Crane Power Quality: What a Single Girder Crane Does to a Weak Grid, 1 to 20 Ton

Written by Li Ming, Senior Applications Engineer at SIEC Cranes. Oct 29, 2026.

A 10 tonne single girder crane draws 17.6 kW. One hoist motor and three much smaller ones. Less than a mid-size air compressor, less than a single welding set. Ship that crane to a plant where the only supply is a 63 kVA diesel generator and there is a real chance the hoist will trip on the first loaded lift, at roughly half its rated capacity, with a fault code the site electrician will read as "crane defective".

It usually is not the crane. The crane is a nonlinear load bolted onto a source whose impedance is ten to twenty times higher than the transformer it was designed against. That sentence explains most of the failures I get called about, and the arithmetic behind it is short enough to put in this article.

Two things make it worse than it sounds. Single girder cranes between 5 and 20 tonnes draw 19 to 49 amps at 400 V, which puts most of them inside the 16 to 75 amp window where IEC 61000-3-12 sets harmonic emission limits. And the fix buyers reach for first, a bigger generator, does not work. I can show that from published generator data rather than from opinion, which is what the middle of this article does.

Sources are marked throughout. Standards and vendor data carry a reference. Crane power ratings come from a published single girder specification table. Anything computed by us is labelled as a planning figure, because no public survey covers a 5 tonne crane on a Nigerian factory generator.

How much power does a 1 to 20 tonne single girder actually draw?

Less than most buyers expect, which is exactly why the electrical problems surprise people. The published ratings below are for standard single girder bridge cranes across a 7.5 to 28.5 m span, with the three mechanisms broken out.

Capacity	Crane travel	Trolley travel	Hoist	Total installed	Crane full load current at 400 V	Hoist direct-on-line start
1 t	2 x 0.8 kW	0.2 kW	1.5 kW	3.3 kW	6.2 A	19 to 25 A
2 t	2 x 0.8 kW	0.4 kW	3.0 kW	5.0 kW	9.4 A	37 to 49 A
3 t	2 x 0.8 kW	0.4 kW	4.5 kW	6.5 kW	12.3 A	55 to 74 A
5 t	2 x 0.8 kW	0.8 kW	7.5 kW	9.9 kW	18.7 A	92 to 123 A
10 t	2 x 1.5 kW	2 x 0.8 kW	13.0 kW	17.6 kW	33.2 A	160 to 213 A
16 t	2 x 2.2 kW	2 x 0.8 kW	13.0 kW	19.0 kW	35.9 A	160 to 213 A
20 t	2 x 2.2 kW	4 x 0.8 kW	18.5 kW	26.1 kW	49.3 A	227 to 303 A

Power ratings are from a published standard single girder specification table, 7.5 to 28.5 m span. Currents are our own arithmetic at 400 V using 0.85 power factor and 0.90 efficiency, and the start column applies the 6 to 8 times full load current that ABB quotes for direct-on-line starting, after the 10 to 12 times initial inrush has decayed.

Read the last two columns together. A 5 tonne crane is an 18.7 amp load that pulls 92 to 123 amps for two or three seconds every time the operator lifts. A 20 tonne crane is a 49 amp load that pulls 227 to 303 amps. On a utility transformer that is a rounding error. On a 63 kVA generator it is the largest transient the set will ever see, and it happens ten thousand times a year.

What does a direct-on-line start do to a generator?

It drops the voltage, and the drop is not small. The relevant published figures: motors are designed to ride through a 30 to 35 percent instantaneous voltage dip before the contactor drops out, and specifying engineers usually impose a stricter 20 percent limit to guarantee the motor actually accelerates (Kohler, Sizing Generators for Motor Starting). Construction site guidance puts the crane-specific number at 15 to 20 percent, because crane control contactors are less forgiving than a plain motor starter. Above that, the contactor opens mid-lift.

That last sentence is the one that costs money. A contactor dropping out under load does not just stop the lift. It drops the load, and the brake has to catch it.

The generator size that holds the dip inside 20 percent depends on how the hoist starts. Direct-on-line needs the most, star-delta less, a soft starter less again, and a drive the least, but a drive brings its own problem, which is the next section.

