Diesel generator cable sizing decides whether the run to the load keeps the terminal voltage inside the maker band under full output. The procedure covers the 3% drop rule, the temperature correction and the record a third party will audit, so site crews can compare suppliers and request a scoped quote. Tesla Power uses the same calculation to review a cable schedule before a set is cleared for shipment. Three things are proved at once: the conductor cross-section carries the full load current, the drop stays inside the 3% limit, and the temperature correction keeps the insulation within rating. A glance at the schedule at the factory gate confirms none of them. The margin between the as-installed drop and the 3% ceiling is built from the run length, the load power factor and the ambient, and the engineer notes it beside every column, then signs the sheet on the day of the test rather than a month later.
- Read the schedule against the 3% drop rule before the cable is ordered. Our cable entry guide lists the glands and seals you need for an export enclosure.
- Record the load current and the run length for every circuit branch.
- Apply the temperature correction before you freeze the cross-section.
- Compare the schedule with the data sheet before you sign the handover. request the template we use if you need the exact columns.
What does diesel generator cable sizing actually decide?
A cable that is too small drops the terminal voltage below the maker band the moment the load steps in. It also heats the conductor past its insulation rating and shortens the life of the run. A cable that is too large wastes copper and makes the tray heavier than the structure was built to carry. A nameplate reading confirms none of them. The schedule settles the argument before the cable is cut.
The fault also exposes weak points a visual check misses. A long run to a remote load silently eats voltage that a short test misses. An undersized neutral carries imbalance current the phase check ignores. A tired lug still reads tight but may cook the joint on a motor start. Each fault is cheap to fix on the drawing and expensive on site. A written schedule turns the fault into evidence rather than opinion. Tesla Power Equipment builds every export set to this sequence and stamps the cable schedule on the panel drawing.
How do you calculate the voltage drop on a generator cable?
Calculate the drop so the installed value stays inside 3% at full load. For a three-phase run the drop equals the current times the loop resistance times the length, divided by one thousand. A 500kVA set at 400V carries about 722 ampere at 0.8 power factor, and a 50 metre run in 240 square millimetre copper lands near 2.4% under that load. The earthing method on our blog covers the bonding that moves with the cable route during a long run.
Note the drop point that the load must hold while the alternator carries full rated output. That load keeps the current steady while the engine works. A purely visual check cannot reproduce that condition. The data sheet states the voltage band, and that band sets the ceiling you should expect at the terminal. A sudden drop step also proves the lug can recover from a load change. Diesel manufacturers publish that recovery curve in the same document.
Which measurements belong in the cable sizing record?
| Measurement | Acceptance limit | Recorded at |
|---|---|---|
| Load current | Within 5% of 722 ampere | Every branch |
| Voltage drop | Not above 3% | Full load |
| Run length | Measured on site | As built |
| Ambient temperature | Below the correction limit | Steady state |
The record is the document a warranty claim will lean on. It should show the conductor size, the insulation rating and the lug part number. Every reading needs a timestamp and the name of the person who took it. Diesel documentation for export buyers lists the exact columns a third party will check each figure against. Generator documentation for the ISO 8528 rating definitions names the rating behind every number.
Keep the raw calculation files as well as the summary table. Modern tools export a drop trace, and that trace settles arguments about a single odd reading. Store the files with the generator serial number in the filename. A shared folder beats an email attachment, because the folder survives staff changes. request the template we use if you need the exact columns. Yangzhou Tesla Power ships that schedule with every export order so the site can verify the cross-section without waiting for air freight.
Sign the record on the day of the test. A signature added a month later carries much less weight with an engine maker service network. Sign the scan and file it with the load steps.
What does 500kVA demand from the cable run?
The arithmetic is simple. Take the alternator rating of 500kVA and multiply by the power factor of 0.8. The result is 500kVA x 0.8 = 400kW of usable real power. At 400V that power corresponds to a load current of about 722 ampere, which a 50 metre copper run in 240 square millimetre carries with a drop near 2.4%. If the site load is mostly resistive, the practical limit is closer to the full 500kVA rating.
This difference matters at the quotation stage. A buyer who sizes a run from a kilowatt figure alone may order a conductor that is too small for a motor-heavy load. A buyer who sizes from the set rating alone may pay for copper the site cannot use. State the power factor in the enquiry, and read our published commissioning notes before you freeze the cross-section. Tesla Power quotes both figures on every offer and stamps the cable schedule beside them.
Ask for the temperature correction as well. That single figure tells you how far the current must fall when the ambient rises, and it costs nothing to request. Diesel for standby duty follows the same sequence, with a longer hold at full load before the drop is proven.
Step-by-step cable sizing sequence
- Isolate the site load and confirm the generator breaker is open.
- Measure the run length from the output terminal to the load point.
- Read the full load current from the data sheet at 0.8 power factor.
- Select the cross-section so the drop stays inside 3% at full load.
- Apply the ambient temperature correction from the insulation table.
- Verify the lug and the gland match the selected conductor size.
- Record the drop and the corrected current in the schedule.
