How to Power a DC-Input PoE Switch from a 24V Bus: Current and Voltage-Drop Checks

Define the boundary: converter power is not PoE port power

The most important early decision is where the power budget begins and ends.

In a normal PoE system, the switch is the PSE and the connected camera, access point, radio, terminal, or sensor is the powered device (PD). Cisco’s current Catalyst PoE configuration guidance describes detection, classification, per-port allocation, and total available power budget for that implementation. Microchip’s official PoE terminology overview uses the same PSE/PD distinction. An IEEE 802.3 working-group PoE system presentation also illustrates the separate PSE and PD roles; it is useful background, but it is not a substitute for the applicable standard or the switch manufacturer’s compliance documentation.

The upstream converter sees the complete switch as one DC load. Prefer a switch-manufacturer value measured or specified at the switch DC-input terminals that already includes the planned PoE allocation. That keeps port-side PSE power, Ethernet-cable loss, internal conversion loss and switch electronics from being mixed across different reference planes.

Avoid double counting. If the switch manufacturer gives a maximum DC-input power or current that already includes its full PoE budget, use that complete input requirement once. If the manual reports switch consumption and port allocation separately, do not automatically add them: first identify whether each value is defined at the PD, PSE port, internal DC rail or upstream DC-input terminals. Apply only the loss or efficiency relationship documented for that exact switch and configuration. If the relationship is absent, request it rather than inventing a conversion factor.

Build a port budget that reflects configuration, not averages

Average field consumption is useful for energy estimates, but it is usually a weak basis for converter capacity. A camera may draw less most of the day and more when heaters, illuminators, motors, or radios operate. A switch may reserve power according to configuration or classification rather than the momentary value shown on a dashboard. Start with the exact switch manual and the maximum allocation that the intended configuration can request.

Use a table like this during requirements review:

The numbers above are hypothetical; they are not ratings for any particular PoE class, switch, or connected product. Their purpose is to show how a buyer can document the load instead of multiplying every physical port by a generic wattage.

Use one reference plane. Let P_switch_dc,max be the exact requirement at the switch terminals, P_48_link_loss,max the verified worst-case loss from converter to switch, and P_other,converter_out any other load expressed at the converter output. Then:

P_converter_target = P_switch_dc,max + P_48_link_loss,max + P_other,converter_out

For a resistive 48V link, check P_48_link_loss,max ≈ I_48_link,max² × R_48_loop y V_converter_out = V_switch_input + I_48_link,max × R_48_loop with values from the same worst-case condition. Confirm the switch-terminal voltage window. Set link loss to zero only when it is verified negligible.

If the project applies a design factor k_design, document what it covers and confirm that it does not duplicate an allowance already included in the switch specification:

P_converter_screen = k_design × P_converter_target

There is no universal factor that fits every switch. A factor may account for planned expansion or uncertainty, but it must not duplicate allowances already built into the switch’s maximum-input specification. Startup and temperature should be checked against time-dependent and derating data, not hidden inside an unexplained percentage.

Translate the 48V load into the worst-case 24V-bus demand

The input current can be much higher than the output current because the converter draws from a lower-voltage bus. A nominal 24V label is insufficient: the calculation needs the minimum voltage at the converter terminals while the load is present.

For an initial power balance:

I_48 = P_converter_target / V_48_design

I_bus ≈ P_converter_target / (η_assumed × V_converter_input)

where η_assumed is an efficiency justified for the actual input voltage, load, and temperature. A single headline efficiency value is not an efficiency curve.

Cable and connection loss makes the problem iterative:

V_converter_input = V_source_min − (I_bus × R_loop)

R_loop must include both current paths and relevant connection resistance. Because I_bus depends on V_converter_input, solve the two equations together or iterate until the values converge. For a long feeder, a battery near its lower operating limit, or a current-limited DC supply, this calculation can change the model decision.

Also estimate conversion loss for enclosure planning:

P_converter_loss ≈ P_converter_target × (1 / η_assumed − 1)

This is only a first-order estimate. Verified thermal and derating data for the exact mounting condition must decide whether the enclosure and ambient conditions are acceptable.

Worked example—with assumptions that must be replaced

Assume the exact switch manual specifies a 180W maximum at its DC-input terminals, including the planned PoE allocation. For this example only, the adjacent converter’s 48V link loss is verified negligible and no other output load exists. Therefore P_converter_target = 180W; the illustrative port total is not added again.

