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# owuitools prompt test result
# prompt dir : medium/m03-dc-wire-gauge (round v4)
# chat id    : 2400a5f0-f0ad-4c79-86b7-b682fe4be407
# chat title : Copper Wire Gauge for 12V 20A Load
# user       : the operator
# model      : qwen3.8:latest
# turns      : 2 messages on selected branch
# tool calls : 2  wiring_calc x2
# dumped at  : 2026-09-12T04:27:46Z
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=== USER  (2026-09-12T04:26:37Z) ===
I need to run 12 V DC to a 20 A load that's 30 feet from the battery. What copper wire gauge keeps the voltage drop under 3%? Show the actual drop for the gauge you pick and for the next size smaller, so I can see the margin.

=== ASSISTANT  (2026-09-12T04:26:44Z, model qwen3.8:latest) ===
usage: prompt_eval_count=11401 eval_count=972 prompt_tokens=11401 completion_tokens=972 total=27.1s prefill=3.4s decode=23.7s 41.0 tok/s

=== [1] THINKING ===
The user wants to know what gauge of copper wire will keep voltage drop under 3% for a 20A load at 12V over 30 feet.

Let me think about this:
- Voltage: 12V DC
- Current: 20A
- Distance: 30 feet (one way), so total wire length = 60 feet (out and back)
- Max voltage drop: 3% of 12V = 0.36V

I need to find the wire gauge where the voltage drop is under 0.36V.

Let me use the wiring_calc tool to check different gauges. I'll start with common sizes and work through them.

Actually, let me calculate first what resistance we need:
- Max drop = 0.36V
- Current = 20A
- Total wire length = 60 feet
- Max resistance = V/I = 0.36/20 = 0.018 ohms
- Resistance per foot = 0.018/60 = 0.0003 ohms/foot

Copper wire resistances (approximately):
- 12 AWG: about 0.001588 ohms/foot → 60ft × 0.001588 = 0.0953 ohms → drop = 20 × 0.0953 = 1.91V → way too much
- 10 AWG: about 0.001 ohms/foot → 60ft × 0.001 = 0.06 ohms → drop = 20 × 0.06 = 1.2V → still too much
- 8 AWG: about 0.000628 ohms/foot → 60ft × 0.000628 = 0.0377 ohms → drop = 20 × 0.0377 = 0.754V → still over 0.36V
- 6 AWG: about 0.000394 ohms/foot → 60ft × 0.000394 = 0.0236 ohms → drop = 20 × 0.0236 = 0.473V → still over
- 4 AWG: about 0.000248 ohms/foot → 60ft × 0.000248 = 0.0149 ohms → drop = 20 × 0.0149 = 0.298V → under 0.36V!

Wait, let me be more careful with the resistance values. Let me use the tool instead of guessing.

Actually, I should use the wiring_calc tool with voltage_drop calculation. Let me check a few gauges.

Let me check 6 AWG and 8 AWG first to see where we are.

Actually, let me just run the calculations for several gauges using the tool. The tool takes a single gauge at a time, so let me start with a reasonable guess.

For DC voltage drop:
- 30 feet one way = 60 feet total (out and back)
- 20A
- 12V

Let me check 6 AWG first since my rough math suggests it might be close.

=== [2] TOOL CALL: wiring_calc  (call_32n1n8ww) ===
{
  "calculation": "voltage_drop",
  "awg": 6,
  "current": "20",
  "length": "9.144",
  "voltage": "12"
}

=== [3] TOOL CALL: wiring_calc  (call_iffyrazj) ===
{
  "calculation": "voltage_drop",
  "awg": 8,
  "current": "20",
  "length": "9.144",
  "voltage": "12"
}

=== [4] TOOL RESULT: wiring_calc  (call_32n1n8ww) ===
# Voltage drop over 9.144 m of 6 AWG

**Drop: 474 mV**  (round trip, 18.288 m of conductor)

Conductor: 6 AWG = 13.30 mm² (4.12 mm diameter), 1.296 Ω/km
Loop resistance: 23.7 mΩ
Power lost as heat: 9.481 W

That is 3.95% of a 12 V supply — above the usual 3% guideline — consider thicker wire or a shorter run.
Voltage at the load: 11.53 V

## Nearby gauges, same run

-   3 AWG ( 26.67 mm²): 236.4 mV = 1.97%
-   4 AWG ( 21.15 mm²): 298.1 mV = 2.48%
-   5 AWG ( 16.77 mm²): 375.9 mV = 3.13%
-   7 AWG ( 10.55 mm²): 597.8 mV = 4.98%
-   8 AWG (  8.37 mm²): 753.8 mV = 6.28%
-   9 AWG (  6.63 mm²): 950.5 mV = 7.92%

Lower AWG numbers mean thicker wire and less drop.


**For mains or building wiring, follow your local electrical code (NEC 310.16 in the US, BS 7671 in the UK) rather than these figures.** They are free-air rules of thumb; real installations derate for bundling, insulation rating, ambient temperature and conduit fill.