Capacity	Hoist motor	Direct-on-line generator	Star-delta generator	Drive, with harmonic margin
1 t	1.5 kW	5 to 6 kVA	4 to 5 kVA	3 to 4 kVA
5 t	7.5 kW	23 to 26 kVA	15 to 19 kVA	12 to 14 kVA
10 t	13.0 kW	39 to 46 kVA	26 to 33 kVA	20 to 24 kVA
16 t	13.0 kW	39 to 46 kVA	26 to 33 kVA	20 to 24 kVA
20 t	18.5 kW	56 to 65 kVA	37 to 46 kVA	28 to 34 kVA

These are our planning figures, derived from the standard 3 to 3.5 times motor kW for direct-on-line, 2 to 2.5 times for star-delta, and 1.5 to 1.8 times for a drive including a 10 to 20 percent allowance for harmonic heating of the alternator. They assume the crane is the only load. Add the rest of the plant and the set grows again.

Note what is not in the sizing basis: total installed crane power. Sizing on the 26.1 kW total of a 20 tonne crane over-sizes the set by about a third, because the crane travel motors and the trolley never run at the same instant as a loaded hoist start, and even if they did they are 0.8 kW each. The largest motor start sets the size. Everything else is noise.

What does the crane do to the generator's voltage waveform?

Here is where the drive becomes a liability instead of a solution. A six-pulse rectifier draws current in short pulses at the crest of the wave, and that pulsed current flows through the source impedance and dents the voltage. On a transformer with a short circuit ratio around 100 the dent is invisible. On a generator it is not.

Cummins publishes the comparison, and it is the cleanest pair of numbers I have found on this. The same six-pulse drive, with 29 percent current THD, produces 34 percent voltage THD on a generator set with 12 percent subtransient reactance, a short circuit ratio of 8. Behind a transformer at short circuit ratio 100, it produces 2.8 percent. Same drive, same load, twelve times the distortion, because of the source.

Run that ratio the other way and the conclusion is unwelcome. Voltage distortion scales with the inverse of the short circuit ratio, so hitting the IEEE 519 limit of 8 percent voltage THD by source impedance alone would need a short circuit ratio of about 34, which means a subtransient reactance near 3 percent. Standard alternators sit at 8 to 14 percent. You cannot buy your way out of this with generator kVA. You would need four times the set and you would still be at the edge.

Source	Short circuit ratio	Voltage THD from a 29 percent THD drive	Against the 8 percent limit
Utility transformer, strong	100	2.8 percent	passes with margin
Generator, typical alternator	8 (12 percent X"d)	34 percent, published	fails by four times
Generator, low reactance option	12.5 (8 percent X"d)	about 22 percent, interpolated	still fails
Generator that would pass	34 (about 3 percent X"d)	8 percent	not commercially available

The first two rows are Cummins' published figures. The last two are our interpolation on the inverse short circuit ratio relation they describe, which is why they are marked as interpolated rather than measured.

In practice, 34 percent voltage THD on a generator does not look like a number on a meter. It looks like a drive tripping on DC bus overvoltage, a PLC rebooting, contactors chattering, and a site electrician replacing perfectly good components for a week.

Which limit actually applies to my crane?

Three different documents, and buyers usually get told about the wrong one.

IEEE 519-2022 sets voltage and current distortion limits at the point of common coupling. It is a system-level standard, applied to the whole installation, not to a single drive. That distinction matters because a 10 tonne crane on its own may sit comfortably inside the TDD limit while the plant as a whole is over it.

Bus voltage at the point of common coupling	Individual harmonic	Total harmonic distortion
V 1.0 kV or below	5.0 percent	8.0 percent
1 kV to 69 kV	3.0 percent	5.0 percent
69 kV to 161 kV	1.5 percent	2.5 percent
Above 161 kV	1.0 percent	1.5 percent

Source: IEEE 519-2022 Table 1, as reproduced by PQ Blog. Voltage limits are absolute, they do not flex with the size of the customer.

Current distortion is the flexible one. It is expressed as total demand distortion, a percentage of your maximum demand current rather than of the instantaneous current, and the limit tightens as the source gets weaker.

