Convention Applied by the app elec.voltage-drop.k-constant
The K constant for voltage drop
The resistivity figure the estimator's single-phase voltage-drop formula uses — 12.9 for copper, 21.2 for aluminum, in ohm-circular-mils per foot.
| When | The app uses |
|---|---|
| copper conductors | 12.9 Ω·cmil/ft |
| aluminum conductors | 21.2 Ω·cmil/ft |
- Source
- NEC · Chapter 9, Table 8 (as the trade rounds it) · read the public text
- Editions checked
- 2017 · 2020 · 2023
- Used by
- Bid Check
- Amendments
No state or local amendment to this rule is on file. Not on file means the model code as written — check your jurisdiction.
A working figure the trade uses and the app defaults to. Shops override these most.
Bid-day voltage drop is VD = 2 × K × I × L ÷ cmil for single phase: twice the one-way length, the current, the conductor's circular-mil area, and K — the resistance of a one-foot, one-circular-mil conductor at operating temperature. The trade's working figures are 12.9 for copper and 21.2 for aluminum; they are rounded from the resistance table in Chapter 9, not a value the Code prints as "K".
What the app does with it
Bid Check's voltage-drop row uses these two figures with the run length it measured (verticals included) and the circular-mil area of the smallest conductor on the circuit. The formula is single-phase; three-phase circuits would use √3 in place of 2, which the app does not yet do.
Cited by section, not reprinted: the code text belongs to its publisher. Company practice is not a rule here — it lives with your pricing.