Feszültségesés és vezeték méretezés dedikált aljzat áramköröknél
Feszültségesés és vezeték méretezés dedikált aljzat áramköröknél. Tervezzen 120V és 240V készülék-, mosókonyha-, konyha-, garázs- és műhelyáramköröket NEC, IEC 60364-5-52 és gyakorlati példák alapján.
Use this localized guide with the same engineering workflow: confirm the required branch circuit first, calculate voltage drop with the real appliance current, and then check terminals, box fill, conduit fill, and local code before upsizing conductors. NEC IEC
Branch-Circuit Workflow for Appliances and Dedicated Receptacles
Identify the load: nameplate amps, watts, motor starting behavior, continuous operation, and whether the receptacle serves one appliance or a required small-appliance/laundry/garage circuit.
Confirm the code minimum: NEC 210.11(C), 210.19(A)(1), 210.21(B), 210.23, 210.52, 422.10, 422.11, and any manufacturer instructions before choosing the breaker and conductor ampacity.
Calculate voltage drop with real current and one-way route length. For a 120V branch circuit, even 4V of drop is already 3.3%. For 240V circuits, the same volts lost are a smaller percentage but still affect motors and heating performance.
If the voltage-drop result forces a larger conductor, verify NEC 314.16 box fill, device terminal range, conduit fill, bending space, and pigtail/device compatibility before installing the larger wire.
Code and Standard Checkpoints
- NEC 210.11(C)(1) requires at least two 20A small-appliance branch circuits for dwelling kitchens, pantries, breakfast rooms, dining rooms, and similar areas.
- NEC 210.11(C)(2) requires a 20A laundry branch circuit. NEC 210.11(C)(4) covers garage branch-circuit requirements in dwellings.
- NEC 210.19(A)(1) sets branch-circuit conductor ampacity requirements. Its informational note is commonly used as a 3% branch-circuit and 5% combined feeder-plus-branch design target.
- NEC 210.21(B) and 210.23 govern receptacle ratings and permissible loads. NEC 422.10 and 422.11 apply to appliance branch-circuit and overcurrent sizing.
- IEC 60364-5-52 treats voltage drop as part of cable selection along with current-carrying capacity, installation method, grouping, insulation temperature, and conductor material.
Typical Dedicated Circuit Planning Cases
Use these examples as planning checks, not permission to ignore local code or manufacturer instructions. Resistance values are approximate copper values at normal design temperature.
| Circuit | Code or load basis | Voltage-drop check | Practical decision |
|---|---|---|---|
| Microwave receptacle, 120V, 12A, 85 ft | Often a 20A kitchen/small-appliance or dedicated circuit; verify appliance instructions | 12 AWG copper is about 3.4V, or 2.8%; 10 AWG is about 2.1V, or 1.8% | 12 AWG may meet a 3% target, but 10 AWG gives margin if the route grows past 90 ft |
| Laundry receptacle, 120V, 16A, 110 ft | NEC 210.11(C)(2) requires a 20A laundry circuit | 12 AWG copper is about 5.6V, or 4.7%; 10 AWG is about 3.5V, or 2.9% | Voltage drop, not breaker size, often controls the conductor on long laundry runs |
| Garage freezer, 120V, 8A running, 95 ft | Dedicated circuit is often chosen for reliability; GFCI rules may apply by location | 14 AWG copper can approach 4.8V at 8A; 12 AWG is about 3.0V | Use 12 AWG on a 20A circuit when allowed, especially where compressor starts are weak |
| Sump pump, 120V, 9A running, 125 ft | Motor load; check pump instructions and required receptacle protection | 12 AWG copper is about 3.6%; 10 AWG is about 2.3% | Upsize for starting voltage and reliability, not just steady-state amps |
| 240V workshop receptacle, 24A, 150 ft | 30A branch circuit may be required by equipment nameplate | 10 AWG copper is near 4.7%; 8 AWG is near 3.0%; 6 AWG is near 1.9% | For welders, compressors, or saws, a larger conductor may prevent dimming and hard starts |
Worked Examples with Specific Numbers
Microwave branch circuit: 12A at 120V, 85 ft one-way
A built-in microwave is 85 ft from the panel after the route around framing is measured. With 12 AWG copper at about 1.588 ohm/kft, the round-trip drop is roughly 12A x 170 ft x 1.588 ohm/kft / 1000 = 3.24V, or 2.7% on 120V. That is close to the usual 3% branch-circuit target, so any added distance, high ambient temperature, or shared loading makes 10 AWG worth checking.
Laundry circuit: 16A continuous-ish appliance load, 110 ft
A laundry receptacle is required as a 20A branch circuit by NEC 210.11(C)(2), but the appliance load is 16A at 120V and the route is 110 ft. With 12 AWG copper, the drop is about 5.6V, or 4.7%. Upsizing to 10 AWG lowers the drop to roughly 3.5V, or 2.9%, while the breaker remains selected from the circuit and equipment requirements.
Workshop receptacle: 24A at 240V, 150 ft
A detached workshop receptacle feeds a 24A machine on a 30A 240V branch circuit. At 150 ft one-way, 10 AWG copper can land near 11.3V drop, or 4.7%. Moving to 8 AWG is close to 3.0%; 6 AWG gives better motor-start margin near 1.9%. The final choice should be checked against device terminals, box size, conduit fill, and equipment instructions.
Checklist Before You Install
- Measure the actual route length, not the straight-line distance from panel to receptacle.
- Use appliance nameplate input current or watts converted to amps; do not blindly enter the breaker rating in the calculator.
- For 120V circuits, treat every lost volt seriously because 3.6V is already 3%.
- Check GFCI, AFCI, tamper-resistant, and location-specific requirements separately from voltage drop.
- When upsizing conductors, confirm box fill under NEC 314.16 and the maximum conductor size accepted by the receptacle or equipment terminals.
Common Questions
Can I use 14 AWG for a dedicated 15A appliance circuit?
Sometimes, if the circuit is truly 15A and all other code rules allow it, but voltage drop may still make 12 AWG the better choice on long 120V runs. At 100 ft and 12A, 14 AWG copper is roughly 4.8V drop, or 4%.
Should voltage drop be calculated from the breaker size?
Usually no. Use the actual load current from the appliance nameplate. A 20A circuit serving a 12A microwave should normally be checked at 12A, while the breaker and conductor ampacity are checked under NEC branch-circuit rules.
What is a good voltage-drop target for receptacle circuits?
Many designers use the NEC informational-note target of about 3% for a branch circuit and 5% total feeder plus branch. Sensitive electronics, compressors, and long workshop runs may justify a lower target such as 2%.
Does upsizing from 12 AWG to 10 AWG let me use a larger breaker?
No. The breaker must still match the circuit rules, receptacle rating, appliance instructions, and conductor ampacity. Upsizing for voltage drop does not automatically permit a 30A breaker on a 20A receptacle circuit.
How does IEC cable sizing differ for appliance circuits?
IEC 60364-5-52 checks current-carrying capacity, installation method, grouping, insulation temperature, and voltage drop together. The calculation may use mm2 conductor sizes, but the same design logic applies: thermal suitability and delivered voltage are separate checks.
Check the Branch Circuit Before Pulling Cable
Run the voltage-drop result with the real appliance current, then compare wire sizes before the boxes, conduit, or cable route are locked in.
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