Load Cell Junction Box: What It Does, Wiring and How to Choose
Short answer: a load cell junction box (also called a summing box or summing card) connects two to four load cells to one indicator. It joins the cells in parallel so the indicator sees one combined signal, gives every cable a sealed terminal block, and, on trimmable boxes, lets a technician match the corners. For a four-cell floor, platform or livestock scale, the MNM Scales JB-4SS-ST stainless steel junction box is designed for 4 load cells. Load cells are quoted by phone: call for a quote with the capacity and type you need.
Shop the JB-4SS-ST Call (832) 290-3120 Request a load cell quote
Most of the junction box questions we get come from people building or repairing a scale: a livestock platform, a tank or hopper, a pallet deck, or a floor scale whose readings change depending on where the load sits. MNM Scales has supplied load cells, indicators and scale parts for over 30 years, and our technicians wire, trim and calibrate these systems in our Cypress, TX shop and on-site. One recent job was a scale kit install at a major logistics center near Houston’s IAH airport. Here is how a junction box works, how to wire one, and what to look for when you buy.
What a load cell junction box does
- Sums the load cells: each load cell’s excitation, signal and shield wires land on a common board, so one cable (the home-run cable) carries the combined signal to the indicator.
- Protects the connections: the enclosure and cable glands keep moisture and dirt off the terminals, which is where many scale faults start.
- Allows corner trimming: no two load cells have exactly the same output. A trimmable junction box uses potentiometers or resistors to reduce the output of the “hot” cells until every corner reads the same.
- Makes service easier: a technician can disconnect one cell at the box to test it without rewiring the whole scale.
Do you need a junction box?
A single load cell connects straight to the indicator. A junction box comes in when two or more load cells share one indicator, as on almost every floor scale, livestock scale, tank and hopper scale. A junction box is not included with bare load cells, so order one with the cells; complete kits such as the 5,000 lb floor scale kit and the 20K floor scale kit include the junction box, load cells, indicator and cable. Choosing the load cells themselves? See our load cell buying guide.
How to wire load cells into a junction box
- Mount the box on the scale frame where every load cell cable reaches it and the home-run cable reaches the indicator. Where you can, point the cable glands down so water runs off.
- Route each load cell cable through its own gland. Do not shorten a load cell cable; coil the extra length and secure it, because cutting the cable can change the cell’s calibrated output.
- Land the wires on each channel’s terminals: +excitation, −excitation, +signal, −signal and shield. Wire colors differ between manufacturers, so follow the color code on the load cell’s data sheet or certificate, not a generic chart.
- Connect the home-run cable to the output terminals and to the indicator. Six-wire cables add sense lines; if your indicator is four-wire, follow the indicator and junction box diagrams for the sense terminals.
- Seal it up: tighten every gland, plug any unused gland, and check the lid gasket before closing the box.
- Calibrate and trim: calibrate the indicator, then check each corner with a test weight and trim if needed (see how to calibrate a floor scale and LP7515 indicator calibration).
How to choose a load cell junction box
- Number of load cells: match the channels to your cells. Four-channel boxes are the most common because most platforms sit on four cells.
- Enclosure: stainless steel suits washdown areas, barns, docks and outdoor installations.
- Cable glands: check that your load cell and home-run cable diameters fit the gland size.
- Trimming: a trimmable box makes corner adjustment possible without swapping load cells.
- Size and mounting: make sure the box fits the frame space and that the lid can be opened for service.
| Product | What it is | Best for |
|---|---|---|
| MNM Scales JB-4SS-ST junction box | Stainless steel junction box / summing card designed for 4 load cells; PG7 cable glands; 7 1/8″ x 2 7/8″ x 1 5/8″ | Replacing a damaged box or building a four-cell scale |
| MNM Scales 5,000 lb floor scale kit | Indicator, four shear beam load cells, mounting hardware, junction box, cable and AC adapter | Warehouse and animal platforms you build yourself |
| MNM Scales 20K floor scale kit | Four shear beam load cells, junction box, LP7515 indicator, feet hardware and cable | Heavier floor and livestock platforms |
| MNM Scales LP7515 indicator | Replacement digital readout (replacement for Optima OP-902), RS232 output, AC adapter included; rated 4.9/5 by Amazon reviewers | Reading the summed signal from the junction box |
| Load cells | Single-ended beam, S-type, compression and other load cells | Call for a quote with the capacity, type and quantity |
Need a matching indicator? Browse scale indicators. For load cells, call (832) 290-3120 or request a quote with the capacity, type, cable length and quantity, and we’ll confirm the right cells for your build.
When the readings change by corner
The problems our technicians see most on multi-cell scales are load cells that go bad after heavy use, cut cables, and indicators where someone got into calibration mode or turned on the wrong setting by accident. If a scale reads differently depending on where the load sits, the junction box is the first place we look. Check for moisture or corrosion on the terminals, a loose wire, a cut or crushed cable, and a load cell that has shifted or been overloaded. Drying the box, replacing a cable or re-trimming the corners fixes many of these. Our technicians repair scales in our shop and on-site (see scale repair in Houston).
Calibration after installing or replacing a junction box
Any change to the junction box, a load cell or the home-run cable changes what the indicator sees, so the scale needs calibration afterwards. MNM Scales is TDA licensed with registered technicians, and in the Houston area we calibrate floor and industrial scales on-site for $295 plus $2 per mile, round trip, charged once per visit, with calibration priced per scale on top (see industrial scale calibration in Houston). Smaller scales can be mailed in, wherever you bought them; mail-in calibration is usually done within 1–3 business days after we receive the scale (see mail-in scale calibration).
Why buy load cell parts from MNM Scales
- One of the largest scale distributors, with load cells, junction boxes, indicators and cables from one company, since 1992 (over 30 years)
- In-house calibration and NIST calibration with a NIST-traceable certificate
- Order pickup at our shop, 21249 FM 529 Rd, Cypress, TX, by appointment
- Phone answered 24/7 at (832) 290-3120
Building or repairing a multi-cell scale?
JB-4SS-ST junction box Load cells Call (832) 290-3120 Request a quote
Frequently asked questions
What does a load cell junction box do?
It connects two or more load cells to one indicator. The load cells are wired in parallel on a common board so the indicator sees one combined signal, and trimmable boxes let a technician match the output of each corner.
Is a summing box the same as a junction box?
Yes. Load cell junction box, summing box and summing card are names for the same part.
Do I need a junction box for one load cell?
A single load cell connects directly to the indicator. A junction box is used when two or more load cells share one indicator.
Is a junction box included with load cells?
No, a junction box is not included with bare load cells, so order one with the cells. Complete floor scale kits include the junction box, load cells, indicator and cable.
How do I adjust a load cell junction box?
On a trimmable junction box, place a test weight over each corner in turn and turn the trim potentiometer for the corner that reads high until every corner reads the same, then recalibrate the indicator.
How much do load cells cost?
Load cells are quoted by phone. Call (832) 290-3120 or request a quote online with the capacity, type, cable length and quantity you need.