P_converter_target = P_switch_dc,max = 180W

At a 48V planning point, the corresponding output current is:

I_48 = 180W / 48V = 3.75A

Now assume, only for a preliminary comparison, constant 0.95 converter efficiency and constant-power load behavior over the relevant range. Assume the source can fall to 21.3V under the operating condition and that the measured or otherwise justified total loop resistance is 0.12Ω. Solving:

I_bus = 180W / [0.95 × (21.3V − I_bus × 0.12Ω)]

gives a low-current mathematical root of approximately 9.39A. That is the only root in the assumed operating domain; the other mathematical root is physically invalid for this scenario. The estimated voltage at the converter terminals is:

21.3V − (9.39A × 0.12Ω) ≈ 20.17V

The first-order converter loss at the assumed efficiency is:

180W × (1 / 0.95 − 1) ≈ 9.47W

This example is not a recommendation to purchase, wire, fuse, or commission a particular converter. It depends on a fictional switch requirement, a measured-or-justified feeder resistance, constant assumed efficiency, constant-power behavior and a steady-state equation whose low root remains inside the assumed input domain. It does not prove converter stability, startup behavior, transient response, thermal capacity, electromagnetic compatibility, source current-limit interaction, battery-management-system behavior or performance during a bus interruption.

The example also shows why the minimum source voltage should not be entered directly into the converter comparison. The relevant value is approximately 20.17V at the converter terminals after the estimated feeder drop, not 21.3V at the source.

What three WEHO product pages establish—and what they do not

Official WEHO WH-B244810 product image from the published catalogue. Confirm the exact model, current revision and complete operating envelope before design release.

Para WH-B244805, the published product page lists a rated input of 24VDC, a rated output of 48VDC, 5A output current and 240W rated output power. Its other page-listed fields differ from the larger candidates: “Voltage Range (V)” is 18–32VDC, “Starting Voltage (V)” is 10VDC at “100% load,” efficiency is 92%, and “Input-Output” is non-isolated. These labels need exact-model clarification before use in an operating envelope.

Para WH-B244810, the published product page lists a rated input of 24VDC, a rated output of 48VDC, 10A output current and 480W rated output power. For the same exact model, the published page lists a field labelled “Voltage Range (V)” as 10–40VDC, a “Starting Voltage (V)” of 18VDC at “100% load,” efficiency of 95%, and “Input-Output” as non-isolated. In the row labelled “Short Circuit Protection,” the page’s only text is “Input requires adding a 25A fuse”; that text is not a complete system protection design.

Para WH-B244820, the published product page lists a rated input of 24VDC, a rated output of 48VDC, 20A output current and 960W rated output power. For the same exact model, the published page lists a field labelled “Voltage Range (V)” as 10–40VDC, a “Starting Voltage (V)” of 18VDC at “100% load,” efficiency of 95%, and “Input-Output” as non-isolated. In the row labelled “Short Circuit Protection,” the page’s only text is “Input requires adding a 45A fuse”; that text is not a complete system protection design.

These are page-listed values, not independent test results. They should be used to form questions for the supplier, not to fill evidence gaps by inference:

• The WH-B244810 and WH-B244820 pages list 10–40VDC in a field labelled “Voltage Range”; the WH-B244805 page instead lists 18–32VDC. Obtain current model documentation and confirm what operating conditions, load, temperature and transient limits each field covers.
• The WH-B244810 and WH-B244820 pages list an 18VDC start value at 100% load; the WH-B244805 page lists 10VDC at 100% load. These fields do not, by themselves, define operating shutdown, hysteresis, restart delay, recovery from a sag or behavior below the stated start value.
• The pages list 95% efficiency for WH-B244810/820 and 92% for WH-B244805. They do not provide complete efficiency maps across input voltage, load and temperature, so worst-case current and thermal review still require up-to-date supporting data.
• The published pages list “Input-Output” as non-isolated. That label alone does not establish the exact revision’s input-negative, output-negative, case or shield connections. Obtain the connection information and confirm compatibility with the switch, source, grounding plan, communications and applicable safety requirements.
• The pages place their external input-fuse statements in rows labelled “Short Circuit Protection.” Those short statements do not select fuse technology, location, interrupt rating, conductor protection or source fault coordination for a finished machine. Those decisions depend on the actual source, prospective fault current, conductors, installation rules and current manuals.

For the 180W hypothetical switch-input requirement, WH-B244805’s page-listed 240W and 5A output values make it an obvious screening candidate; its different voltage-range, start and efficiency fields also make exact-model evidence essential. WH-B244810 and WH-B244820 are higher-capacity candidates, but a larger rating does not prove better system fit and can imply greater source and protection demands. The correct question is which model, if any, passes the complete operating envelope and staged acceptance test.

