Crash course: Ohm's Law for electricians what journeymen forget (part 4)

Ohm's Law is the first thing you learn and the first thing you stop thinking about. That is exactly when it bites you. Voltage drop on a long run, a motor pulling more amps than the nameplate, a breaker that nuisance trips on a cold morning. All of it traces back to V = IR and the two siblings most journeymen can recite but rarely apply: P = VI and P = I R. This is the field version, not the textbook version.

The three forms you actually use

You only need three relationships to solve 90% of field problems. Voltage equals current times resistance. Power equals voltage times current. Power equals current squared times resistance. The last one is the one people forget, and it is the one that explains why your lugs are hot.

Memorize the wheel if you want, but in practice you rearrange on the fly. If you know two values, you can find the other two. The trick is knowing which two you actually have on a live job, not which two the test gave you.

• V = I x R ... sizing conductors, checking drop
• P = V x I ... loading panels, sizing services
• P = I squared x R ... heat in conductors, terminations, splices

Voltage drop is Ohm's Law in a work shirt

NEC 210.19(A) Informational Note 4 recommends branch circuit voltage drop not exceed 3%, with a 5% combined feeder and branch limit. That is a recommendation, not a rule, but the inspector who is also a journeyman will ask. More importantly, the motor or the LED driver at the end of that run will tell on you.

For a single phase run, drop equals 2 times length times current times resistance per foot of the conductor. For three phase, swap the 2 for 1.732. Resistance values live in NEC Chapter 9, Table 8. Keep that page bookmarked. A 200 foot run of #12 copper at 16 amps drops about 6 volts on 120V, which is 5%. Bump to #10 and you are back under 3%.

If a customer complains that lights dim when the AC kicks on, do not chase the AC. Measure voltage at the panel and at the load while it starts. Nine times out of ten the feeder or the service conductors are undersized for inrush, not the equipment.

Why I squared R matters more than you think

Heat in a conductor or termination scales with the square of the current. Double the current, quadruple the heat. That is why a loose lug on a 200 amp feeder destroys itself in a week while a loose lug on a 15 amp lighting circuit might sit there for a decade.

This is also why NEC 110.14(C) terminal temperature ratings exist. A 75C lug on a 90C conductor still has to be sized using the 75C column. The conductor can take the heat, the lug cannot. Thermal imaging on a panel under load will show you every termination that is fighting Ohm's Law and losing.

1. Torque every termination to the manufacturer spec, NEC 110.14(D)
2. Re torque after the first heat cycle on large feeders
3. Scan with an IR camera under load, not at idle
4. Anything more than 10C above ambient on a termination is a flag

Motors lie, and Ohm's Law tells the truth

Nameplate full load amps assume rated voltage and rated load. Drop the voltage 5% and the current goes up to maintain the same horsepower output, because power is roughly constant. That is why undervoltage cooks motors. The winding resistance did not change, but I squared R did.

NEC 430.6(A)(1) tells you to size conductors and overload using Table 430.250 values, not the nameplate. But for overload protection, NEC 430.32 uses the nameplate. Two different numbers for two different purposes. Ohm's Law explains why: conductor sizing is about worst case current, overload is about protecting that specific motor from itself.

If a 3 phase motor is humming and pulling unbalanced current across the three legs, check voltage balance first. NEMA says more than 1% voltage imbalance causes roughly 6 to 10 times that in current imbalance. Ohm's Law, ruthless as ever.

Quick field checks that save callbacks

Before you energize, before you leave, before you sign the sticker, run the numbers in your head. Not on paper. The journeyman who can estimate voltage drop on a 150 foot run of #10 in five seconds is the one who never gets called back.

Use round numbers. #12 copper is about 2 ohms per 1000 feet. #10 is about 1.2. #8 is about 0.78. Double the length, double the resistance. Double the current, double the drop. Square the current, quadruple the heat. Those four facts cover most of what you need on a service call.

• Long run of small wire feeding a motor or heater ... check drop
• Warm breaker or warm lug under normal load ... check torque and current
• Equipment misbehaving on one leg of 208V ... check voltage balance
• Nuisance trips in cold weather ... check inrush and conductor length

The habit that separates journeymen from masters

Anyone can pass the Ohm's Law questions on the journeyman exam. The masters are the ones who run the math before they pull the wire, not after the customer calls back. Every conductor, every termination, every motor circuit is an Ohm's Law problem waiting to be solved or waiting to fail.

Keep NEC Chapter 9 Table 8 and Table 9 within reach. Table 8 gives you DC resistance for voltage drop math. Table 9 gives you AC impedance for the same calculation on larger conductors where reactance starts to matter. Both tables exist because the Code writers know what happens when you ignore the law.