may satisfy ampacity, but use the calculator to see whether 8 AWG copper better protects 120 V freezer starting voltage
フィーダ計画比較
| ガレージ計画 | 標準ブレーカ | 初期導体確認 | 電圧降下リスク | 計画メモ |
|---|---|---|---|---|
| Storage only | 30 A 120/240 V | 10 AWG Cu or 8 AWG Al check | Medium after 100 ft | Good for lights, outlets, opener, freezer |
| Hobby shop | 60 A 120/240 V | 6 AWG Cu or 4 AWG Al check | High after 120 ft | Common point where voltage drop drives upsizing |
| Heavy workshop | 100 A 120/240 V | 3 AWG Cu or 1 AWG Al check | High after 150 ft | Check terminals, conduit fill, and future EVSE |
| Single welder circuit | 50 A branch or feeder allowance | 6 AWG Cu branch check | Medium after 75 ft | Welder duty cycle may affect sizing under NEC 630 |
| IEC outbuilding | 32-63 A distribution circuit | 6-16 mm2 check by method | Project specific | Use IEC 60364 installation method, grouping, and Clause 525 voltage drop |
コード参照
リンクとコード項目は設計確認用であり、許可審査の代わりではありません。地域の補正や電力会社条件で詳細が変わります。
NEC 225
Outside feeders and branch circuits to a separate building, including disconnect and supply rules for detached structures
NEC 250.32 and 250.122
Grounding electrodes, bonding, neutral isolation, and equipment grounding conductor sizing for feeders to separate buildings
NEC 310 and Chapter 9
Ampacity tables, adjustment factors, terminal temperature limits, and raceway fill checks for THWN-2 or cable assemblies
IEC 60364-5-52
IEC projects check current-carrying capacity, installation method, grouping, ambient temperature, and voltage drop under Clause 525
現場チェックリスト
- Document one-way feeder distance and voltage at the source
- List every 120 V and 240 V load before choosing panel amp rating
- Verify four conductors when a neutral is required and keep neutral isolated in the garage panel
- Size the equipment grounding conductor separately from the neutral
- Check conduit fill, pull length, sweeps, expansion fittings, and wet-location conductor marking
- Confirm permits, burial depth, GFCI rules, and local amendments with the AHJ
離れたガレージフィーダ FAQ
Can I install a 100 A panel if my load is only 45 A?
Often yes, but the feeder breaker and conductor ampacity still govern usable load. A larger panel gives spaces, not extra amps without a matching feeder
Does a detached garage need ground rods?
Under NEC 250.32, a detached building supplied by feeders or branch circuits typically needs a grounding electrode system such as rods, plus an equipment grounding conductor back to the source
Can I bond neutral and ground in the garage subpanel?
For modern 4-wire feeders, no. Keep neutral isolated and bond the equipment grounding conductor and grounding electrode conductor to the panel enclosure
What wire size is common for a 60 A garage subpanel?
6 AWG copper is a common ampacity starting point at suitable terminals, but a 120-150 ft run may need 4 AWG copper or larger to meet a 3% voltage-drop target
Should DIYers use this guide for final installation?
DIYers can use the numbers to plan questions and material estimates, but detached-building feeders involve permits, trenching, grounding, and inspection, so final work should follow the AHJ and a licensed electrician where required