What it is

A loose, corroded, or burnt connection is one of the most common and most overlooked failures in the trade. The sneaky part: a bad connection often ohms out fine with the power off, then falls apart under load. Voltage-drop testing is how you catch it — you measure the voltage across a connection while current is actually flowing through it. A healthy joint drops almost nothing. A bad one drops a chunk and gets hot. This is the test that separates a real troubleshooter from a parts-changer.

How it works

Ohm's law again: any voltage dropped across a point equals the current through it times the resistance at that point. A good connection has near-zero resistance, so even at high current it drops almost no voltage. A degraded connection — oxidized lug, loose terminal, pitted contact — has measurable resistance. Push real current through that resistance and you get a real voltage drop, and that dropped voltage turns into heat right at the joint. Heat makes the connection worse, which makes more heat. That runaway is why a slightly loose lug ends up as a melted, blackened mess.

The reason an ohmmeter lies here is that it tests with a tiny current. A connection might read a fraction of an ohm cold, which looks fine, but at 30 amps that same tiny resistance drops real voltage and cooks. You have to test it under load to see the truth.

In the field

Set the meter to AC volts and run the system so the circuit is loaded. Now put one lead on each side of the connection you suspect — the two sides of a contactor's line terminal, the two ends of a lug, across a fuse, across a disconnect blade.

A good connection reads a few hundredths of a volt, basically zero. A bad one reads anywhere from tenths of a volt to several volts. The bigger the drop, the worse the joint. You're essentially asking "how much voltage is this connection wasting?" — and a connection shouldn't waste any.

Walk the whole current path the same way: across the disconnect, across each fuse, across the contactor poles, across the wire nuts and lugs to the compressor. The connection eating the voltage is the one to clean, tighten, or replace. While you're at it, feel for heat (carefully) and look for discoloration — a browned terminal or melted insulation marks the spot.

Normal values & targets

  • Across a solid connection under load: roughly 0.00–0.05V. Essentially nothing.
  • Across a single set of closed contactor contacts: should be only a few hundredths of a volt; more than ~0.1–0.2V means pitted or burnt contacts.
  • Across a good fuse or disconnect blade under load: near 0V; a few tenths or more means a corroded clip or a fuse element going high-resistance.
  • A rule of thumb for the whole line side from disconnect to load: keep total wasted drop down in the small fraction of a volt. Any single point dropping 0.5V or more under load is a problem joint.
  • Once you're seeing a volt or more across a connection that should be solid, it's actively making heat and on its way to failure.

Common faults & what they mean

  • Several volts dropped across one contactor pole under load → burnt/pitted contacts; replace the contactor.
  • Notable drop across a fuse that tests good cold → corroded fuse clip or a fuse going bad under heat.
  • Drop across a lug with a discolored or loose ring terminal → loose/oxidized connection; clean, re-torque, or re-terminate.
  • Compressor pulling high amps with low voltage at its terminals but full voltage at the disconnect → the lost voltage is being dropped somewhere in between; walk the path to find it.
  • Connection ohms out fine cold but the equipment nuisance-trips under load → classic high-resistance-under-load joint that only voltage-drop testing reveals.

Tech tips & gotchas

The whole game here is "test under load." If you power down to ohm it, you've thrown away the one condition that exposes the fault. Make your measurements with current flowing.

Heat and discoloration are your free clues. Before you even meter, scan the connections — a browned terminal, melted insulation, or a faint burnt smell points you straight at the bad joint.

Don't just tighten a burnt lug and call it fixed. Once a terminal has overheated, the metal is oxidized and the spring tension or thread may be compromised; it'll come back. Cut it back to clean copper and re-terminate, or replace the component.

A bad neutral or a bad ground can also show up as a drop where you don't expect one — if line-to-line looks fine but the equipment behaves oddly, check the drops on the grounded conductor too.

Safety / code notes

You're working a live, loaded circuit for this test, so it carries shock and arc-flash exposure — use properly rated leads and PPE and keep one hand discipline. Terminations and torque matter: NEC Article 110 covers conductor terminations and the requirement to torque lugs to spec, which is exactly the kind of loose connection this test finds. After repair, re-torque to the manufacturer's value rather than gorilla-tightening.