RTU Electrical Checklist

When performing maintenance on a rooftop HVAC unit, the electrical inspection is one of the most critical steps yet often overlooked. Over 70% of equipment failures originate from electrical issues, so verifying this part isn’t optional; it’s essential. In this post, let’s break down what to look for, how to document it, and what proves a proper electrical check has been done.

 

Before Opening the Panel

Before opening any panel, always switch the disconnect to OFF and fully de energize the unit. Touching wires while power is still supplied poses a serious risk of electric shock.

 

Once the power is off, open the panel and verify that all wires are securely connected and free of movement. If there’s visible dust, corrosion, or buildup inside, blow it clean using compressed air (I recommend carrying a can type air duster for quick and easy cleaning during rooftop inspections) only while power remains off. If wiring is messy or loose, tidy it up and secure it with cable ties. A well organized panel not only looks professional but also makes future inspections much easier.

 

What to Check Around the Compressor Wiring

The wiring condition should always be the first thing you inspect visually. If you find cracked, pinched, or melted insulation, it’s an early sign of electrical insulation failure. Compressor and blower motor areas are especially prone to damage due to heat, vibration, and moisture. In these cases, don’t patch with electrical tape, the proper fix is wire replacement.

 

Another key indicator is discoloration. If copper conductors appear dark brown or black, it usually indicates overheating caused by loose terminals or poor contact. Cut back the wire until you see clean copper, then recrimp or reconnect the terminal.

 

Abrasion from vibration is another common problem. When wires rub against fan housings, brackets, or duct edges, insulation wears down and eventually leads to a short circuit. To prevent this, always secure wires with clamps and install rubber bushings or protective sleeves where cables pass near metal edges.

 

Loose or partially tightened terminals create electrical resistance, which generates heat as current flows. This accumulated heat melts insulation and can ultimately cause a short or fire. Check each terminal by gently wiggling it. If it moves, tighten it with a torque screwdriver.

 

If terminals show bluish discoloration or a burnt smell, that’s evidence of overheating and copper oxidation. In such cases, both the wire and the terminal should be replaced.

 

What to Look for on the Contactor

The contactor is a switching device that delivers power to the motor and compressor. Over time, its contacts can wear out, pit (pitting), or carbonize, which leads to poor conductivity, voltage drop, and excessive heat. During a visual inspection, if the contact surfaces look rough, uneven, or dark brown to black in color, the contactor should be replaced. When the surface is rubbed lightly and a rough texture or black dust is felt, it indicates that the contact is already pitted or carbonized. These are early signs of failure as contact resistance increases, it causes further arcing, heat buildup, and eventually serious system damage.

 

What is Pitted and Carbonized?

▪️ Pitted

Small craters or dents formed on the contact surface due to electrical arcing each time current flows.

▪️ Carbonized

When repeated arcing burns the contact surface, leaving blackened, non conductive residue. Both of these increase contact resistance, leading to voltage drop, heat buildup, and failed startups.

 

Check Wire Routing

Finally, inspect the wire routing throughout the unit. Wires must not touch fan belts, pulleys, coils, or sharp metal edges. Constant vibration can cause insulation to wear away and eventually short to ground.

 

If wires are tied together with refrigerant lines, the heat transfer can soften or melt the insulation. Always separate electrical wiring from hot refrigerant piping, and secure everything along a stable, low vibration path using cable clamps and insulation guards.

Below is a checklist summarizing the points mentioned above. 

Rooftop Unit Electrical Maintenance Checklist

Section	Inspection  Name	Checkpoint	OK	NG
Panel	Power Isolation	Disconnect switch turned OFF, power completely isolated
	Panel Cleanliness	No dust, corrosion, or debris inside the panel
	Wire Tightness	All wires are firmly secured; no loose connections
	Wire Organization	Cables neatly arranged; no dangling or tangled wires
	Internal Cleaning	Dust removed using compressed air
Compressor	Insulation Condition	No cracks, pressure marks, or melted insulation
	Discoloration	Copper conductors not dark brown or black (no overheating)
	Terminal Tightness	All terminals properly crimped and tightened
	Overheating Signs	No bluish discoloration or burnt smell at terminals
	Vibration Interference	Wires not touching fans, pulleys, brackets, or sharp metal edges
	Wire Protection	Cable clamps and rubber bushings installed in vibration zones
Blower	Insulation Condition	No cracks, pressure marks, or melted insulation
	Discoloration	Copper conductors not dark brown or black (no overheating)
	Terminal Tightness	All terminals properly crimped and tightened
	Overheating Signs	No bluish discoloration or burnt smell at terminals
	Vibration Interference	Wires not touching fans, pulleys, brackets, or sharp metal edges
	Wire Protection	Cable clamps and rubber bushings installed in vibration zones
Contactor	Contact Surface	Contact surface is smooth and clean
	Discoloration / Carbonization	No dark brown color or carbonized surface
	Surface Contamination	No black dust or rough residue when touched
	Terminal Tightness	All contactor terminals are tight and secure
	Physical Clearance	No contact between wires and fan belts, pulleys, or metal edges

