Sawmill or joinery on a small connection: what works?

Often, yes. Many joineries, small sawmills and concrete plants do not need more energy, they need more power at a few moments. First stagger the starts and put drives on the extraction fan and the heaviest motors. Then a semi off grid system can carry the peaks from a battery, with your existing connection as the charger. When the heavy load runs flat out all day, a bigger connection or a cabin stays the honest answer.

You run a joinery, a sawmill or a concrete plant. At seven in the morning the extraction starts, the planer follows, someone switches on the panel saw and the compressor kicks in. The main breaker trips, or the lights dip. Or the grid operator writes back about the new moulder or the bigger mixer. You need a heavier connection, maybe a cabin of your own. Owners describe the same wall online. A weekend woodworker started "the dust collector and switched on the table saw right behind it". The inverter shut off before the saw was up to speed. An electrician on a French forum wanted to wire an old sawmill with a 30 hp motor. The answer: "on ne peut pas démarrer le gros moteur en direct", the big motor cannot start direct on line.

What hurts is what you do not see on the bill. You risk paying for years of reserved power because four machines happen to start in the same minute. And the extraction fan, the one machine nobody thinks about, runs all day at full speed, even when only one saw is cutting.

A flax-processing company in Wielsbeke faced the extreme version: one heavy machine meant a large medium-voltage cabin on paper. Today a generator starts that machine and keeps running while it works, as is also common at a concrete plant. The generator is loaded a little extra to charge the battery. With 3 x 8 kVA of charging power and 30.8 kWh of storage, it supplies the rest of the day. This article looks at the whole site: what drives the peak in these three trades, and what fixes it in which order.

Why does a joinery, sawmill or concrete plant hit the limit of its connection?

Because the power comes in short, heavy blocks, not as a steady flow. A few big motors start close together, the extraction runs with all of them, and the connection has to carry the sum at that moment. The rest of the day the same site may use a fraction of it.

Take an example, not a customer case. A joinery has an 11 kW extraction fan, a 7.5 kW planer and a 5.5 kW moulder. Add a 5.5 kW panel saw and a 5.5 kW compressor. Running together, that is about 35 kW, plus lighting, heating and the office. On a connection of 3 x 400 V 63 A, which delivers about 44 kVA, that already leaves little room. Now add the start currents of the next paragraph, and the breaker has no margin left.

In Flanders, Fluvius decides between low and medium voltage for a connection between 25 and 250 kVA. Above 250 kVA it is always medium voltage with a customer cabin (Fluvius connection tariffs). What each grid level changes for your business is explained in our article on low voltage or medium voltage for a business.

What uses the most electricity in a joinery or sawmill?

Usually the dust extraction, because it runs the longest at the highest steady load. A planer or a saw cuts for minutes at a time. The extraction fan often runs from the first cut to the last, at full speed.

That makes two different questions, and many owners mix them up. Energy (kWh) is what you use over the day; power (kW) is what you ask at one moment. The extraction mainly drives your energy bill. The starts and the moments when everything runs together drive your peak, your breaker and the connection size.

• Extraction fan: the biggest steady load in most joineries; it runs as long as any machine runs.
• Compressor: starts and stops on its pressure switch, often many times per hour, and every start is a small peak.
• Planer, moulder and saws: short heavy loads, with a start current well above their running current.
• Sawmill extras: chippers and debarkers with heavy starts, conveyors that run for hours, and drying fans that can run day and night.
• Concrete plant: the mixer and the belts in short cycles, one batch after another, all day long.

How do starts and shifts build your quarter-hour peak?

Your grid bill in Flanders counts your highest quarter-hour of the month, so what matters is what runs together for fifteen minutes. A start of a few seconds weighs little on that average. What weighs is the extraction plus three machines running through the same quarter-hour.

Fluvius defines the monthly peak as "the highest quarter-hour power of the month". For small businesses with a digital meter, the bill uses the average of twelve monthly peaks, with a minimum of 2.5 kW (Fluvius, capacity tariff). Sites on medium voltage pay per month on half their reserved capacity and half their actual monthly peak (Fluvius, peak metering). How to calculate it for your site is explained in our article on the capacity tariff for businesses.

Shifts make the pattern predictable. The worst quarter-hours are the start of the day and the minutes after each break, when everyone switches on at once. One cold morning in January, with the heating on too, can set the peak you pay for the next twelve months. Our article on peak demand in a business shows how to find those moments in your own data.

What can you do before adding a battery or a bigger connection?

Tame the starts, slow the extraction and spread the loads first. These steps cost far less than extra power, and they make any battery you add later smaller.

We fit frequency drives ourselves as part of a project. At a plant nursery in Jabbeke we placed them on the heaviest pumps so they would run smoothly. The physics of start currents, star-delta and soft starters is explained in more depth in our article on the start-up current of a heavy machine.

When does a battery or semi-off-grid system carry the rest?

When the peaks are short and the connection can refill the battery in between. Then a battery with inverters delivers the extra power at the busy moments, and your existing connection charges it during breaks, evenings and nights.

That is how a semi-off-grid system works for a small or medium site on 3 x 400 V. It is built for connections up to 3 x 400 V + N, 63 A. In Jabbeke the nursery kept its 3 x 400 V 32 A street connection. It now serves only as a charger. The system of 45 kVA and 92.4 kWh delivers a heavier 63 A supply for the pumps. The full calculation is in our article on more power without a grid upgrade.

Two limits decide the size. The inverters must carry the highest moment, including the starts that are left after the drives. The battery must carry the energy of the busy hours minus what the grid refills. Solar panels on a large workshop roof help during the daytime shift. But the peak you pay for often falls on a dark winter morning.

Bigger site, or already on a heavier connection? Then a business battery behind the connection can shave the peaks instead, in modular systems up to 1 MW. How peak shaving works is explained in our article on peak shaving with a battery.

That generator route suits a sawmill or a concrete plant with one dominant machine, for example on a site far from a strong grid. How a generator and a battery share the work is explained in our article on a generator with a battery. In Maasmechelen a generator that was only kept for emergencies became a charger for the semi-off-grid system, next to the street connection.

When is a bigger connection or a medium-voltage cabin still the right answer?

When the heavy load runs most of the day, every day, and keeps growing. A battery moves energy in time; it does not create it. If the site needs close to full power for eight hours or more, that energy must come from the grid or a generator.

Whether building your own cabin pays off is the question of our article on a medium-voltage cabin or a battery. Already on medium voltage, with a transformer that is too small for a new line? Read our article on a cabin transformer that is too small.

What do we need to size a system for your workshop or plant?

We need your machine list, how they start, when they run, and your meter data. With that we can tell you what drives your peak, what a drive or a sequence changes, and whether a battery is still needed.

1. Step 1: Send us the list of motors with their rating plates (kW, amps) and start method: direct, star-delta, soft starter or frequency drive.
2. Step 2: Describe a normal day: shift times, breaks, which machines run together, and how the extraction and the compressor are switched.
3. Step 3: Add your connection size and a year of quarter-hour data from your digital meter, or your peak readings.
4. Step 4: Where possible we measure the site over a period, so inverters, battery and cables are sized on real numbers.
5. Step 5: We put the options side by side: drives and gates, a semi-off-grid system, a business battery, or the heavier connection. You can also request a quote for frequency drives on their own.