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Generator Sizing and Installation

FW Power Step-by-Step Guide

Generator Sizing and Installation

Welcome to our step by step guide designed to help you through the process of selecting and sizing  your Diesel Generator. We have tried to keep the document as simple as possible and to explain some of the commonly used industry terms. Should you have any further queries then please contact FW Power at

Generator Rating Definitions:

One of the first terms you will come across for your Diesel Generator will be the rating, Prime or Standby. The below definitions are important and should be used to enhance the life of your generator, to ensure adherence  to manufacturer’s warranty conditions and to prevent overloading of your generator.

Prime Power: The maximum power available for a varying electrical load for unlimited hours. A 10% overload is available for 1 hour in 12.

Restrictions are often placed upon the average value of the varying load, typically 70% of the prime power and also on the overload operation which typically is for a maximum of 25 hours per year.

Standby Power: The maximum power available with a varying load for the duration of the interruption of the normal power source. No Overload is available for a Standby rating.

Restrictions are generally placed upon the average load, typically 70% of the standby ratings and hours limitations are imposed of usually 500 hours per year but sometimes 200 hours per year.

For more information on Prime and Standby Generators please refer to the FW Power Guide- Differences Between Prime Generators and Standby Generators.

What Size Generator Do I need?

Simply you need a generator that is sufficient to power your loads and run your business but not too large that it will cost you unnecessary money in purchasing and running costs. So the first step is to determine all of the site loads and there are a number of options available to obtain this information:

Option 1 – Manual Process:

Record all of the equipment that the generator will power e.g. lights, sockets, computers, printers, fridges, microwaves, pumps, motors and air con units etc.

Each item of equipment will have a rating plate detailing the kW’s and often the Amps. If the equipment also has its starting current then record this as well (more on starting currents later in this guide). If you are unable to find the rating plate then look in the instruction manual or online. Use our Power Calculator (that you will find on our home page in a blue box) or view our Common Equipment Ratings to assist you.

Once all of the kW’s are known, multiply this number by 0.8 to obtain a generator running kVA – please note that this will not give you any starting currents.

Option 2 – Electricity Company Reports:

Ask your electricity supplier to give a breakdown of the last 3 months electricity usage. To do this, give them your business details and your MPAN number (Metering Point Administration Number). The resulting information will provide a detailed analysis of the average loads and the peak currents. Check that the peak currents have recorded any large starting currents which sometimes only last for milliseconds. It is important to ensure that whatever system is used that the “cold start” loadings are recorded. i.e. turn everything off and then turn it on again. This time pay particular attention to the maximums as this will include the starting currents.

Option 3 – Use a Data Logger:

This can be undertaken by a competent electrician over the course of a number of days to give a detailed breakdown of the electricity usage in Amps and kW’s. Very similar to the report from the electricity company but it can be tailored to your own requirements.

Use the Data Logger to record all of the equipment from a “cold start”  since this will duplicate a mains failure and record the start-up (in rush) currents. This allows the sizing of the generator for both the running loads and the start up currents.

When sizing a generator it is important to take into consideration the running loads and the starting currents. For example, the running current of a 3 phase 10 HP induction motor will be approximately 13 Amps, however the starting current will be between 3 and 9 times the running current i.e. 39 to 117 Amps. It is easy to see how in an installation with a number of motors the currents can become quite large and therefore the required kVA of the generator itself will be quite large.

Modern alternators are designed to take up some of the slack when it comes to starting currents – normally around twice the generators rated kVA (the exact details will be found in the relevant manufacturers’ alternator manual).

There are ways to offset such large starting currents by using “soft start” systems or “variable speed drives”. Ask your electrician to guide you in this process.

So in summary to size your generator effectively you should determine all of the running loads, the starting currents and size accordingly.

A generator that is too small will trip out on overload and not power the site it is intended for.

A generator that is sized exactly to the running loads may not cope with the large start-up currents of motors, will be running at continually high loads stressing the components and will leave no room for future expansion.

A generator that is too large for its application will run very inefficiently and cost the owner in higher fuel costs as well as the initial purchasing costs of the equipment.

