Understanding Voltage Drop - Short #251

HVAC School - For Techs, By Techs

Bryan Orr

Training, Careers, Airconditioning, Self-improvement, Hvac, Business, Education, Refrigeration, Heating, Ac, Apprenticeship

4.8 • 985 Ratings

🗓️ 5 August 2025

⏱️ 17 minutes

🔗️ Recording | iTunes | RSS

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Summary

In this short podcast episode, Bryan drops some knowledge to help with understanding voltage drop, a few different causes of it, and NEC recommendations.

Voltage is electrical potential or "pressure," and voltage drop is the reduction in electrical potential energy. We often think of it happening across conductors (which add resistance), but it also happens across loads (like contactor coils). Voltage drop across loads is usually designed, but voltage drop across conductors is usually undesigned and undesirable.

Several factors contribute to conductor resistance. Length is a major one; long runs of wire introduce more resistance to the circuit than shorter runs. Size/gauge also matters; smaller-gauge wire has more resistance than larger-gauge wire. Copper is the most common material for wiring, but we use other materials (including steel or aluminum), and those have different resistance values. Temperature also affects resistance, as they both increase and decrease as the other one does.

When we measure voltage drop, we want to make sure we're doing it under load, NOT on startup. Voltage drops that happen on startup can be mitigated with more suitable infrastructure (including larger wires) or soft starts.

Undersized conductors don't have sufficient cross-sectional area for the applied load. To avoid voltage drop due to undersized conductors, we should size conductors based on minimum circuit ampacity (MCA), not breaker size (MOCP). Poor connections can also cause resistance to jump, which reduces voltage; we need to pay attention to the connection design (including torque specs and proper lugging) to avoid making mistakes. Long wires don't cause overheating by themselves, but they still contribute to voltage drop and cause reduced performance (including drawing higher current on startup).

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Transcript

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0:00.0	Yo, yo, yo, yo, this is the HVAC school podcast.
0:07.1	I'm Brian.
0:07.6	This is a short episode about voltage drop.
0:11.0	In other words, we're going to drop some knowledge about voltage drop.
0:16.4	Some of this is going to be real applicable.
0:18.0	Some of it's going to be sciencey, but just giving you a way to improve your mental model around electricity, which is always helpful, helps you understand
0:25.7	things you're seeing out there in the field. But before I do that, we want to thank our great
0:30.4	brand sponsors. Sensi by Copeland and proor branding on their thermostats.
0:38.4	Carrier and Carrier.com. I've been a carrier dealer for many years. Carrier has their new
0:44.7	green speed extreme super high efficiency heat pumps out on the market now. Find out more by
0:50.5	going to Carrier.com. Navak at navakglobal.com.
0:56.5	Refrigeration technologies, and specifically viper wipes, the all-purpose wipe that is
1:03.0	really amazing what it can cut through. Everything from mastic to oil grease tar,
1:08.7	grime, and it's nice on the hands.
1:12.6	Make some nice and soft when you're done.
1:19.3	So that way, you can go home and head out a dancing when you get back to the house.
1:23.4	Find out more at refrigetect.com slash viper dash wipes.
1:27.9	Refragitech.com slash viper dash wipes.
1:32.3	All right, so core concept.
1:35.8	What is this voltage drop we talk about?
1:40.0	Voltage drop is the reduction in electrical potential energy,
1:45.4	also known as voltage, along a conductor typically due to resistance. So that's like a dictionary definition. Reduction of potential energy of electrical potential across a conductor due to
	...

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