The temperature correction step protects the insulation and the joint. A hot enclosure forces the current down, and a skipped correction cooks the lug over time. Ten minutes of checking is enough to confirm the margin. Log the correction in the same schedule, because a missing correction is a common cause of early joint failure on prime power sites. Diesel for export sites runs longer, because the duty cycle changes the true load on the conductor.
Repeat the sequence at the end of the run to prove the schedule matches the as-built route. This is the step that most installers skip, and it is the one that finds a longer-than-drawn run. A schedule that passes the drawing may still fail the measured route, so check both. Tesla Power keeps every trace in the project folder for five years and reviews them before each export.
How do you read the temperature correction curve?
The temperature correction curve shows how far the permissible current falls as the ambient rises. The horizontal axis is the ambient temperature in degrees. The vertical axis is the current as a percentage of rating. A sound schedule keeps the corrected current inside the conductor rating across the site ambient range. Plot the route ambient against the insulation limit. A 30 C ambient should correct more than a 20 C ambient. The correction table in IEC 60364-5-52 lists the de-rate factor for every insulation class, so keep the column beside the curve. If the shape changes between runs, the insulation or the route has changed too.
The curve also shows the de-rating shape. A sharp fall at high ambient points to a conductor that was sized for a cool room. A flat line that does not correct points to a schedule that ignored the enclosure. A current that exceeds the curve points to a drop that will breach 3%. Keep every trace, not just the last one. Diesel for the data centre keeps the same curve in the handover pack and compares it with the original factory reading.
Common cable faults and what they indicate
A drop larger than 3% on a long run usually means the conductor is too small for the starting load. The fix is a larger cross-section or a shorter route, not a higher alarm setting. Measure the current with a clamp meter and compare it with the schedule. Diesel manufacturers publish a maximum ambient for each frame size, and that figure settles the argument about a hot-day drop.
A rising temperature at constant load points to a loose lug or a poor joint. Both are cheap parts and both are easy to verify before shipment. Our installation requirements page lists the checks that fail most often on export sites, and the torque we apply to every lug. Tesla Power ships that torque sheet with every export order so the site can re-check the joint without waiting for air freight.
A voltage that wanders under steady load points to the neutral imbalance or the lug aeration. Air in a crimped joint will cause the same reading, so re-crimp the joint before any gauge work. Alarm settings follow emergency supply wiring rules, so check the trip points against that document before you change them. Diesel in hot climates runs longer, because the de-rate changes the true load on the conductor at 45 C.
Site safety and documentation
Treat the live run as live equipment for the whole test. The conductor carries full load current and it is hot. Barricade the area and keep one person at the emergency stop at all times. Use a lockout tag on the site breaker so nobody reconnects the building load while the test runs. Ask any Chinese diesel suppliers for the same three documents before you compare quotations. the contact page carries the template request form.
Record the ambient temperature, because a hot day changes the correction. An enclosure that holds current at 20 C may breach the band at 45 C. The data sheet states the de-rate curve; note where the site sits on it. Read the build standard for the altitude limits we build to, then check the site altitude before you accept the rating. Tesla Power publishes the same curve in the handover pack. Conformity assessment for the EU market is a separate file naming the standards the set was checked against.
- Keep a fire extinguisher rated for fuel and electrical fires at the test point.
- Confirm the earthing before the first start.
- Log every alarm, even the ones that clear on their own.
Frequently asked questions about diesel generator cable sizing
Keep the drop at or below 3% from the output terminal to the load at full rated current, because a larger drop pulls the terminal outside the maker band. The exact figure sits in the installation standard and on the nameplate. A run that reaches 4% will trip protection on a motor start, while one held at 2% leaves margin for a longer future route. Keep the drop inside the 3% ceiling.
Only if the site has a real load that can be applied in controlled steps. Resistive heaters and large motor loads work if they can be switched. A building load that cannot be controlled will not produce a usable drop record, and it will not prove the run at rated output. A no-load reading hides every fault the running test finds.
Show about 722 ampere for a 500kVA set at 400V and 0.8 power factor, with the drop held at 3% over a 50 metre run. The schedule should also capture the ambient, because it moves the correction. A third party will check each figure against the data sheet before the set is accepted for export.
Annual checks suit standby sets that run a few hours a month. Prime power and continuous duty sets need a shorter interval, often quarterly. Sets in dusty or hot locations need a shorter interval again, because the de-rate changes the true load on the conductor and the joint every season.
A clamp meter, a thermocouple, a tape measure and a data logger cover most tests. Every instrument should carry a calibration certificate that is still in date, and the certificate number belongs in the record. A logged trace beats a hand-written column when a single odd reading is challenged later.
Stop the test and find the cause before the next step. Repeat the step after the fix and record both attempts. A record that shows the failure and the correction is stronger evidence than a clean sheet with no notes. Most excess drops trace to a longer run or a loose lug, both of which are cheap to correct.
No, provided the schedule follows the manufacturer procedure and the record is complete. Read the engine manual before the test. Some makers set a limit on how long the run may run above the band, and exceeding that limit is what causes a warranty dispute. Tesla Power keeps that limit in the commissioning file so the site team can check it against the logged trace.