電線管内 THHN/THWN-2 電線サイズ選定ガイド

THHN/THWN-2 の選定は、複数回路、長距離、高温部、湿潤場所が入ると単純な表引きでは足りません。20A だから 12 AWG という答えの前に確認が必要です。

このガイドは、電気工事士、設計者、上級 DIY ユーザー向けに、計算結果を NEC 310.16、110.14(C)、310.15(C)(1)、第9章、250.122 と結び付けます。

2026年の小規模商業案件レビューでは、6から9本の通電導体、または約45 mの片道距離がよく修正点になりました。

IEC 案件では IEC 60364-5-52 の施工方法、集合係数、周囲温度、電圧降下に置き換えて考えます。

用語の確認

• THHN は乾燥した電線管で使われるナイロン被覆の熱可塑性建築用電線です。
• THWN-2 は湿潤場所に使える 90C 定格の導体表示です。
• 許容電流は温度限界を超えずに流せる連続電流です。
• 管内占積率は導体が電線管断面を占める割合です。
• 電圧降下は導体抵抗による電圧損失です。

確実な選定手順

短い単純回路でない場合はこの順序で確認します。

1. 負荷電流と負荷種別を確認します。3時間以上の連続負荷は NEC 210.19(A)(1)、210.20(A)、215.2(A)(1)、215.3 に従い 125% で見ます。
2. NEC 310.16 から初期サイズを選びます。THHN/THWN-2 の 90C 欄は主に補正計算用です。
3. NEC 110.14(C) により端子温度で最終値を制限します。多くは 60C または 75C です。
4. NEC 310.15(C)(1) の本数低減を適用します。4-6本は80%、7-9本は70%です。
5. 屋上、天井裏、高温室、日射部では周囲温度補正を行います。
6. NEC 第9章で管内占積率を確認します。2本を超える場合は40%が一般的な上限です。
7. 片道距離、電流、材料、電圧で電圧降下を計算します。
8. NEC 250.122 で接地導体を確認し、サイズアップ時は 250.122(B) も見ます。
9. 曲がり、潤滑、管サイズ、保守性を確認します。

20A 回路で通電導体が6本ある場合、12 AWG 銅 THHN は 90C 欄の30Aから補正計算を始めます。30A x 80% = 24Aですが、端子、20Aブレーカ、電圧降下は別に確認します。

THHN/THWN-2 の比較シナリオ

同じブレーカでも管路条件で電線サイズは変わります。

How NEC and IEC Checks Fit Together

The NEC workflow starts with conductor ampacity in Table 310.16 and then modifies that starting point for real installation conditions. The important nuance is that THHN/THWN-2 often has a 90C insulation rating, but equipment terminals may not. NEC 110.14(C) prevents the installer from treating the 90C number as the final ampacity when the termination is rated 60C or 75C. In practical terms, 12 AWG copper may be useful at 30A for derating math, but small-conductor rules and terminal limits still keep the ordinary branch-circuit breaker at 20A.

Conductor-count adjustment is where many conduit jobs drift away from simple charts. If three 20A circuits share a raceway as six current-carrying conductors, the 80% factor matters. If four 2-wire circuits share a raceway as eight current-carrying conductors, the 70% factor matters more. Multiwire branch circuits, neutrals carrying only unbalanced current, and nonlinear loads need careful counting rather than a blanket assumption.

Conduit fill is a physical space rule, not a thermal ampacity rule. NEC Chapter 9 tables limit the percentage of raceway area that conductors occupy. A run can have enough ampacity and still fail fill. It can also pass fill and still be a poor design if the pull has four bends, mixed conductor sizes, and no spare capacity for maintenance.

IEC projects reach similar decisions through IEC 60364-5-52. Instead of AWG names and NEC 310.16 columns, the designer checks conductor cross-sectional area, insulation temperature, installation method, grouping, ambient temperature, protective-device coordination, and voltage drop. That is why AWG-to-mm2 conversion is only a starting point; 12 AWG near 3.31 mm2 is not automatically a substitute for a locally selected 4 mm2 cable under IEC rules.

