Conductor sizing · NEC 310.16

Wire Size Calculator

Estimate the conductor that satisfies both NEC Table 310.16 ampacity — with ambient and more-than-three-conductor derating — and your voltage-drop limit. See which requirement drives the size, with the supporting row evidence.

100% free
Ampacity + voltage drop
Ambient & bundling derate
240.4(D) cap applied

Conductor sizing

NEC Table 310.16 + derating

Formula

Effective A = base × ambient × bundling

Field inputsConductor sizing

Ampacity: Table 310.16
Derate: ambient × fill
Driver: A · VD
Limits: 240.4(D)

Picks the first size that clears ampacity and your drop limit.

Circuit inputs

Describe the circuit

Conductor material

Phase

Load current (amps)

One-way length (ft)

System voltage (V)

Max voltage drop (%)

Termination temp (°C)

Ambient temp (°C)

Current-carrying conductors

Parallel sets

Assumptions: ampacity from NEC Table 310.16 (≤3 conductors, 30°C); ambient correction from NEC Table 310.15(B)(1); more-than-three-conductor adjustment from NEC Table 310.15(C)(1); voltage-drop resistance from NEC Chapter 9, Table 8. The effective ampacity is capped by the NEC 240.4(D) small-conductor overcurrent limit (#14/#12/#10 copper, #12/#10 aluminum), and parallel sets are restricted to 1/0 AWG and larger per NEC 310.10(H)(1). This tool does not apply continuous-load 125% factors or overcurrent-device rounding.

Recommended conductor

Wire size

Ampacity-driven

Recommended size

Ampacity-only size: #3
Sizing driver: Ampacity
Derate factor (ambient × bundling): 1.00
Base ampacity (#3 @ 75°C): 100 A
Derated ampacity: 100.0 A
Effective ampacity (used to size): 100.0 A
Voltage drop: 4.90 V
Voltage drop (percent): 2.04 %

Row evidence (NEC 310.16)

Size	Effective A	VD %	OK
#14	15	25.58	—
#12	20	16.08	—
#10	30	10.08	—
#8	50	6.37	—
#6	65	4.09	—
#4	85	2.57	—
#3	100	2.04	✓
#2	115	1.62	✓
#1	130	1.28	✓
#1/0	150	1.02	✓
#2/0	175	0.81	✓
#3/0	200	0.64	✓

Sources: NEC Table 310.16, Table 310.15(B)(1), Table 310.15(C)(1), Chapter 9 Table 8.

Safety & accuracy notice

This wire size calculator is an estimator and check only — a planning aid, not engineering, a permit, or design approval, and not legal or code certification. It applies the NEC 240.4(D) small-conductor cap and the 310.10(H)(1) parallel minimum, but it does not apply every NEC rule (for example continuous-load 125% factors, tap rules, or overcurrent-device rounding).

Always verify the final conductor and overcurrent protection against the current adopted NEC (National Electrical Code), the equipment listings and manufacturer instructions, and your local AHJ (Authority Having Jurisdiction — the inspector or department that enforces the code). The final conductor and overcurrent protection should be reviewed and approved by a licensed electrician or professional engineer.

Wire sizing FAQ

How does this wire size calculator pick a conductor?

It walks NEC Table 310.16 from the smallest conductor up. For each size it applies ambient correction (NEC Table 310.15(B)(1)) and a more-than-three-conductor adjustment (NEC Table 310.15(C)(1)), then checks both that the derated ampacity carries the load and that voltage drop stays within your limit. The recommended size is the first that satisfies both.

What does 'driver' mean?

The driver tells you which requirement forced the size up. If the ampacity-only size already meets the voltage-drop limit, ampacity is the driver. If voltage drop forced a larger conductor than ampacity alone, voltage drop is the driver.

Is this an engineered conductor selection?

No. It is an estimator and check only — a planning aid, not engineering, a permit, or design approval. Verify against the current adopted NEC, the equipment listings, and your local AHJ.

Related electrical tools