Episode Description:
A secondary power loop can look completely healthy while creating problems that technicians chase for months.
A valve struggles to reach set point. A controller randomly reboots. An actuator behaves inconsistently. Nothing trips, and the panel looks normal.
If you work in building automation, these symptoms can point to an electrical load problem that was built into the system long before the service call arrived.
In Episode 562 of The Smart Buildings Academy Podcast, you’ll learn how to think through secondary power loops before installation and how to evaluate existing loops when intermittent problems start appearing.
The goal is not simply to make the equipment work today. It is to build power systems that remain reliable as devices are added, loads change, and systems operate under real conditions.
Topics Covered
• Why overloaded secondary power loops can be difficult to recognize in the field
• What needs to be considered when estimating transformer capacity
• How startup conditions and voltage drop affect BAS devices
• Common secondary power loop mistakes that create intermittent failures
• How field measurements can validate your design assumptions
A little planning can prevent years of troubleshooting problems that never look like power problems.
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Some of the most difficult building automation problems do not trigger an obvious failure.
A breaker does not trip. The panel looks normal. The controller LED stays green. Yet an actuator struggles to reach its commanded position, a controller occasionally reboots, or a zone consistently misses set point.
These symptoms can lead technicians toward programming, communications, controllers, actuators, or mechanical equipment. The actual problem may be the secondary power loop.
An overloaded or poorly designed secondary power loop can operate well enough to avoid an obvious failure while still creating unstable conditions throughout the system. Understanding how to estimate electrical loads, account for real operating conditions, and verify the completed installation is an important BAS engineering skill.
A secondary power loop can be undersized without completely failing.
That distinction matters.
When a circuit fails completely, troubleshooting has a clear starting point. When voltage only sags under certain operating conditions, the symptoms can appear unrelated to the power supply.
You may encounter:
A technician checking the panel during a low-load condition may see acceptable voltage and conclude that power is not the problem.
That is why secondary power loop design needs to begin with a load estimate rather than a transformer selected based on convenience or what happens to be available.
Before sizing a transformer, identify everything that will receive power from the secondary loop.
This includes controllers, actuators, relays, transducers, sensors that require external power, and other active devices. Planned devices should also be included when future expansion is already known.
Spare capacity deserves consideration as well. Building automation systems change. Additional devices are often connected to existing panels years after commissioning.
For each device, record its running VA from the manufacturer's nameplate or technical documentation.
Avoid estimating a value when the documentation is unclear. A small assumption repeated across several devices can significantly distort the final load calculation.
Passive devices should not be assigned an artificial electrical load. Record them appropriately so the device inventory remains complete without inflating the calculation.
Once every active device has been documented, sum the running VA to establish the base load.
The transformer should not be sized to match the calculated load exactly.
A design that consumes the transformer's full available capacity leaves little room for operating variation, additional devices, or other real-world conditions.
A practical approach is to add 25 percent to the calculated running load and then select the next appropriate standard transformer size.
For example, if the connected running load totals 30 VA:
30 VA × 1.25 = 37.5 VA
The transformer would then be sized to the next appropriate standard capacity rather than selecting a transformer that barely meets the calculated load.
Common transformer sizes may include 20, 40, 50, 75, and 100 VA, depending on the equipment and application.
When the required capacity grows beyond the practical limits of a Class 2 secondary loop, the solution should not simply be a larger transformer. Consider splitting the loop into functional sections.
Dividing loads logically can improve reliability, troubleshooting, documentation, and future serviceability.
A transformer can appear adequately sized based on steady-state load and still experience problems when equipment starts.
Coils, motors, actuators, and similar loads can require greater current during startup than they consume while operating normally.
If several devices energize at the same time, their combined inrush can create a temporary condition that the transformer cannot support effectively.
This is why a design needs to consider which loads may start simultaneously.
The steady-state calculation answers one question:
Can the transformer support the equipment while it is running?
The inrush evaluation answers another:
Can it support the equipment while those loads are starting?
Both conditions matter.
A system that operates normally after startup but experiences resets or erratic behavior every morning may be telling you that the steady-state load is acceptable while the startup condition is not.