A Practical Example Using Large Motors:

Operating large industrial machinery is often the most difficult when sizing a generator. Below is an  example of a Metal Cutter and Bailer.

The system has 4 motors (for simplicity motors and pumps are described as motors, alternator efficiencies are not included in the calculations and it is assumed that all motors start at the same time and are not sequenced). Often you will need to convert the HP figures of motors to kW’s.

2 x 100 HP Motor = 73.3 kW.
1 x 25 HP Motor = 18.3 kW.
1 x 4 HP Motor = 3 kW.
Total  = 168 kW.

All of the motors are 3 phase. A generator is rated at 0.8 Power factor (PF).

168 kW = 210 kVA (168/0.8).

The motors are wired Star Delta and have a starting current of 3.5 x the running current. The alternator can take twice its running current to assist with motor starting.

To start the motors requires a generator of (210 x 3.5)/2.0 = 367 kVA.

The engine has an initial load step of 60% of its kW rating:

Net Engine kW’s required = 168/0.6 = 280 kW = 350 kVA (280 kW / 0.8 PF)

So a generator of 367 kVA will start the equipment.

However, a bailer has rams that when they get to the end of their travel and “dead end” they effectively call on the motors and pumps to provide more power and the Power Factor drops and lags. In this instance the PF drops to 0.38. The equipment requires 210 kVA at full load and 0.8 PF.

When the PF drops to 0.38 the effective kVA required by the alternator is :

210 kVA x 0.8 = 168 kW.

168/0.38 = 442 kVA.

So the equipment will require 442 kVA to ensure that it can cope with the required power when the PF drops.

The example above shows how difficult it can be when sizing a generator correctly. Oversized alternators can overcome some of the problems associated with large start-up currents.

Transfer Switches:

Transfer Switches transfer the load from the mains supply to the generator supply and back again. They consist of a mains contactor, a generator contactor, interlocks to prevent both mains & generator being connected at the same time and a series of relays, timers and lamps.

With a manual transfer switch the operator has to physically switch over the selector from mains supply to generator supply.

With an automatic transfer switch (ATS) the system senses that the mains supply has failed and after a preset time delay switches the supply over to the generator side of the switch to power the installation. Care should be taken for equipment that resets automatically to the off position upon a mains failure, these will need to be switched on manually – it is useful to have a procedure that indicates what to do in the event of a mains failure.

When sizing a transfer switch attention should be given to the current rating of both the generator supply and the mains supply. The generator may only have been rated for emergency loads whereas the mains supply will cover all loads. Generally the transfer switch will have two contactors of the same size inside. So if the transfer switch is sized to the generator and the mains supply is much larger, the contactor in the transfer switch will continually trip out due to an overload. The Transfer Switch must be sized to the larger of the mains or the generator.

For more information please refer to the FW Power Guide  – Automatic Transfer Switches.

Positioning of the Generator:

Generators should be positioned on flat level solid ground, ideally a cast concrete base and  be secured to the ground with bolts. In some instances generators are positioned on hardcore, this is OK as long as the ground is very compact and stable. Although generators are isolated for vibration, not all the vibration can be removed and unless you want your generator wandering down the road make certain it is secure!

The generator needs lots of air to breath and hot air makes it underperform. Therefore site the generator where it has plenty of room to suck in cold air and extract hot air. Do not position the generator 1 metre away from a wall with no means for the hot air to escape and expect it to run and cool efficiently. Equally do not position the hot air or exhaust outlet where they may get mixed with the cool air supply.

Also be careful that on some generators in acoustic canopies there are often air inlet vents at the sides of the generator and not just at the control panel end – be sure not to block these.

Exhausts are hot !! In fact a few hundred degrees hot. Take care when positioning to prevent anything that might get too close to the exhaust and catch fire. Trees that seem OK today may become a problem once they have grown in a few years time.

Generator Cabling:

The generator is now positioned and sized perfectly, apart from fuel what is required to make it work?

Firstly, the power cables need to be sized and connected. For a 3 phase generator you will have 3 power cables (L1, L2 and L3) a neutral cable and an earth cable. Most installations also require an earth rod.

