What is the maximum allowable voltage drop for landscape lighting?

While NEC 210.19(A)(1) Informational Note No. 4 suggests a 3% maximum drop for branch circuits, for 12V landscape lighting, a practical maximum of 8.3% (1V drop) to 12.5% (1.5V drop) is recommended to prevent LED driver brownout and flickering.

Does NEC 411 require landscape lighting wire to be buried?

Yes, under NEC 411 and NEC 300.5, low-voltage landscape wiring must be buried at least 6 inches deep for cables rated 30V or less, protecting it from physical damage unless otherwise permitted for specific wiring methods.

What does IEC 60364-7-714 say about outdoor lighting?

IEC 60364-7-714 specifies requirements for fixed outdoor lighting installations, including protection against electric shock and ensuring that voltage drop limits do not impair the proper functioning of the equipment, aligning with IEC 60364-5-52 wiring standards.

Can I use 14 AWG wire for a 200-foot landscape lighting run?

No, a 200-foot run using 14 AWG wire carrying just 5A will result in a voltage drop of 5.06V, leaving only 6.94V at the fixture. You must use at least 8 AWG wire to keep the drop under 1.5V for that distance and current.

How does a 120V primary voltage drop affect the 12V secondary?

A 5% drop on the 120V primary (down to 114V) will proportionally reduce the 12V secondary to 11.4V, assuming a standard transformer ratio. This leaves only 0.9V of acceptable drop on the 12V side before fixtures fail.

Do LED fixtures experience the same voltage drop as halogen?

LED drivers maintain constant current, but if voltage drops below the driver's minimum threshold (often around 10.5V or 10.8V), the driver will shut down or flicker. Halogen bulbs simply dim linearly, but LEDs experience abrupt failure below their rated voltage.

ランドスケープ照明の電圧降下：12V LEDおよび120Vトランスの選定

Landscape lighting is one of the easiest places to see voltage-drop mistakes. On a 12V system, losing 0.5V to 1.0V is not a rounding error; it changes brightness, color consistency, and driver behavior immediately.

The practical workflow is straightforward. Total the fixture load, convert watts to current for each transformer tap, measure the actual one-way route to the farthest luminaire, and compare a daisy-chain layout against center-fed or split runs before burying cable. For US work, the useful checkpoints are NEC 411, NEC 110.3(B), NEC 300.5, and the familiar 3% / 5% design guidance in the informational notes to NEC 210.19(A)(1) and 215.2(A)(1). For IEC-style work, IEC 60364-7-714 and IEC 60364-5-52 are the practical references. NEC IEC

Why Landscape Lighting Needs a Dedicated Voltage-Drop Check

A 12V system has very little voltage margin. Losing 1V means about 8.3% of the source voltage is already gone.

Fixture spacing matters. A long daisy-chain can leave the first fixtures bright and the last fixtures visibly dim or warm in color.

Transformer location often matters as much as conductor size. Moving the transformer closer to the load can outperform one more wire-size jump.

Burial depth, wet-location splices, and cable type still matter for safety and durability, but none of them correct an undersized lighting run.

Code and Standards Points Worth Marking on the Plan

NEC 411: low-voltage lighting systems need listed low-voltage power units, luminaires, and the correct field installation approach.
NEC 110.3(B): follow the transformer and fixture instructions, especially for secondary taps, maximum load, and connection method.
NEC 300.5: underground outdoor wiring still needs the correct burial depth, raceway protection, and cable routing method.
IEC 60364-7-714 with IEC 60364-5-52: check requirements for outdoor lighting installations, cable sizing, grouping, and design voltage-drop limits before finalizing the cable route.

Worked Lighting Examples with Specific Numbers

12V LED path-light run, 8A, 80 ft one-way

With 12 AWG copper at about 1.98 ohm per 1000 ft, the round-trip drop is roughly 2.53V, or about 21.1% at 12V. Even 10 AWG still drops about 1.59V, or 13.2%. That usually points to a split run, a center-feed layout, or a transformer moved closer to the middle of the load.

12V accent-light circuit, 4A, 60 ft one-way

With 10 AWG copper, the drop is about 0.60V, or 5.0%. Moving to 8 AWG lowers it to about 0.37V, or 3.1%. That is a far healthier target when the owner expects even beam intensity across multiple fixtures.

120V primary feed to a landscape-lighting transformer, 5A, 220 ft one-way

A 12 AWG copper primary loses about 4.36V, or 3.6% at 120V. A 10 AWG primary drops about 2.73V, or 2.3%. If the transformer primary starts weak, the entire 12V secondary system begins with less headroom.

