Category
Electrical System Design Calculators - Voltage Drop, Wire Size, Grounding
Free electrical system design calculators. Voltage drop, wire sizing per NEC 310.16, motor wire sizing per NEC 430, grounding conductor sizing per NEC 250, 12V DC wire sizing. For electricians, MEP engineers, and solar installers.
Electrical system design calculators are essential for electricians, MEP engineers, solar installers, and electrical contractors. Our free tools cover voltage drop, wire sizing per NEC 310.16, motor circuit conductor sizing per NEC 430.22, equipment grounding conductor sizing per NEC 250.122, and 12V DC cable sizing — all verified against the National Electrical Code (NFPA 70) and IEEE standards.
Voltage drop is the silent constraint in electrical design — it often governs conductor size more than ampacity. For a 200 A 480V 3-phase circuit over 500 ft, voltage drop (not ampacity) dictates the required conductor size. NEC recommends ≤3% for branch circuits and ≤5% total (feeder + branch). Low-voltage DC systems (12V, 24V, 48V) are especially sensitive — a 2V drop at 12V represents a 16.7% loss, dimming LED lights and starving inverters of voltage.
Wire sizing per the NEC starts with ampacity (NEC 310.16), applies derating for temperature and conduit fill, and then checks voltage drop. Motor circuits have specific rules under NEC 430.22 (125% of FLA), and equipment grounding conductors follow NEC 250.122 based on the overcurrent protection device rating. Every circuit in every building passes through these calculations.
Common Electrical Design References (NEC-Based)
| Design Task | NEC Reference | Key Rule |
|---|---|---|
| Branch circuit VD | NEC 210.19 Informational Note | ≤3% branch, ≤5% feeder+branch |
| Conductor ampacity | NEC Table 310.16 | 75°C column for most applications |
| Motor conductor sizing | NEC 430.22 | 125% of NEC Table 430.250 FLC |
| Motor OCPD | NEC 430.52 | Inverse-time breaker: max 250% of FLC |
| EGC sizing | NEC Table 250.122 | Based on OCPD rating, not load amps |
| DC voltage drop | ABYC E-11 (marine) | ≤3% for navigation lights, ≤10% general |
| Solar PV wiring | NEC 690.8 | 1.56 × I_sc (1.25 continuous × 1.25 irradiance) |
Electrical System Design Calculators - Voltage Drop, Wire Size, Grounding (7)
Voltage Drop Calculator | NEC & IEEE 141
Voltage drop is the reduction in voltage along a conductor as current flows through it, dictated by Ohm's Law...
Wire Size Calculator (NEC 310.16) | AWG & kcmil Ampacity
Wire size selection is a two-constraint optimization problem: the conductor must satisfy BOTH (1) ampacity —...
12V Wire Size Calculator | DC Cable Sizing for Auto, RV & Solar
12V DC wire sizing is fundamentally a voltage-drop-driven calculation, not an ampacity-limited one. The core...
DC Voltage Drop Calculator | 12V/24V/48V Solar & Automotive
DC voltage drop is the purest application of Ohm's Law to conductor sizing: with no reactance, no skin effect,...
Ground Wire Size Calculator (NEC 250.122)
The equipment grounding conductor (EGC) -- commonly called the 'ground wire' -- is the safety conductor that...
Motor Wire Size Calculator (NEC 430.22)
Motor circuit conductor sizing is governed by NEC Article 430, which has specific, different rules from...
Resistor Voltage Drop Calculator | Ohm's Law Series Circuits
The voltage drop across a resistor is the most fundamental calculation in electronics: a resistor converts...
Frequently Asked Questions
What size wire for 100 amps?
Per NEC 310.16 (75°C copper): #3 AWG for 100 A non-continuous, #1 AWG for 125 A (continuous load at 125% factor). At 100 A continuous — the common scenario — you need 125 A ampacity minimum, which requires #1 AWG copper (130 A) or #1/0 AWG aluminum (120 A). This does not include voltage drop or temperature derating, which may require larger conductors.
How do I reduce voltage drop on a long run?
Four options: (1) Upsize the conductor (one extra AWG size reduces resistance by ~26%). (2) Increase system voltage (doubling voltage halves current for the same power, reducing VD proportionally). (3) Reduce distance (relocate panel or transformer closer to load). (4) Parallel conductors. For existing installations, a boost transformer at the load end can compensate for steady-state VD but not starting inrush dips.
What ground wire size do I need?
The equipment grounding conductor (EGC) is sized per NEC Table 250.122 based on the overcurrent device rating: 15 A → #14 Cu, 20 A → #12, 60 A → #10, 100 A → #8, 200 A → #6, 400 A → #3, 800 A → #1/0. If circuit conductors are upsized for voltage drop, the EGC must be proportionally upsized per NEC 250.122(B).
What wire size for DC solar systems?
12V DC solar: voltage drop is the dominant constraint. For a 200W panel (I_mp ≈ 11 A at 18V), a 30 ft run at 3% VD requires #10 AWG minimum — and #8 AWG for 50+ ft. At 48V: the same power at 1/4 the current needs only #14 AWG for 30 ft. This is the strongest argument for higher-voltage solar arrays — dramatically smaller, cheaper, and easier-to-route conductors.