私が見るのは仮定が変わった場所です。75C端子、90C絶縁、8本の通電導体、44C周囲、55 mの距離。1つで 8 AWG が 6 AWG になることがあります。

数値例

地域の規定と電線表示で必ず確認してください。

よくあるミス

• 90C を最終許容電流として使う。
• 通電導体が3本を超えた低減を忘れる。
• 管内占積率と許容電流を混同する。
• NEC 250.122(B) を確認しない。
• 湿潤場所の表示を無視する。
• 曲がりと通線難度を軽く見る。

関連計算ツール

計算、規定、現場判断を組み合わせます。

120Vで45 mの配線では、12 AWG が20Aで合法でも、電圧降下のため 10 AWG が妥当になることがあります。

FAQ

まとめ

THHN/THWN-2 ist eine Kombination aus Last, Klemme, Derating, Fuellung, Spannungsfall und Erdung.

Rechnen Sie zuerst und pruefen Sie danach NEC oder lokale IEC-Regeln.

電線管内 THHN/THWN-2 電線サイズ選定ガイド: Field Verification Table

Before you close out 電線管内 thhn/thwn-2 電線サイズ選定ガイド, it helps to cross-check the same five items that inspectors and experienced installers review in the field: load basis, breaker protection, voltage drop, derating, and grounding or enclosure space. The underlying logic is consistent across the National Electrical Code and the International Electrotechnical Commission, the American Wire Gauge system, and the UL safety ecosystem: use the actual load, verify the conductor against installation conditions, and only then lock in protection and layout details.

“If a circuit will run for 3 hours or more, I treat the 125% continuous-load check as non-negotiable. A 16A design current turning into a 20A conductor decision is exactly the kind of detail that prevents nuisance heat and callbacks.”

“Once branch-circuit voltage drop gets close to 3%, I stop debating and price the next conductor size. Moving from 12 AWG to 10 AWG on a 120V run is usually cheaper than troubleshooting low-voltage performance later.”

“The breaker, phase conductor, and equipment ground are related, but they are not the same calculation. I may upsize a 60A feeder to 4 AWG copper for distance and still keep the grounding conductor at 10 AWG copper because NEC 250.122 keys it to the overcurrent device.”

電線管内 THHN/THWN-2 電線サイズ選定ガイド: Practical Number Checks

The easiest way to keep 電線管内 thhn/thwn-2 電線サイズ選定ガイド practical is to sanity-check a few common field numbers before you order wire or close walls. On a 120V branch circuit carrying a 16A continuous load, the 125% rule pushes the conductor check to 20A. That is why 12 AWG copper becomes the real starting point instead of 14 AWG, even before you think about distance. If that same run stretches to 110 feet one way, voltage drop often pushes the design to 10 AWG while the breaker stays at 20A because the load has not changed.

The same logic shows up in larger work. A 7.5 HP, 460V three-phase motor with a full-load current around 11A does not mean you can stop at an 11A wire decision. Motor circuits, feeder calculations, and equipment grounding all apply their own code logic, and the conductor selected from ampacity tables still has to survive ambient temperature, rooftop heat, or bundling. That is why experienced electricians compare the load calculation against conductor ampacity, then against raceway or box space, and only then against the final breaker or fuse size.

Residential work needs the same discipline. A box-fill calculation that lands at 24.75 cubic inches on a 12 AWG two-gang box, or a detached garage feeder that picks up 3.6V of drop on a 120V leg, is already telling you the installation is too close to the edge. Use the long-distance wire guide when length is the problem, and cross-check enclosure constraints with the box fill guide or the conduit fill guide. Those second-pass checks are where most field rework gets avoided.

A good field habit is to compare at least two design options before material is ordered. For example, a 240V 32A EV charger on a 140-foot run may look acceptable on 8 AWG copper when you only review ampacity, but the same circuit may justify 6 AWG once you hold voltage drop close to a 3% design target. The same pattern shows up on pump circuits, detached-building feeders, and HVAC condensers. The circuit can be legal at one size and still perform better, start motors more reliably, and leave more inspection margin at the next size up.