Create the complete bus envelope before model selection

A useful RFQ gives WEHO more than “24V in, 48V out.” Record the following operating envelope:

1. Minimum bus voltage at the source and at the converter terminals. Include steady full-load conditions and relevant cable and connector drop.
2. Maximum bus voltage. Include battery charging, regenerative events, supply tolerance, and any credible transient condition identified by the source manufacturer.
3. Source behavior. Record continuous current capability, current-limit mode, battery-management-system limits, interruption behavior, and restart logic.
4. Switch startup demand. Obtain inrush, startup duration, input capacitance or manufacturer-specified source requirements, and the order in which the switch enables PoE ports.
5. Load changes. Consider simultaneous port enablement, device restart, heaters, illuminators, motors, radios, and future allocations.
6. Environment. Provide minimum and maximum ambient temperature, enclosure details, airflow or cooling method, altitude if relevant, contamination, vibration, and moisture exposure.
7. Electrical architecture. State the required isolation, permitted grounding scheme, EMC environment, and any machine, vehicle, railway, marine, or other compliance obligations.

A nameplate voltage and a summed wattage cannot replace this envelope. The converter, switch, feeder, source, and protection system interact most strongly at low bus voltage and during transitions—the same conditions that a simple nominal calculation hides.

Fit signals and stop signs

A candidate may be worth advancing when

• the switch’s complete DC-input range includes the converter’s verified output envelope;
• the measured minimum input at the converter and credible maximum input fit current model documentation;
• required output watts and amps fit the model with a documented project basis;
• the source, feeder, connectors, and protective devices can support the calculated low-voltage input demand;
• non-isolated conversion is acceptable in the reviewed system architecture;
• startup, sag recovery, thermal behavior, and load steps pass a staged acceptance test.

Stop and resolve the gap when

• the team expects the converter to add PoE capability to a non-PoE switch;
• the switch manual does not state whether maximum DC input includes the PoE budget;
• the actual 48V input window of the switch is unknown;
• the voltage at the converter terminals can cross an unverified startup or operating boundary;
• a fuse value copied from a product page is being treated as complete system protection design;
• the application requires galvanic isolation but the candidate page lists non-isolated construction;
• the converter would operate near a rating without verified temperature, load, and input-voltage data;
• environmental, EMC, or regulatory requirements have been assumed from marketing language rather than confirmed for the exact model.

Convert the design into a supplier-ready RFQ

Send one structured package rather than a generic request for a “24V to 48V PoE converter.” Include:

• exact PoE switch manufacturer, model, hardware revision, and manual;
• permitted switch DC-input range and maximum input current or power;
• whether that maximum already includes all PoE port power;
• port-by-port maximum allocation and simultaneous-use assumption;
• other loads sharing the 48V output;
• minimum and maximum voltage measured or specified at the converter terminals;
• source type, continuous current, current-limit or BMS behavior, and credible transients;
• feeder length, conductor information, connectors, and justified loop resistance;
• switch startup/inrush requirement and intended startup sequence;
• ambient range, enclosure, cooling, altitude, vibration, and moisture conditions;
• isolation and grounding requirements;
• EMC, safety, and market-access requirements;
• requested quantity, project stage, and acceptance-test conditions.

Ask for confirmation against the current datasheet and application guidance for the exact model. A useful response should close the unknowns around the voltage-range label, start and restart behavior, efficiency at the proposed operating point, output tolerance and transients, thermal limits, protection coordination, and environmental suitability.

Frequently asked questions

Does a 24V-to-48V converter turn a normal Ethernet switch into a PoE switch?

No. The converter supplies DC power to equipment that already has a compatible DC input. PoE detection, classification, allocation, and delivery must be performed by a compliant PSE such as a PoE switch or a separate injector designed for that purpose.

Can I size the converter from the switch’s PoE wattage alone?

Usually not. Include the switch’s own consumption and any shared 48V loads, unless the switch manufacturer provides a complete maximum DC-input requirement that already includes the port budget. Use the complete value once to avoid double counting.

Does a page-listed starting voltage define the shutdown threshold?

No. A start value is not automatically a continuous operating or shutdown threshold. The pages list different start values for the three models discussed here, but none of those fields alone defines operating threshold, shutdown threshold, hysteresis, restart or sag behavior. Confirm those functions from current exact-model documentation and testing.

Which WEHO model should I choose from the number of PoE ports?

Do not choose from port count alone. Use the switch’s verified DC-input requirement, complete voltage windows, worst-case converter-terminal input voltage, source capability, startup behavior, temperature and system architecture. The 240W/5A, 480W/10A and 960W/20A page-listed output ratings are screening inputs, not a complete suitability decision.

A better next step than guessing

If you are evaluating a 24V battery or industrial bus for a DC-input PoE switch, prepare the RFQ fields above and share them through the relevant WH-B244805 product page, WH-B244810 product page, or the WEHO contact page. Ask for exact-model confirmation and a wider catalogue review when required. The goal is not merely to find a converter with a larger wattage number; it is to prove that the source, feeder, converter, switch and PoE load remain compatible at the conditions that matter.