Падіння напруги тимчасового живлення будмайданчика: NEC 590, розподільчі бокси та генератори
Плануйте тимчасове живлення з NEC 590, IEC 60364-7-704, розподільчими боксами, подовжувачами та генераторами.
Плануйте тимчасове живлення з NEC 590, IEC 60364-7-704, розподільчими боксами, подовжувачами та генераторами.
Резюме
- Check temporary feeders and final cord branches as separate voltage-drop segments.
- Use NEC 590 for temporary installations and IEC 60364-7-704 for construction-site context.
- A 120 V, 15 A tool on 150 ft of 12 AWG cord can lose about 7.1 V, or 5.9%.
- Moving a spider box closer often fixes more circuits than upsizing one cord.
Temporary construction power is a limited-duration electrical supply for construction, demolition, maintenance, emergency, or testing work. Voltage drop is the voltage lost in conductors as current flows. A spider box is a portable distribution unit for temporary receptacle circuits.
NEC 590 governs temporary wiring practice, while National Electrical Code background and IEC context help explain the standards family.
Temporary does not mean casual: a 20 A saw 200 feet from the spider box on 12 AWG cord can lose enough voltage to slow the job.
— Hommer Zhao, Technical Director
Feeder and cord examples
A 100 A, 120/208 V spider-box feeder carrying 70 A over 180 ft of 2 AWG copper drops about 4.2 V, or 2.0%.
A 120 V, 15 A tool on 150 ft of 12 AWG cord has about 0.476 ohms loop resistance and drops about 7.1 V, or 5.9% before feeder drop is added.
Use the AC voltage drop calculator, the wire resistance calculator, and the extension cord guide together.
Comparison table
| Option | Best use | Risk | Decision |
|---|---|---|---|
| Move spider box | Changing work zones | Low | Best when several cords are long |
| Upsize feeder | Large rack far from source | Medium | Use when feeder exceeds about 2% to 3% |
| Upsize final cord | One critical 120 V tool | Medium | Good for 75 ft to 150 ft runs |
| Use 240 V equipment | Pumps and saws | Lower percent drop | Use when equipment supports it |
| Daisy-chain cords | Emergency reach only | High | Avoid as normal design |
On temporary 120 V circuits, a cord branch over 3% by itself deserves review because feeder drop and motor starting current still add on top.
— Hommer Zhao, Technical Director
FAQ
What voltage drop should temporary power allow?
Many designs target about 3% branch and 5% total feeder plus branch circuit.
Does NEC 590 give a different drop limit?
No; NEC 590 governs temporary installations, while voltage-drop targets come from design practice and equipment limits.
How do I calculate through a spider box?
Calculate the feeder to the box first, then calculate each final cord branch separately.
Why do 120 V tools fail first?
The same 6 V loss is 5% at 120 V but only 2.5% at 240 V.
What should be documented?
Record source voltage, feeder size, GFCI/RCD protection, cable type, route length, calculated drop, and spider-box location.
Before deploying temporary feeders, spider boxes, generator cords, or long tool extensions, calculate feeder and cord segments separately.
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