Short circuit ratio	2 to 11th harmonic	11 to 17th	17 to 23rd	23 to 35th	Above 35th	TDD limit
Below 20	4.0	2.0	1.5	0.6	0.3	5.0
20 to 50	7.0	3.5	2.5	1.0	0.5	8.0
50 to 100	10.0	4.5	4.0	1.5	0.7	12.0
100 to 1000	12.0	5.5	5.0	2.0	1.0	15.0
Above 1000	15.0	7.0	6.0	2.5	1.4	20.0

Source: IEEE 519-2022 Table 2, 120 V to 69 kV systems. Even harmonics below the sixth order are held to half these values.

Here is the useful part. A 6-pulse drive with no reactor has 101.5 percent current THD. At full load that equals the TDD, and against a 5 percent limit for a weak source it fails by a factor of twenty. With a 5 percent line reactor it is 33.9 percent, still over. With a line harmonic filter it is 4.9 percent, which passes. The values are Siemens' published table for drive front ends, and they tell you the answer in one line: the reactor is not optional on a generator.

IEC 61000-3-12 is the other document, and it is the one that applies lower down the chain. It limits harmonic current injected into public low voltage systems by equipment drawing more than 16 and up to 75 amps per phase. Which means the size of the crane decides whether it applies at all.

Capacity	Crane current at 400 V	Which IEC emission standard
1 to 3 t	6 to 12 A	Below 16 A, IEC 61000-3-2 applies to the equipment
5 t	18.7 A	Inside IEC 61000-3-12, 16 to 75 A
10 t	33.2 A	Inside IEC 61000-3-12, 16 to 75 A
20 t	49.3 A	Inside IEC 61000-3-12, 16 to 75 A

So a 20 tonne single girder sits squarely in the 16 to 75 amp band. That is not a coincidence, it is where most of this market lives, and it is why a drive maker's EMC declaration is worth reading before you sign rather than after the crane lands. IEC 61000-3-12 sets per-order limits in a table, and whether a given drive passes depends on which table row and which reactor the vendor fitted. Ask for the declaration. It exists, and vendors who have it will send it in an hour.

The third document is your customer's specification, and this is the one that actually governs. A typical public crane tender reads: 380 V three-phase 50 Hz, supply variation plus or minus 6 percent on voltage and plus or minus 2 percent on frequency (Hong Kong Works and Services Department standard specification EM-01-01, which is a fair example). Tighter than either standard, and it tells the supplier exactly what the crane has to ride through.

Why does the crane keep tripping the RCD?

Because a drive is not a motor as far as a residual current device is concerned, and because somebody fitted the wrong type.

Doepke's application guidance is blunt about it. Multi-phase equipment with frequency converters, and it names cranes specifically, may only be operated on Type B or B+ residual current devices, because a fault can produce smooth DC residual current that a Type A device cannot see. The alternatives are an isolating transformer or a fixed connection. Type AC or Type A on a crane drive is a device that will either nuisance trip or fail to protect, and possibly both on different days.

The second half of the problem is leakage current, which grows with motor cable length. Integrated EMC filters in a drive are typically only declared valid for 5 to 10 metres of shielded motor cable. Cable lengths of 50 to 100 metres are also quoted, but those relate to the capacitive load the output stage can drive, not to EMC conformity. Above about 10 metres, leakage current rises significantly and the manufacturer has to be asked what RCD sensitivity will hold.

Now think about a 20 tonne crane on a 25 metre span with a festoon system. The hoist sits in the middle of the bridge, the panel sits at one end, and the travel motors are at both ends. Cable runs of 20 to 30 metres are normal, before any consideration of the festoon. This is not a fault in the crane. It is a physical consequence of the geometry, and it is why the specification needs to say dv/dt filter, shielded cable and Type B device rather than leaving it to site.

One more, from the same source, and it catches people who have done everything else right. If the drive's switching frequency happens to match the resonant frequency of an upstream EMC filter, or a multiple of it, the filter can oscillate and leakage current rises far above normal, tripping the device. Changing the switching frequency fixes it. Nobody finds that by reading a manual at 2 a.m.

Does the line reactor change my power factor bill?

Yes, and by more than most people realise, because the power factor a drive advertises is not the power factor the meter records.

Displacement power factor is the cosine of the angle between the fundamental voltage and fundamental current. A drive's front end gives you something around 0.96 to 0.98, which is what the datasheet means when it says high power factor. True power factor includes distortion, and the relationship is straightforward: true power factor equals displacement power factor divided by the square root of one plus current THD squared.

