How to Read HVAC Wiring Diagrams? (Control Panel)

Wiring diagrams are basic HVAC drawings that every engineer must learn to read. They contain information about the electrical aspect of the HVAC system. So, how do you read HVAC wiring diagrams?

To read HVAC wiring diagrams, start by identifying the HVAC equipment connected to the control panel and the power requirement. Then, find the circuit breaker and wire size. Afterward, determine the type of starter used to power the equipment and finally, check the control circuit to see the equipment operating sequence.

For people who have never seen an HVAC wiring diagram before, they may not understand the symbols and icons used. Hence, I’ll explain section-by-section on how to read HVAC wiring diagrams.

Single Line Diagrams

Single line diagrams are one of the HVAC wiring diagrams. They show the wire (cable) size, the equipment load and specifications of important electrical components such as MCCBs, the earth leakage protector and starters.

Cable Size

Usually, cable sizes are indicated in electrical single line diagrams together with how they are run (by conduit/trunking/cable tray). There is a standard way of writing the cable size in single line diagrams.

Cable sizes often start with the quantity followed by the number of cores, the cable cross-sectional area, the cable type (fire-rated/PVC/armour/screen), the earth wire and the cable tray/conduit/trunking.

For instance, 6x1C / 4mm2 FR CABLE C/W E / WIRE ON CABLE TRAY read as:

Six numbers of one-core four square millimeters fire-rated cable complete with earth wire on cable tray.

In another word, it means that the equipment uses 6 nos. of single-core 4mm2 cross-sectional area fire-rated cable. It follows by “complete with earth wire” meaning that the earth wire size was not specified and thus, you can decide an appropriate size for the earth wire.

Usually, this equipment will end up using 6 nos. of 4mm2 for the power cable and 1 no. of 2.5mm2 as the earth wire (earth wire is usually one size smaller than power wire). So, a total of 7 cables.


As the name suggests, starters are devices that startup the HVAC equipment. Different HVAC units required different types of starters.

By looking at the cable quantity and size, immediately we know that it is connecting to a star-delta starter because only star-delta starters required 6 cables instead of 3 cables (excluding earth wire).

A star-delta starter is usually used for medium capacity fans. Low capacity fans are using a DOL or direct-online starters. As for large capacity fans like the one shown in the above drawing (30 kW), an auto-trans or auto-transformer starters is used.

Equipment Tag and Power Requirement

As mentioned earlier, you can see that SFAF-B2-Z1-1 with 30/7.5 kW is using an auto-trans starter while SFAF-B2-Z1-2 with 4.0/1.0 kW is using a star-delta starter. One quick test here, what is SFAF?

If you remember from my post How to Read HVAC Duct Drawings?, SFAF is likely an acronym for Smoke Spill Fresh Air Fan. You see, there are many ways to write a tag name for the equipment. However, they are more or less follow the same pattern.

Since it is a smoke spill fan, it needs to run in two modes. Hence, we have two different power requirements. Nevertheless. the cable must be sized based on the higher power requirement.

Circuit Breaker

Circuit breakers have two types; a) molded case circuit breaker (MCCB) and b) miniature circuit breaker (MCB). MCCBs are used to break large currents while MCBs are used to break small currents in case of an overload to protect cables.

Standard circuit breaker sizes are 6A, 16A, 20A, 32A for MCBs and 40A, 50A, 65A, 75A, 80A, 100A for MCCBs. Certainly, there are other breaker sizes which I didn’t mention. I just want to let you know that breaker have standardized sizes.

The following table is the common acronyms used for circuit breakers:

MCCBMolded Case Circuit Breaker
MCBMiniature Circuit Breaker
SPSingle Pole
SPNSingle Pole + Neutral
2P or DPTwo Pole or Double Pole
TP or 3PThree Pole
TPNThree Pole + Neutral

Apart from MCCB, TPN and 100A shown in the above drawing, the maximum current breaking capacity is usually included. The maximum current breaking capacity is expressed in kA. The higher the kA (kilo Amp) rating, the more expensive the MCCB/MCB.

If an overload current passes through a 100A MCB with 25kA is 30,000A, the MCCB will trip but it may be damaged because the overload current is more than the maximum current breaking capacity of the MCCB.

Electrical Monitoring Device

Good HVAC control panels should have an electrical monitoring device such as a voltmeter, an ammeter or a digital power meter. Panels with a monitoring device allow users can see the electrical load without using a measuring device such as a clamp meter.

A voltmeter/ammeter combo also have a voltmeter switch and an ammeter switch. In the above drawing, the voltmeter has a maximum range of 500V which is appropriate for 415V equipment. The ammeter reading range should consider the starting and running current of all associated equipment.