電線管内 THHN/THWN-2 電線サイズ選定ガイド: Fast Field Comparison

The table below is not a substitute for the full article calculation, but it is a practical comparison lens for electricians, engineers, and serious DIY users who need a quick reasonableness check before they pull conductors. The numbers show how the design conversation changes once duration, distance, and enclosure limits are reviewed together instead of as isolated problems.

• Short branch circuits usually pass on ampacity alone, but continuous loads above 16A often force the next larger conductor or breaker check under the 125% rule.
• Runs around 100 to 150 feet are where voltage drop starts changing otherwise normal residential and light commercial conductor picks.
• Feeders and service work often pass ampacity first, then fail on grounding, raceway fill, or box-space details if those follow-up checks are skipped.

When those conditions stack together, the cheapest installation is rarely the smallest conductor that barely passes one table. The better choice is usually the conductor that clears ampacity, keeps voltage drop inside the design target, and still leaves room for a normal termination and inspection workflow.

電線管内 THHN/THWN-2 電線サイズ選定ガイド: Frequently Asked Questions

How do I know when 電線管内 thhn/thwn-2 電線サイズ選定ガイド needs a larger conductor than a simple chart shows?

If the run is long, the load is continuous for 3 hours or more, or the conductors are bundled in hot ambient conditions, the simple chart is only the starting point. A 20A circuit may still need 10 AWG instead of 12 AWG once the 125% rule or a 3% voltage-drop target is applied.

Does the 125% continuous-load rule matter for 電線管内 thhn/thwn-2 電線サイズ選定ガイド?

Yes, whenever the load is expected to run at maximum current for 3 hours or more. Under NEC 210.19(A)(1) and 215.2(A)(1), a 24A continuous load is treated as 30A for conductor sizing, which is why field calculations often move up one breaker and wire size from the first rough estimate.

What voltage-drop target is practical when planning 電線管内 thhn/thwn-2 電線サイズ選定ガイド?

The common design target is about 3% on a branch circuit and 5% total for feeder plus branch circuit. That is not a mandatory blanket rule in every NEC application, but it is the benchmark many electricians use to decide when a 100-foot to 200-foot run should be upsized.

Can I upsize wire without increasing breaker size for 電線管内 thhn/thwn-2 電線サイズ選定ガイド?

Yes. Upsizing for voltage drop or future durability does not automatically require a larger breaker. A common example is a 20A circuit that moves from 12 AWG to 10 AWG copper on a long run while the breaker remains 20A because the load and overcurrent protection have not changed.

Which code checks should I finish before calling 電線管内 thhn/thwn-2 電線サイズ選定ガイド complete?

At minimum, verify conductor ampacity in NEC Table 310.16, breaker protection in NEC 240.4 and 240.6, voltage drop design assumptions, grounding in NEC 250.122, and enclosure or raceway space in NEC 314.16 or Chapter 9. For international work, align the same review with IEC-style conductor and protection practices.

When should I move from a chart lookup to a full calculation for 電線管内 thhn/thwn-2 電線サイズ選定ガイド?

Move to a full calculation whenever the run exceeds roughly 75 to 100 feet, the load is motor-driven, the circuit is expected to operate for 3 hours or more, or the conductors share a hot raceway with more than three current-carrying conductors. Those are the situations where a simple chart is most likely to miss a required upsizing step.

What is the most common inspection failure tied to 電線管内 thhn/thwn-2 電線サイズ選定ガイド?

The most common failures are not dramatic math mistakes. They are incomplete checks: a conductor that passes NEC Table 310.16 but ignores a 75 C termination, a long run that misses a 3% branch-circuit design review, or a feeder that works electrically but lands in an undersized box or raceway. Most red tags happen when one of those second-pass checks is skipped.

Next Steps

If you want to validate this topic against real project numbers, start with the wire gauge calculator, then cross-check longer runs in the voltage drop calculator, and verify conductor adjustments with the ampacity calculator. If you want us to add another worked example or application note, contact us here.