Apply that to the Siemens current distortion values and the picture changes completely.

Drive front end	Current THD at full load	Displacement power factor	True power factor at the meter
6-pulse, no reactor	101.5 percent	0.98	0.69
6-pulse, 2 to 3 percent reactor	43.6 percent	0.98	0.90
6-pulse, 5 percent reactor	33.9 percent	0.98	0.93
6-pulse with line harmonic filter	4.9 percent	0.98	0.98
12-pulse	8.8 percent	0.98	0.98
Active front end	1.3 percent	0.99	0.99

THD values are Siemens' published figures for each front end. The power factor column is our arithmetic on the standard relation, rounded.

A drive with no reactor has a true power factor of 0.69 while its datasheet says 0.98. Both statements are true. That gap is the difference between a utility that charges you for reactive power and one that does not.

How that lands on a bill depends on the tariff, and the structures are not subtle. South Central Indiana REMC's industrial schedule, effective February 2024, adjusts billing demand upward whenever measured power factor at maximum demand falls below 97 percent, multiplying the recorded demand by 97 divided by the measured power factor. It then adds an excess reactive energy charge of 0.01009 dollars per kVARh once the monthly average falls below 95 percent.

Put numbers on that. A plant with 80 kW of real demand paying 12 dollars per kW of billing demand:

Plant power factor	Billed demand	Monthly demand charge	Extra per year against 0.97
0.97	80.0 kW	960 dollars	none
0.90	86.2 kW	1,034 dollars	about 890 dollars
0.69	112.5 kW	1,350 dollars	about 4,680 dollars

Our arithmetic on the published tariff formula, applied to a hypothetical plant. It is not a quotation and it is not your tariff. But the shape is right, and it explains why a 300 dollar reactor on a 15 kW drive is the cheapest electrical decision on the crane.

What happens when the supply is not 400 V at 50 Hz?

This is where a crane that was perfect in the factory becomes a warranty claim in another country, and the mechanism is a single ratio: volts per hertz.

An induction motor's magnetic flux, and therefore its torque, is set by supply voltage divided by frequency. Keep the ratio and the motor behaves. Change it and you change the machine.

Motor nameplate	Actual supply	Volts per hertz	Available torque	Speed	Verdict
400 V, 50 Hz	400 V, 50 Hz	8.00	100 percent	100 percent	rated
400 V, 50 Hz	480 V, 60 Hz	8.00	100 percent	120 percent	fine, 20 percent more power available
400 V, 50 Hz	400 V, 60 Hz	6.67	69 percent	120 percent	will not lift rated load
400 V, 50 Hz	380 V, 50 Hz	7.60	90 percent	100 percent	marginal at rated load, watch the hoist
460 V, 60 Hz	400 V, 50 Hz	8.00	100 percent	83 percent	saturates, runs hot, 17 percent slower

Torque scales with the square of the ratio, which is why the third row is the dangerous one. Drop the ratio by 17 percent and you keep 69 percent of the torque, but the motor is also spinning 20 percent faster, so the same hoist speed needs more power than the motor can produce. The failure is not dramatic. The hoist simply will not lift nameplate load, and it gets hot doing it.

The consolation is that most of these cranes ship with a drive, and a drive makes the 50 and 60 Hz problem disappear, as long as it is programmed for the motor rather than for the supply. Set the drive's base frequency and base voltage to the nameplate, and it delivers the right volts per hertz from whatever the line gives it. The catch is that the drive's output voltage cannot exceed its input voltage, so a 50 Hz motor that needs 400 V at base speed cannot be served from a 380 V supply without losing 10 percent of its torque curve.

And the tolerance question is separate again. IEC 60034-1 divides supply conditions into two zones. Zone A is plus or minus 5 percent voltage with plus or minus 2 percent frequency, where the motor delivers rated torque with a modest temperature rise. Zone B is plus or minus 10 percent voltage with plus 3 minus 5 percent frequency, where rated torque is still available but the temperature rise is higher and operation should be short. NEMA MG 1 is looser on paper at plus or minus 10 percent voltage and plus or minus 5 percent frequency, with the combined variation capped at 10 percent. A crane that spends eight hours a day in zone B is a crane with a shortened insulation life, whatever the standard permits.