Field Checklist Before the Cable Is Buried

Add the actual fixture wattage on each run, then convert that load to current at the transformer tap you plan to use.
Measure the one-way route to the farthest fixture, not just the straight trench distance on the sketch.
Compare daisy-chain, hub, and center-fed layouts before upsizing blindly; layout changes often save more voltage than one conductor step.
Verify transformer tap selection, direct-burial or raceway method, splice rating, and wet-location hardware before closing the trench.
When appearance matters, keep 12V lighting runs close to the 3% to 5% range. Around 10% drop is usually visible in the field.

Run the Lighting Numbers Before You Bury the Cable

Enter fixture current, source voltage, conductor size, material, and one-way distance so you can compare cable size against transformer placement before the landscaping is finished.

計算機を開く関連する電線サイズガイド

“12Vランドスケープシステムを扱う場合、1.5Vを超える電圧降下は灯具が10.5V未満の電圧しか受け取れないことを意味し、これによりLEDドライバのブラウンアウトが引き起こされ、ルーメン出力が30%以上低下することがよくあります。”
— Hommer Zhao, Technical Director

“12Vシステムでは、トランスを160フィートの配線の端から中央に移動させることで、銅線に1ドルも費やす前に最悪の電圧降下をほぼ半分に減らすことができます。”
— Hommer Zhao, Technical Director

“120V一次側が114Vで到達した場合、公称12Vのトランスは二次側を11.4V付近で開始する可能性があり、低電圧側には現場損失の余地がほとんど残されていません。”
— Hommer Zhao, Technical Director

比較表：12V二次側配線の決定

レイアウト	負荷	電線	片道距離	概算降下	現場結果
Daisy chain from one end	96W at 12V	12 AWG copper	80 ft	≈ 2.0V	Last fixtures visibly dim and often below stable LED-driver voltage
Daisy chain from one end	96W at 12V	10 AWG copper	80 ft	≈ 1.3V	Usually workable, but brightness still drifts from first fixture to last
Center feed	96W at 12V	10 AWG copper	40 ft per leg	≈ 0.6V	Much tighter brightness spread without jumping to extreme copper cost
Hub layout	60W at 12V	8 AWG trunk + short leads	120 ft trunk	≈ 0.75V	Strong field result with more even output across fixtures
120V transformer primary	300W load	14 AWG copper	150 ft	≈ 1.9V	Acceptable primary drop and healthier secondary starting voltage

よくある質問

12Vランドスケープ照明で許容される電圧降下はどれくらいですか？

NEC 411.4および一般的な業界慣行に基づき、12Vランドスケープ照明システムの最大許容電圧降下は通常1.5Vであり、灯具の電圧を10.5V以上に保つ必要があります。

NEC 300.5はランドスケープ照明ケーブルの浅い埋設を許可していますか？

NEC 300.5は低圧ランドスケープ照明ケーブルの浅い埋設を許可していますが、NEC 300.5(C)(3)に基づき、供給電圧が30V以下の場合、埋設深は600mm（24インチ）未満でよいとされています。

120Vトランス給電は12V側とは別々に計算すべきですか？

はい、120V一次側給電は別々に計算する必要があります。NEC 210.19(A)の情報では120V分岐回路の電圧降下を3%以下にすることが求められており、12V側の許容割合とは異なるためです。

デイジーチェーン配線は長い12V LED配線に適していますか？

お勧めしません。デイジーチェーン配線は累積的な電圧降下を引き起こします。長い12V LED配線では、電圧降下を1.5V以内に抑えるためにスターまたはT型配線を採用することをお勧めします。

いつ12 AWGから10 AWGまたは8 AWGに切り替えるべきですか？

12 AWG電線が長距離配線で1.5Vを超える電圧降下を引き起こす場合、10 AWGまたは8 AWGにアップグレードする必要があります。例えば、負荷が100Wで距離が50フィートを超える場合などです。

電圧が低下した場合、LED灯具はハロゲンランプと異なる反応を示しますか？

はい、ハロゲンランプは電圧低下時に単に暗くなるだけですが、LED灯具は電圧が10.5Vを下回るとドライバのブラウンアウトにより完全に消灯したり点滅したりする可能性があります。

掘削前の計算を

穴を掘る前に、当社の計算機を使用してランドスケープ照明の電圧降下を正確に計算し、システムの安定稼働を確保してください。

電圧降下計算機 120V / 12V 3% / 5% 電線サイズ選定ガイド