Crash course: Ohm's Law for electricians code-compliant version (part 2)

Why Part 2 exists

Part 1 covered the basics: V=IR, P=IE, and the wheel. This installment puts those equations to work against the NEC. Knowing Ohm's Law is table stakes. Using it to size conductors, set overcurrent protection, and survive an inspection is the job.

Every calculation below assumes copper conductors at 75 degrees C terminations per NEC 110.14(C) unless noted. If your terminations are rated 60 degrees C, drop to that column in Table 310.16. Mixing them is how you fail an inspection.

Voltage drop: the 3% and 5% rule

The NEC does not mandate voltage drop limits in the enforceable sections. It recommends them in Informational Note No. 2 to 210.19(A) and Informational Note No. 2 to 215.2(A)(1): 3% on branch circuits, 5% total including feeders. Ignore it and your motors run hot, your lights dim, and your callback rate climbs.

Single phase voltage drop: VD = (2 x K x I x L) / CM. K is 12.9 for copper, 21.2 for aluminum. L is one-way length in feet. CM is circular mils from Chapter 9, Table 8. For three phase, swap the 2 for 1.732.

• 120V branch circuit, 3% max drop = 3.6V
• 240V branch circuit, 3% max drop = 7.2V
• 480V feeder, 5% max drop = 24V total system

Field tip: if a run is over 100 feet at 20A, upsize one wire gauge before you pull it. Cheaper than pulling it twice.

Sizing conductors with the 80% rule

NEC 210.19(A)(1) and 215.2(A)(1) require branch circuit and feeder conductors to carry no less than 125% of continuous load plus 100% of non-continuous load. A continuous load per Article 100 is one expected to run 3 hours or more. That 125% factor is the inverse of the 80% rule everyone quotes on the job.

Run the math with Ohm's Law first, then apply the multiplier. A 40A continuous load needs conductors rated for 50A minimum. Check Table 310.16, the 75 degree C column, and you land on 8 AWG copper. Do not forget the OCPD sizing in 210.20(A) follows the same 125% rule.

Ampacity correction and adjustment

Ohm's Law gives you the load current. The NEC makes you derate that conductor for ambient temperature (Table 310.15(B)(1)) and for more than three current-carrying conductors in a raceway (Table 310.15(C)(1)). Skip this step and you undersize the wire.

The sequence matters:

1. Calculate load current with I = P / E
2. Apply the 125% continuous-load multiplier per 210.19(A)
3. Pick a conductor from Table 310.16 at the termination temperature rating
4. Apply ambient correction from 310.15(B)(1)
5. Apply conduit fill adjustment from 310.15(C)(1)
6. Verify the derated ampacity still covers the load

Example: nine 12 AWG THHN conductors in a conduit running through a 40 degrees C attic. Base ampacity at 90 degrees C is 30A. Ambient correction is 0.91. Conduit fill for 7-9 CCCs is 0.70. Final ampacity is 30 x 0.91 x 0.70 = 19.1A. Your 20A breaker is now non-compliant.

Motor calculations where Ohm's Law bends

Motors do not follow Ohm's Law at startup. Locked rotor current is 6 to 8 times full load amps. NEC Article 430 handles this by using FLA from Table 430.250 (three phase) or Table 430.248 (single phase) instead of nameplate, and by sizing branch circuit protection at 175% to 250% of FLA per Table 430.52(C)(1).

Conductor sizing uses 125% of FLA from 430.22. Overload protection uses nameplate at 115% to 125% per 430.32. Three separate calculations, three separate values. Confusing them is the top motor circuit fail on inspection.

Field tip: always use the table FLA for circuit sizing, never the nameplate. The code writes it that way on purpose. Nameplate is for overloads only.

Grounding and fault current math

Ohm's Law tells you what a fault current will do. NEC 250.4(A)(5) requires the grounding path to be low impedance enough to clear the OCPD quickly. Short version: I = E / Z, where Z is the loop impedance of the fault path. If your equipment grounding conductor has too much resistance, the breaker never trips and the enclosure stays energized.

Equipment grounding conductors are sized from Table 250.122 based on the OCPD, not the load. A 100A breaker needs 8 AWG copper EGC. Upsize the phase conductors for voltage drop and 250.122(B) tells you to upsize the EGC by the same ratio. That one gets missed on almost every long feeder.

Putting it together on a real job

A 208V three phase, 30A continuous load at 250 feet from the panel. Minimum conductor ampacity is 30 x 1.25 = 37.5A, so 8 AWG copper at 75 degrees C from Table 310.16 (50A). Voltage drop check: VD = (1.732 x 12.9 x 30 x 250) / 16510 CM = 10.1V or 4.8% on a branch circuit. Too high. Upsize to 6 AWG (26240 CM) and drop to 6.3V or 3%. EGC goes from 10 AWG to 8 AWG per 250.122(B) because you upsized the phase conductors.

Every number on that job came from Ohm's Law and two NEC tables. That is the pattern. Load current, code multiplier, table lookup, derate, verify. Same steps every time.