Brake resistor or regenerative drive?

Brake resistor, on a single girder. I will show the arithmetic, and then show the case where it flips.

Lowering a load makes the motor a generator. The energy has to go somewhere. A brake chopper dumps it into a resistor as heat. A regenerative front end puts it back on the supply. ABB's engineering guide works a 90 kW crane hoist through both options: chopper and resistor in the cabinet at 7,500 euros, versus an IGBT front end costing 2,000 euros more and recovering 4,000 euros of energy. On that machine, regeneration wins, and ABB recommends it for cranes.

That machine is a 90 kW hoist on 20 percent annual duty with three minute braking periods. A single girder hoist is 1.5 to 18.5 kW, and the energy per lift is smaller by the mass ratio squared in practical terms, because both mass and height shrink.

Capacity and lift	Energy per loaded descent at 60 percent load	Recovered at 65 percent	Annual recovery, A5 duty	Value at 0.12 per kWh	Payback on a 1,500 to 3,000 dollar premium
5 t, 6 m	0.049 kWh	0.032 kWh	384 kWh	46 dollars	33 to 65 years
10 t, 6 m	0.098 kWh	0.064 kWh	766 kWh	92 dollars	20 to 38 years
20 t, 9 m	0.294 kWh	0.191 kWh	2,292 kWh	275 dollars	9 to 16 years

Our arithmetic. Assumes 2,000 operating hours, 12 lifts per hour, half of them lowering a load, closed loop regeneration efficiency of 65 percent, and a 1,500 to 3,000 dollar premium for a regenerative front end over a chopper and resistor at these ratings.

Nine to sixteen years on a 20 tonne crane is not a business case, it is a rounding error on a twenty year asset that might be replaced first. So specify the chopper and spend the difference on the line reactor, which returns something every single hour of every shift rather than only when the load comes down.

The exception is worth stating, because it is where the export market is heading. If the buyer already pays a demand charge on a weak supply, or the site runs on a generator where every regenerated kilowatt-hour is diesel not burned, the value per kWh is two to three times the grid tariff, and the 20 tonne case moves into the five to eight year range. Ask what the electricity actually costs before you rule it out. On a generator site the answer is usually not 0.12.

What does the fix cost, and in what order should I spend?

Cheapest first, and the cheap end does most of the work. Costs below are indicative ranges for a 13 to 18.5 kW hoist drive, which covers a 10 to 20 tonne single girder. They are our planning figures from project quotations, not a price list.

Measure	Current THD after	True power factor after	Indicative cost	When it is the right answer
5 percent line reactor or DC link choke	33.9 percent	0.93	200 to 600 dollars	the default for any export crane
3 percent line reactor	43.6 percent	0.90	150 to 450 dollars	stiff utility supply with a short run to the panel
Passive line harmonic filter	4.9 percent	0.98	1,200 to 3,500 dollars	generator supply, or a contractual THD figure
Active front end drive	1.3 percent	0.99	1,800 to 4,500 dollars premium	regeneration is wanted anyway, or the limit is strict
12-pulse with phase shifting transformer	8.8 percent	0.98	6,000 to 15,000 dollars	above roughly 75 kW per drive, where reactors are not enough
Active filter at the point of common coupling	under 5 percent at the bus	close to 1.0	4,000 to 12,000 dollars	retrofit, or several drives already installed
Oversizing the generator	unchanged	unchanged	15,000 to 40,000 dollars extra	never, for harmonics. It reduces voltage distortion, it does not remove it

Two things worth saying about this table. The first is that the bottom row is the one most buyers take, because it is the one that sounds like engineering rather than accessories, and it is the worst value on the list. The second is that the top row is not really a harmonic measure at all. A line reactor protects the drive from supply transients and capacitor switching, which is why Pacific Gas and Electric's application note recommends a 3 percent reactor as standard practice for nuisance overvoltage trips. The harmonic reduction comes free.

If the crane is going onto a generator, skip the ladder and go straight to the third row. A passive filter costs about the same as the difference between a 46 kVA and a 65 kVA set, and it fixes the problem the set cannot.

What should go in the electrical section of the RFQ?