 

Compressor Electrical Check

Once the visual inspection is complete, it’s time to move on to the electrical check, measuring both voltage and amperage. Every compressor, condensing unit, rooftop unit (RTU), or package unit used in HVAC systems has a manufacturer’s nameplate on the exterior of the cabinet. This label lists the electrical ratings of the compressor, and the two most important ones to verify are RLA and LRA.

 

RLA (Rated Load Amps)
The rated current the compressor draws under normal operating load. This value serves as the reference for determining whether the compressor is running at normal load or under overload.

 

LRA (Locked Rotor Amps)
The maximum current drawn by the compressor at startup the instant the rotor is locked. This value is useful for diagnosing start up load issues or checking whether the contactor and relay are properly sized.

 

Measuring Average Current

While the compressor is running( he unit must be operating in cooling mode), set your clamp meter to Amp mode and measure one phase at a time by clamping around each conductor L1, L2, and L3

 

Average = (L1 + L2 + L3) / 3

 

If this average current is within ±10% of the rated RLA, the compressor is operating normally.

 

For example:
If RLA = 12.0A, acceptable range is 10.8A to 13.2A.

A lower amp reading means the compressor is doing less work than normal (underload), (possible low refrigerant or low airflow),
while a higher amp reading means it’s doing more work than normal (overload), (overcharge, restricted condenser, or high head pressure).

 

Checking Current Imbalance (%)

A three phase motor operates properly only when all three phases carry balanced current. If one phase draws higher or lower current than the others, the internal magnetic field (flux) becomes distorted, causing vibration, noise, and overheating. Over time, this leads to bearing wear, insulation breakdown, and winding damage inside the compressor.

 

The degree of this imbalance is expressed as Imbalance Percentage (%):

 

Imbalance (%) = (Highest amp reading − Average amp) ÷ Average amp × 100

 

Recommended limits,
≤ 5% : Ideal (Normal)

≤ 10% : Acceptable in field operation

 

If the imbalance exceeds 10%, suspect electrical unbalance or mechanical issues such as bearing wear, wiring defects, or power supply irregularities.

 

Checking the Blower Motor

The same procedure applies to the blower motor. Measure current for each phase, calculate the average, and compare it to the FLA (Full Load Amps) rating on the nameplate. If the average current is within ±10% of FLA, operation is normal.

 

If it’s lower, the blower is underloaded, which may indicate low airflow, loose belts, or low system pressure.

If it’s higher, check for issues such as restricted airflow, dirty filters, or excessive belt tension.

 

Contactor Voltage Check

Finally, measure the voltage across the contactor terminals. This step confirms whether the power supply is balanced and whether the contact points are in good condition without voltage drop. Set your multimeter to AC Voltage mode, and use the two test leads to touch each pair of terminals (for example, L1–L2, L2–L3, and L3–L1). This gives you the line to line voltage readings.

 

Line Side (Input)
Measure L1-L2, L2-L3, and L3-L1.
Normal voltage should be 208V or 460V ±10%, depending on the system.

 

Load Side (Output) 
Measure the lines going out to the compressor or fan.

Measure L1-L2, L2-L3, and L3-L1.
The voltage difference between input (Line) and output (Load) should be within 1-2 volts. If the difference is greater, the contactor contacts are likely pitted or carbonized.

 

 

Rooftop Unit Electrical Check Sheet

Section	Measurement	Note				OK	NG
Compressor	RLA (Rated Load Amps)
	Measured Current (Running Amps)	L1:	L2:	L3:	(L1+L2 +L3)/3
	Current Imbalance	(Highest amp reading − Average amp) ÷ Average amp × 100
	Connections / Bearings / Power Condition
Blower	FLA (Full Load Amps)
	Measured Current (Running Amps)	L1:	L2:	L3:	(L1+L2 +L3)/3
	Current Imbalance	(Highest amp reading − Average amp) ÷ Average amp × 100
	Vibration / Noise / Heat
Contactor	Line Voltage	L1–L2:	L2–L3:	L3–L1:
	Load Voltage	T1–T2:	T2–T3:	T3–T1:

 

In the following post, you’ll find a calculator that performs these checks automatically. Click here to see it in action.