Sizing of the power cables is complex and should be undertaken by our installation teams or a competent electrician. BS7671 provides guidance on the cable sizes suitable for the installation. Below is a general list  of cable current ratings that can be used as a guide for installation. However be aware – dependent upon the type of  cable and installation – ducts, ground, grouped, spaced, clipped, cable tray, ambient temperatures, operating temperatures, aluminium conductors, copper conductors, armoured cables, number of cores  –  the size of cables required can vary considerably from the table below:

Conductor area mm2Maximum Current Ratings in AmpsConductor area mm2Maximum Current Ratings in Amps

Example is for a good quality multi-core armoured cable in perforated tray.

Secondly, if the generator is expected to start automatically when the mains fails then a signal is required to start the generator. The Automatic Transfer Switch will have a pair of  contacts inside the panel, usually “volt free”. A two core cable is required from the contacts to the remote start terminals within the generator. Some generator control systems require a 12 or 24 Volt signal in place of the “volt free” signal which can be achieved through additional wiring and relays using the generator battery supply as a source for the additional voltage. This cable is merely a signal cable so 0.5 mm2is sufficient but often 1.5 mm2to 2.5 mm2is used. (Note: some systems also require an “up to speed” signal from the generator).

Thirdly, standby generators (and some prime generators) will require an auxiliary supply to power the battery charger and jacket water heater. Battery chargers are there to keep the batteries charged when the generator is not running and the jacket water heaters keep the engine at a reasonable temperature so that it both starts well and takes load quickly. To do this an AC auxiliary supply is  taken off the main switchboard or more usually the Transfer Panel and connected inside the generator control panel. A battery charger often requires 3 – 5 Amps and a jacket water heater up to 20 Amps (some have two!). So the required current can be as high as 50 Amps – much larger than your standard 13 Amps plug sockets at home. We would suggest 6.0 mmtwin and earth cable, however you may only require  2.5 mm2 depending upon the generator requirements. As with the signal cable it is good practice but not essential to use armoured cabling.

Noise and dB(A) – What does it all Mean?

Generators in acoustic enclosures are called silent generators. Generally silent generators range from 75-85 dB(A) at 1m but bespoke generators can be built as low as 50-60 dB(A) at 1m.

To see some examples of common sound levels as a comparrison to a generator please view our guide on common sound levels in dBA.

Noise levels are logarithmic, so a 40 dB(A) noise is not twice as loud as a 20 dB(A) noise, in fact it is more than 9 times as loud.

Noise can be reduced in a number of ways but the most common are absorption and distance:

Absorption is how most silencers and generator acoustic housings work. Noise is an energy wave and each time the noise is passed through a material or bounced off a solid structure a little is absorbed and converted to another energy type e.g. heat – just like a football being kicked against a wall – it is returned to you slower than when you kicked it i.e. with less energy.

Distance is quite simple, the further from a noise source the quieter it is. A sound wave radiates so the further from the source the more the radiated area increases and the noise reduces. As a guide the noise will reduce by approximately 1 dB(A) per metre for the first 2 metres from the source and then by 3 dB(A) for every doubling of the distance .

So 85 dB(A) at 1m would work as follows:

85 dB(A) @ 1m

83 dB(A) @ 3m

80 dB(A) @ 6m

77 dB(A) @ 12m

74 dB(A) @ 24m

71 dB(A) @ 48m

Please note this is a very rough guide and should not be used as a rule of thumb.

Please refer to the FW Power Guide –  Common Sound Levels in dB(A).

Useful 3 Phase Conversions:

kW = (V(L-L)x 1.732 x PF) / 1000.

kW = kVA x PF.

kVA = (V(L-L)x A x 1.732) / 1000.

A = kW x 1000 (V(L-L)x PF x 1.732).

HP = kW x 0.7335.

kVA =  kWe / PF = (kWm x Eff) / PF.

(PF = Power Factor, Eff = Alternator Efficiency, KWm = Engine kW Mechanical, kWe = kW Electrical).

For more information please refer to the FW Power Guides in our Knowledge Centre.