Ten lines. Each one has cost us or a customer a week at some point.

1. Supply voltage, frequency and measured tolerance. Not the nominal figure. A site that measures 355 V at the crane panel is a different crane from one that measures 400 V.

2. Supply source, and its short circuit ratio. Utility transformer with the kVA and impedance, or a generator with the kVA and subtransient reactance. Without the reactance the harmonic question cannot be answered.

3. Hoist starting method, stated as a requirement. Direct-on-line, star-delta, soft starter or drive. Do not leave it to the supplier's standard build.

4. Drive front end specification, with current THD at full load. A number, not the word "filtered". Ask for the EMC declaration under IEC 61000-3-12 if the crane current is above 16 A.

5. Whether a line reactor or DC link choke is included, and its percentage. If it is not on the drawing, it is not in the panel.

6. Motor nameplate matched to the actual supply. Volts per hertz, not just voltage. This line prevents the 60 Hz trap in the table above.

7. Duty class and starts per hour, per mechanism. A hoist rated for 180 starts per hour at 30 percent duty is a different motor from one rated for 300 at 50 percent, and the price difference is real.

8. Braking arrangement. Chopper and resistor, or regenerative. Plus the resistor's duty rating if the site has long descents.

9. Earthing and residual current protection. Type B or B+ device, shielded motor cable, maximum cable length, and whether a dv/dt filter is needed. State who supplies what.

10. Who commissions the drive, and against what. Parameter set at the factory with the motor nameplate, or at site with the actual supply. This is the single most common gap between a working crane and a crane with a commissioning report nobody can reproduce.

Frequently Asked Questions

Can I run a single girder crane from a generator?

Yes, and plenty of them do. Size the generator on the largest motor start rather than the connected load, keep the voltage dip inside 20 percent, and fit a line reactor to the hoist drive. A 10 tonne crane needs 39 to 46 kVA behind a direct-on-line hoist or 20 to 24 kVA behind a drive. What you cannot do is bolt a bare six-pulse drive onto a 63 kVA set and expect the voltage waveform to stay inside IEEE 519. See the single girder crane range for the standard electrical build.

Why does a 17.6 kW crane need a 46 kVA generator?

Because the hoist motor draws six to eight times its full load current for two or three seconds on every lift, and the generator has to hold voltage through that. The generator is not sized for the 17.6 kW of running load, it is sized for the transient. A drive removes most of the transient, which is why the drive-fed column in our table is less than half the direct-on-line figure. That saving usually pays for the drive.

Is a bigger generator cheaper than a line reactor?

Rarely, and often it does not work at all. Reducing voltage distortion by source impedance alone needs a short circuit ratio near 34, which means a subtransient reactance around 3 percent. Standard alternators are 8 to 14 percent, so you would be buying four times the set to stay at the edge of the limit. A 5 percent line reactor takes current THD from 101.5 to 33.9 percent for a few hundred dollars and does the job at any generator size.

Does the crane need a Type B residual current device?

If it has a frequency converter, yes. Doepke's guidance is explicit that multi-phase equipment with frequency converters, naming cranes, may only run on Type B or B+ devices, because a fault can produce smooth DC residual current that a Type A device will not detect. An isolating transformer or a fixed connection is an acceptable alternative. A Type AC device on a crane drive is a nuisance trip waiting to happen.

Will a 50 Hz crane work on a 60 Hz supply?

The motor will, the hoist may not. Keep the volts per hertz ratio and the motor delivers the same torque at 20 percent higher speed. Run a 400 V 50 Hz motor at 400 V 60 Hz and the ratio drops from 8.00 to 6.67, which costs 31 percent of the torque, and the hoist will not lift nameplate load. Specify the motor to the supply, or fit a drive and set its base voltage and base frequency to the nameplate.

Should I buy a regenerative drive for the hoist?

On a single girder, no. The recovered energy on a 5 tonne crane at A5 duty is about 384 kWh a year, worth 46 dollars, against a premium of 1,500 to 3,000 dollars. On a 20 tonne crane at heavy duty the payback is nine to sixteen years on grid power, which is not a case. It flips on generator sites, where the marginal kilowatt-hour is diesel, and on process cranes above 50 tonnes where ABB's own worked example shows regeneration paying back. Spend the money on the reactor first.