=== [5] TOOL RESULT: wiring_calc  (call_iffyrazj) ===
# Voltage drop over 9.144 m of 8 AWG

**Drop: 753.8 mV**  (round trip, 18.288 m of conductor)

Conductor: 8 AWG = 8.37 mm² (3.26 mm diameter), 2.061 Ω/km
Loop resistance: 37.69 mΩ
Power lost as heat: 15.08 W

That is 6.28% of a 12 V supply — above the usual 3% guideline — consider thicker wire or a shorter run.
Voltage at the load: 11.25 V

## Nearby gauges, same run

-   5 AWG ( 16.77 mm²): 375.9 mV = 3.13%
-   6 AWG ( 13.30 mm²): 474 mV = 3.95%
-   7 AWG ( 10.55 mm²): 597.8 mV = 4.98%
-   9 AWG (  6.63 mm²): 950.5 mV = 7.92%
-  10 AWG (  5.26 mm²): 1.199 V = 9.99%
-  11 AWG (  4.17 mm²): 1.511 V = 12.59%

Lower AWG numbers mean thicker wire and less drop.


**For mains or building wiring, follow your local electrical code (NEC 310.16 in the US, BS 7671 in the UK) rather than these figures.** They are free-air rules of thumb; real installations derate for bundling, insulation rating, ambient temperature and conduit fill.

=== [6] THINKING (8s) ===
Let me analyze the results:

- 30 feet = 9.144 meters one way, so round trip is 18.288 m
- 6 AWG: 474 mV drop = 3.95% — just over 3%
- 5 AWG: 375.9 mV drop = 3.13% — still just over 3%
- 4 AWG: 298.1 mV drop = 2.48% — under 3% ✓

So the smallest gauge that keeps the drop under 3% is **4 AWG**.

The next size smaller (thinner) would be 5 AWG at 375.9 mV = 3.13%.

Wait, let me re-read the question. "Show the actual drop for the gauge you pick and for the next size smaller." The "next size smaller" means the next thinner wire (higher AWG number), which is 5 AWG.

So my answer is:
- **4 AWG**: 298.1 mV drop = 2.48% of 12V → voltage at load = 11.70 V
- **5 AWG** (one size smaller/thinner): 375.9 mV drop = 3.13% of 12V → voltage at load = 11.62 V

This gives a clear margin comparison. 4 AWG gets you under 3%, and 5 AWG just barely misses.

Let me also note that the tool already confirmed 6 AWG is 3.95% and 8 AWG is 6.28%, so 4 AWG is definitely the right pick.

Let me present this clearly.

=== [7] ASSISTANT TEXT ===
## Answer: **4 AWG copper**

Here's the comparison you asked for, both gauges at 20 A over 30 ft (round-trip 60 ft / 18.3 m):

| Gauge | Loop resistance | Voltage drop | % of 12 V | Load sees |
|-------|----------------|-------------|-----------|-----------|
| **4 AWG** (21.2 mm²) | 14.9 mΩ | **298 mV** | **2.48 %** | 11.70 V |
| 5 AWG (16.8 mm²) | 18.8 mΩ | **376 mV** | **3.13 %** | 11.62 V |

### Why 4 AWG and not 5?

Your 3 % ceiling is 0.36 V. 5 AWG lands at 0.376 V — just 0.016 V over the line. That's a razor-thin miss, and in practice it gets worse because:

- Battery sag under load (a "12 V" battery at 20 A is often closer to 11.6–11.8 V before you even start running wire).
- Temperature: copper resistance rises ~0.4 %/°C, so a warm run adds a few percent on top.
- Connector and terminal resistance at both ends, which the wire calc doesn't include.

4 AWG gives you ~40 mV of headroom under the 3 % limit, which absorbs all of that.

### For context, the gauges that clearly fail:

| Gauge | Drop | % |
|-------|------|---|
| 6 AWG | 474 mV | 3.95 % |
| 8 AWG | 754 mV | 6.28 % |
| 10 AWG | 1.20 V | 9.99 % |

### Practical notes

- 4 AWG is the smallest gauge that meets your spec. If cost or stiffness is an issue, 5 AWG *might* work in a cold, well-ventilated installation with short, low-resistance connections — but it's not meeting the stated 3 % target.
- Make sure the battery fuse/PTC is rated for 20 A continuous (a 30 A fuse is common on 4 AWG runs and is fine here).
- These are free-air copper numbers. If the cable is bundled or in conduit, add a few percent to the drop.

=== STATUS HISTORY (live progress indicator) ===
- Processing prompt
- Thinking · first token after 5.3 s
- Thought: The user wants to know what gauge of copper wire will keep voltage drop under 3% for a … 6 AWG first since my rough math suggests it might be close.
- Calling wiring_calc
- Calling wiring_calc
- Thinking
- Thought: Let me analyze the results: - 30 feet = 9.144 meters one way, so round trip is 18.288 m … AWG is definitely the right pick. Let me present this clearly.
- Writing response
- Done · 2 tool calls · prefill 5.1 s for 12,955 tokens · 1,911 generated