Earth Leakage Protection

Panels should always have earth leakage protection. It can be an ELR (earth leakage relay) or RCCB or ELCB depending on the equipment load and the project specification. Other types of earth leakage protectors may not be drawn the same as the above drawing but they are all located at the same position which is after the main circuit breaker.

Total Connected Load

Total connected load (TCL) is the total load possible in a panel. It’s usually for electrical engineers within a project to calculate their cables, breakers and supplies.

If your panel has an interlocking control that only allows one fan to run at a time, the TCL should be the maximum load of that one fan instead of the sum of the two fans. However, if you have any sort of controls that may enable two fans to run at the same time, the TCL should be the sum of the two fans.

Control Circuit Diagrams

The control circuit diagram shows how the system is controlled. Electrical components such as overload relays, selector switches, indicating lamps and timers are found in the control circuit diagram.

Following is an HVAC outdoor air fan control circuit diagram with the function of each component labeled:

HVAC outdoor air fan control circuit diagram explained

A typical control circuit diagram starts with the control MCB. Then, a fire relay is usually included to trip the entire control and thus, stop all fans and air conditioners in case of a fire. When it comes to individual equipment control circuits, the components are shown in the above drawing.

O/L is an overload relay that cuts off the respective control circuit and therefore, stopping the outdoor air fan when there is too much current flowing through. Auto/off/manual selector switches are very common in HVAC control panels for users to toggle between auto mode and manual mode.

Each component is labeled in the above drawing and I’ll skip on individual explanation. Instead, let’s take a look at the actual control logic in 4 different scenarios.

Control logic of different scenarios

Scenario 1: Manual Start

In this scenario, the current flowing through the control circuit is normal and hence, the overload relay is not tripping. Then, the selector switch is toggled to the manual position which allows the fan to run when the start button is pushed. The SP or stop button may also be an emergency stop button.

So, the current flows all the way to the LC1 which can be a contactor that energizes to provide power to the outdoor air fan. At the same time, the run indicating lamp will lid.

Scenario 2: Overload Trip

If there is too much current flowing through the overload relay, it’ll trip and stop the operation of the outdoor air fan. At the same time, it’ll trigger and lid the trip indicating lamp. With a tripped overload relay, pressing the start button or the selector switch won’t have any effect as you can see from the above drawing.

Scenario 3: Auto Timer

By default, the outdoor air fan should be running in auto mode using a timer. So, the selector switch is toggled to auto position. There is an open in the control circuit expressed as T/S (time switch). Hence, the outdoor air fan will activate only when the 24-hrs time switch gives the signal to close the control circuit.

Scenario 4: Remote Start

Notice that when the selector switch is toggled to the manual position, there are two methods to start the outdoor air fan. One way is to manually press the start button on the control panel like what we discussed in scenario 1. The other method is to give a signal to close LC4-1.

The signal can be a remote dry contact signal which the signal source could be located in the building control room, far away from the control panel itself. Hence, we called it remote signal or remote start.

Panel Layout Drawing

If you’re dealing with a control panel that powers the HVAC unit, you’ll have an additional page of drawing which is the panel layout drawing. It shows the construction of the control panel.

The panel layout drawing usually has 4 views; a) front view, b) side view, c) top view, and d) inside view. Here is an example of the panel layout drawing with almost everything labeled and the top view is excluded:

HVAC panel layout drawing explained

How all these electrical components are arranged is heavily dependent on the panel manufacturer. A good panel manufacturer knows how to use the panel space effectively and thus, not wasting on big size panels. However, they more or less are following the same principle.

Generally, the incoming power supply indicating lamps and ammeter/voltmeter is located at the top of the panel. Running/tripping lamps are located at the center of the panel while selector switches are put at the lowest position.

From the panel layout drawing, we know that this panel is a wall mounted type because of the brackets. If your panel has a “hat” or double-door, you may have a weatherproof floor mounted outdoor panel.

The main incoming breaker is usually located at the top of the panel. A large power panel often has a big MCCB on the side in a separate compartment. Some panels have a built-in thermostat and ventilation fan to dissipate heat through the louvre.

Notice that there is a separate compartment for BMS (building management system) to tap on the signal of the equipment. Usually, such a design can avoid disputes and arguments by clearly splitting the scope of work.

Other acronyms used in HVAC control panels:

TOLThermal Overload Relay
DOLDirect Online
A/TAuto Transformer
VSDVariable Speed Drive
ELREarth Leakage Relay
RCCBResidual Current Circuit Breaker
ELCBEarth Leakage Circuit Breaker
EF/OCEarth Fault Over Current
MCBMiniature Circuit Breaker
MCCBMolded Case Circuit Breaker
RYBRed Yellow Blue
VacVolt Alternating Current
VdcVolt Direct Current
FRFire Relay

More posts on how to read HVAC drawings:

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