Voltage drop is one of those topics we often mention but seldom think about in-depth. From a very basic standpoint, we need to know whether or not the rated voltage is being delivered to the device or appliance while under full load, which is as simple as running the equipment and measuring the voltage at the equipment feed conductors. If the measured voltage is within the rated range while under load, then we are in pretty good shape—but there is still more to consider.

The voltage drop across a wire can ONLY be measured under load; simply measuring the potential at the end of a circuit without it being under load tells you almost nothing because the circuit is open.

The voltage drop measured is equal to the percentage of the total circuit that resistance is being measured across.

In other words, if the total applied voltage at the main panel is 240V, and you measure 216V at the condenser while it is running, that means that 90% of the resistance in the circuit is in the condenser (216V), and 10% of the total circuit resistance is in the conductors (24V) leading to the condenser (which is way too high).

You will also find that the voltage drop increases when there's a higher current on the circuit. This phenomenon happens for two reasons:

1. A higher running current is due to lower electrical resistance in the load. When the resistance in the load is lower, the resistance of the load makes up a lower percentage of the total circuit resistance, and the wiring makes up more of it. NOTE: Some of you get confused and think that the resistance in a load increases as the current increases, but it doesn't. Just look at Ohm's law again. When amperage increases, the electrical resistance must go down if the voltage remains constant.
2. When most metals get hot, their resistance increases. So, as the wiring current increases, it heats up and increases in resistance, further increasing the wires' share of the voltage drop.

We care about voltage drop for two reasons:

1. It can be bad for our equipment, resulting in poor performance and efficiency.
2. It can be an INDICATOR of other conditions that can lead to overheating and arcing, which can be a safety hazard.

This article includes many references to the NEC (National Electrical Code) because it is the nationally adopted set of rules for high-voltage electrical work in the USA. The excerpts here are for training and commentary reference and should only be used by licensed professionals who have training in the entire code, found on the NFPA website (HERE). The NEC (NFPA) 70 is all about fire and shock prevention, and 310.15(A)(3) sums up conductor design pretty nicely. I sum it up (further) as:

Don't install anything in a way that's going to result in it getting hotter than it's supposed to get. 

So, a high voltage drop occurs because the amperage is higher than it is supposed to be or because the resistance in the circuit is higher than it should be (or both).

What is Acceptable Voltage Drop?

The NEC recommends no more than a 5% voltage drop from the main panel all the way to the appliance under load with a 2% drop allowable on the “feeder” circuits and 3% on the “branch” circuits (NEC 210.19(A) informational note #4). This is only a recommendation for design so long as all the other rules regarding conductor (wire), over-current protection, and connections are followed due to the fact that it is in an “informational note” in the NEC rather than a code.

From a practical standpoint, we really shouldn’t see more than a 5% voltage drop on a properly sized conductor when measured under load other than during motor inrush (locked rotor). It’s most critical that we remember that voltage drop measurements are only valid when UNDER LOAD. If the equipment isn’t running, then there will be no voltage drop, and the measurement becomes almost meaningless.

In practice, there are four primary causes of objectionable voltage drop:

• Undersized Conductors
• Poor Connections (Terminations)
• Higher Than Design Circuit Current
• Long Conductors (Long Wire Length)

Let’s look at each one individually to see what we can do to diagnose, repair, and prevent these issues.

Undersized Conductors
In HVAC, we need to size the majority of our conductors (wires) according to NEC Table 310.15(B)(16), which is where we get rules of thumb about wire size, primarily by looking at basic copper conductors in the 60 degree Celsius category.

When conductors are undersized for the rated ampacity of the system, the result will be an overheating conductor and voltage drop, which is a dangerous issue. Many techs and electricians aren't aware that section 440 of the NEC allows air conditioning system wiring to be sized according to the MCA (minimum circuit ampacity) listed on the equipment, EVEN when the brakes or fuse is larger and sized according to the listed MOCP (maximum over-current protection). No matter what we do, we must abide by 310.15(A)(3) and ensure that we do not install conductors in such a way that they will overheat, whether that is due to the amperage, the ambient conditions they are exposed to, or the number of conductors run in a conduit.

Poor Connections

When wires are connected using wire nuts, lugs, splices, etc., they should be made with the best possible contact with low resistance and compatible materials that won't wear or corrode. If the connection is poor, then the resistance will increase, resulting in heat at that point, which can lead to more resistance. The issue becomes worse and worse. Poor connections not only cause voltage drop but can also cause a safety hazard. All high-voltage electrical connections and terminations should be made with NEC/UL-approved materials and according to instructions. Common causes of poor connections are:

• Connecting too many wires under a lug
• Using an unapproved connector
• Connecting dissimilar metals in an unapproved connector for that use (such as copper and aluminum)
• Failing to tighten lugs or screws to the rated torque rating

Higher than Design Circuit Current

In some cases, the wiring and connections are correct, but the device itself is drawing above its rated current. That will lead to a high voltage drop and should be corrected at the root cause in the system causing the high current.

Long Conductors

There are some interesting ramifications to long conductors, with the first being that the NEC doesn't really address it—at least not directly. As we already mentioned, NEC 210.19(A) makes suggestions to keep the total voltage below 5%, including a drop due to wire length. A voltage drop due to wire length isn't as large of an issue because it doesn't cause wire overheating. If the wire is long but still the correct size, it WILL have higher resistance, which WILL result in greater voltage drop, but since the resistance is spread across the entire wire, it won't get any hotter in one spot like a poor connection. The result will LOWER circuit amperage and possibly poor performance of the device, but it won't result in a dangerous condition in the conductor.

We are often responsible for upsizing conductors to prevent voltage drop for the sake of the system, but that's not because we are REQUIRED to do so. That means that when wire lengths are long, we need to pay special attention to the under load voltage drop, especially in new construction environments.

—Bryan