Transformer capacity is not the only electrical consideration.
A correctly sized transformer can still produce poor device performance if excessive voltage is lost along the conductor.
Voltage drop becomes increasingly important as conductor length increases, wire size decreases, and current rises.
One common calculation mistake is using only the one-way conductor distance.
The electrical path includes the outgoing and return conductors. If the device is located 200 feet from the panel, the voltage-drop calculation needs to account for the complete circuit path.
Ignoring the return conductor can produce a calculated voltage drop that is roughly half of the actual value. The design may look acceptable on paper while the installed device receives inadequate voltage.
For a nominal 24 VAC system, the goal discussed in Episode 562 is to ensure the farthest device receives at least 21.6 volts under load.
This makes conductor sizing and circuit length part of the power-loop design, not an afterthought.
Measurements are only useful when they represent the conditions that produce the problem.
An unloaded transformer may read higher than its nominal voltage. A technician measuring a 24 VAC circuit while most equipment is idle may therefore see a healthy reading.
That measurement does not tell you what happens when the system is loaded.
Measure the secondary voltage at the panel and at the farthest device while the connected equipment is operating. Comparing these measurements helps identify voltage loss between the transformer and the load.
Current should also be measured under operating conditions. A clamp meter on the appropriate secondary conductor allows the technician to compare actual current against the design estimate.
The calculation establishes what the system should do.
The meter establishes what the system is actually doing.
Both are necessary.
Transformer relationships can create another category of failures.
If a device has separate power inputs intended to remain isolated, maintain that isolation. Do not bond transformer commons unless the manufacturer specifically documents that configuration as acceptable.
Improperly connecting transformer commons can damage equipment and create faults that are difficult to diagnose after the failure occurs.
Consistent phasing and a single intentional ground reference also help maintain predictable electrical behavior throughout the BAS installation.
These details may appear minor during installation, but they can determine whether a system remains stable over its operating life.
A consistent process reduces the chance that important electrical considerations will be missed.
For every new or existing secondary power loop:
Most of this work happens before equipment is purchased or installed.
The final step happens in the field.
That final measurement converts an engineering estimate into documented operating data.
Several mistakes repeatedly create problems in building automation systems.
One is sizing based on an incomplete understanding of device current requirements. A system can appear stable during normal operation while struggling during startup.
Another is calculating voltage drop using only the distance from the panel to the device. Forgetting the return path creates an inaccurate result.
Existing systems also become vulnerable when new devices are added without recalculating the load budget. The technician adding the new device may have no information about how much spare transformer capacity was originally available.
Improperly connecting transformer commons can introduce equipment damage and difficult electrical faults.
Finally, measuring voltage only while the system is idle can hide the exact voltage sag responsible for the complaint.
Each mistake has the same underlying problem: the system is being evaluated without considering how the complete circuit behaves under actual load.
Once the loop has been tested, record the results.
Document the measured secondary voltage at the panel, voltage at the farthest device, and actual operating current. Include the information on the panel schedule or other appropriate project documentation.
This gives the next technician a baseline.
If another device is added two years later, the technician can see how much load existed when the system was commissioned. If a voltage problem develops, measured commissioning values provide a useful comparison.
Without documentation, every future technician starts from zero.
Estimating a secondary power loop does not require an elaborate engineering process. It requires disciplined electrical fundamentals.
Identify the loads. Calculate the demand. Add headroom. Evaluate inrush. Check voltage drop. Maintain proper transformer relationships. Then measure the completed system under operating conditions.
These steps can prevent failures that otherwise appear as intermittent controller problems, slow actuators, premature equipment failures, and zones that never quite maintain set point.
The difference is not whether the system powers up after installation.
The difference is whether it continues to operate reliably when every connected device is doing the work it was designed to do.
Listen to Episode 562 of The Smart Buildings Academy Podcast for the complete discussion on estimating electrical loads for secondary power loops and applying these principles to building automation systems.
For a deeper discussion and insights from the field, listen to this episode on the Smart Buildings Academy podcast.