SERケーブル選定ガイド

SERケーブルは住宅のサービス機器、盤交換、集合住宅フィーダー、屋内サブパネルでよく使われます。間違いの多くは、住宅用の簡略導体ルール、一般の許容電流表、接地導体ルールを混同することから始まります。

正しい選定では、負荷計算、条文、許容電流、中性線、接地導体、電圧降下を分けて確認します。熱的に足りるケーブルでも、サブパネルの中性線と接地の扱い、150フィートの距離で不合格になることがあります。

2026年に18件の住宅図面を見直した際、100Aに2-2-2-4アルミSERと書きながらNEC 310.12の根拠がない例が複数あり、135〜165フィートのガレージフィーダー4件は電圧降下計算が必要でした。

SER選定前の重要用語

• SERケーブルはNEC 338のType SE、Style Rのサービス入口ケーブルで、共通外装内に複数の絶縁導体を持ちます。
• 住宅フィーダーは住戸全体の負荷、または認められた住宅負荷を供給するフィーダーで、NEC 310.12の可否を決めます。
• 許容電流は温度、束ね、端子制限を考慮した導体の使用可能電流です。
• 機器接地導体は中性線ではなく、NEC 250.122により過電流保護装置から選びます。
• 電圧降下は導体抵抗による損失で、長距離ではサイズアップの理由になります。

実務的なSER選定手順

Antes de elegir 2-2-2-4, 1-1-1-3 o 4/0 de aluminio, sigue esta secuencia.

1. NEC 220で負荷を計算します。
2. NEC 310.12が使えるか確認し、不可ならNEC 310.16を使います。
3. 材料、アルミ端子、締付トルクを確認します。
4. NEC 110.14(C)で端子温度を確認します。
5. 中性線は実際の負荷で決めます。
6. 接地導体はNEC 250.122で決めます。
7. NEC 338の支持、保護、湿気、地下制限を確認します。
8. 片道長さ、電流、電圧、材料で電圧降下を計算します。

SERでは計算書にコード経路を書きます。NEC 310.12の100AとNEC 310.16の100Aは、別のアルミ導体になることがあります。

SERケーブル比較表

Estos ejemplos son referencias de planeación; la respuesta final depende del NEC adoptado, terminales, material, carga e inspección local.

NEC 338、310.12、310.16の関係

NEC 220で負荷を計算します。

NEC 310.12が使えるか確認し、不可ならNEC 310.16を使います。

材料、アルミ端子、締付トルクを確認します。

高いSERミスは、許容電流は合うのに施工が間違うことです。例はサブパネルで中性線と接地を結ぶケースです。

数値で見るSER例

Estos ejemplos muestran cómo cambian la respuesta la carga, la norma, la distancia y la puesta a tierra.

SERでよくある間違い

• NEC 310.12が使えるか確認し、不可ならNEC 310.16を使います。
• 材料、アルミ端子、締付トルクを確認します。
• NEC 110.14(C)で端子温度を確認します。
• 中性線は実際の負荷で決めます。
• 接地導体はNEC 250.122で決めます。
• NEC 338の支持、保護、湿気、地下制限を確認します。

関連する計算機とガイド

Usa estas páginas para verificar lo que una tabla de SER no resuelve por sí sola.

SERフィーダーは負荷、許容電流経路、中性線と接地、電圧降下が一致して初めて完了です。

SER選定FAQ

結論

SER選定はコード経路の判断です。負荷、NEC 310.12または310.16、NEC 338、端子、中性線、接地、電圧降下を確認します。

計算機で許容電流と電圧降下を比較し、採用NECと所轄機関で最終確認します。

SERケーブル選定ガイド: Field Verification Table

Before you close out serケーブル選定ガイド, 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.”

SERケーブル選定ガイド: Practical Number Checks

The easiest way to keep serケーブル選定ガイド 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.

SERケーブル選定ガイド: 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.

SERケーブル選定ガイド: Frequently Asked Questions

How do I know when serケーブル選定ガイド 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 serケーブル選定ガイド?

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 serケーブル選定ガイド?

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 serケーブル選定ガイド?

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 serケーブル選定ガイド 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 serケーブル選定ガイド?

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 serケーブル選定ガイド?

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.