How to Size a Diesel Generator

In this Power Continuity knowledge base article, we will explain how to size a diesel generator as well as other aspects you wouldn’t of thought about.

Generator Rating – Prime or Emergency Standby

Diesel Generators are termed either Prime or Standby rated.
These definitions are important to prevent overloading of your installed generator.

Generator Terms Explained.

Prime Power:

This term indicates both the maximum continuous running that the generator can support as well as the generator manufacturer has built the generator to operate unlimited hours supporting variable loads. A by-product of a prime generator is its ability to accept a short-term overload of 10% above its prime capacity as long as it is no more than for 1 hour in any 12 hour period.
Typically, should this overload happen regularly we would advise upgrading the generator or reducing the extra load and placing this on another generator.

A Prime power generator can run for up to 8000 hours per year as long as regular service and maintenance is adhered to.

NOTE:- To ensure a long working life for your generator never continually overload the genset and preferably run the set around 75-80% of its capacity.

Standby Power:

An emergency standby diesel generator provides electrical power during a GRID power outage. The maximum power available for the duration of the GRID interruption is the size rating of the generator.
NOTE:- No Overload capacity for a Standby rated generator.

Typically, standby generators run for up to a maximum of 200 hours per year.

How to size a Diesel Generator

Simply put you need to size the generator to ensure its sufficient to power your load. Hence, the first step is to determine all of the site loads, that require a diesel generator.

Bear in mind, that any generator can only accept approximately 65-70% of its maximum capacity at any one time. Also review any future growth to ensure that your generator will be big enough without the need for a larger generator.

Similarly, take into account the power factor is 0.8. This applies to all diesel generators.

How to size a Diesel Generator Example Sizing a Deisel Generator

This example presents how to calculate the Diesel Generator Size.

The rated power Pn (kVA) of a Generator is the maximum output power given an output power factor of 0.8.
It is necessary to calculate the corresponding input power to determine the Diesel Generator power rating

Therefore a Diesel GENERATOR of  100 kVA  supplies the load with a maximum output power of 100 x 0.75 = 75 kW

How to explain Diesel Generator Power Factor

Diesel Generators have a power factor of around 0.75-0.8
Gas Generators have a power factor of around 0.50
LPG Generators have a power factor of around 0.45

Another helpful example

If the increase is 18% (value supplied by the manufacturer), the necessary input power is Pe = 80 x 1.18 = 94.4 kW.

Therefore the Diesel Generator must be sized to supply power Pg.
Such that, if the input power factor is 0.8, then Pg > 94.4 / 0.8 = 118 kVA.

In this case, where both the input and output power factors are equal to 0.8,
it is possible to calculate the result as 100kVA x 1.18 = 118 kVA

Unsure of your calculations then use our FREE Power Calculator app

Behaviour of a Diesel Generator with harmonics

Diesel Generators are designed to supply linear loads.
Should you notice the  presence of harmonics it’s necessary to take action to ensure compatibility.
Otherwise, there will be possible repercussions down stream on the Diesel Generator and its Power Control.

Electricity Supplier Reports

Ask your electricity supplier to provide the maximum peak draw during the last 12 months electricity usage. To require this ensure you have your MPAN number (Metering Point Administration Number) to hand as well as your account details. Check that the peak currents have recorded any large startup spike loads which sometimes only last for milliseconds but will cause a generator to fail, if they are above the diesel generator capacity. Allow for any new equipment or adverse weather conditions during the last 12 months as these will affect the power usage.

Manual or Automatic choice

Option 1 – Manual Operation – Start/Stop

Record all the equipment that the generator will power e.g. lights, sockets, computers, printers, fridges, microwaves, pumps, motors and air con units. Are they variable loads? How you allowed for any startup spike load? Allow for all the equipment starting at once.

Each equipment will have a rating plate detailing the kW’s and often the Amps. But this will not provide the startup current. For that you need to go back to the manufacturer.

Option 2 – Automatic Operation

A fully installed automatic diesel generator system requires an AMF or Automatic Mains Transfer panel. This takes the headache away as the system will operate autonomously in the event of a power outage or power drop. No need for any human intervention. Transfer switches both AMF and ATS explained.

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. This can be achieved through additional wiring and relays using the generator battery supply as a source for the additional voltage. This signal cable is vital to allow the generator to start. Should the generator batteries go flat the ATS/AMF won’t operate as the signal won’t be receive by the generator.

Make sure the battery charger and engine block heater auxiliary supplies are always ON. Never turn OFF or disconnect by accident, as this not allow the ATS /AMF to transfer load to the generator.

Transfer Switches Explained:AMF Panel considering the Generator power factor

Transfer Switches transfer the load from the mains supply to the generator supply and back again. They consist of a mains contactor, either contactor based or rotary generator contactor. In the event of a power outage, this interlocks to prevent both mains & generator being connected at the same time. The mains is isolated to allow the generator power only to run. The ATS/AMF prevents feed back to the GRID. These are fully automatic.

On the other hand a manual transfer switch has to physically switch over from mains supply to generator supply. This is termed MAUANUAL Start/Stop.

An automatic transfer switch (ATS) or Automatic Mains Transfer Panel ( AMF ) switch continually monitors the mains supply. Should a failure happen the ATS/AMF will immediately sense the GRID power has disappeared. At this moment it springs into action. Pre-set time delay switches immediately activate. Within 20 seconds the failed incomer supply is switched over to the generator side of the switch to power the equipment. An ATS/AMF is operation is termed, ‘BREAK before MAKE’. Therefore, any equipment that can’t handle a BREAK before MAKE requires an Uninterruptible Power Supply (UPS) to hold up the power whilst the generator is starting up to power to the site.

Our Advice

Remember that any equipment that resets automatically to the off position upon a mains failure, will need to be switched on manually. We advise that you check all equipment is able to accept the generator power supply without the need for manual intervention. If you have such sensitive equipment, then we advise installing UPS to ensure power to such items is never lost prior to generator supply. Likewise when the generator supply stops, and the load is transferred back to GRID supply. Again, there will be a one or two second break in power supply as the ATS/AMF reverses back to GRID supply.

When sizing a ATS/AMF transfer switch consider the current rating of both the generator supply and the mains supply, as well as future capacity. Likewise, ensure the Transfer Switch is sized to the larger AMPS of the mains breaker, never to the smaller size of the generator.

Positioning of the Generator:

Position the Generator on level hard standing, ideally reinforced concrete of at least 300mm depth. Check the noise levels tolerance of the location. Also security, do you require a steel palisade fence or simply a wooded hit and miss fence. Vibration may also be an issue. The generator can be placed on rubber to prevent/reduce vibration creep.

With the Diesel Generator size it must have free space around them on all sides. At least 3 metre on all side to allow FREE FLOW air. Remember, generators suck in cold air and extract hot air. NEVER position a generator 1 metre away from a wall as this will restrict air flow as well as vibration travel. Equally, the exhaust outlet should never be placed under windows

Dependant on the design of the generator, the air inlet / outlet vents at the sides of the generator must never be blocked. Whether by being to close to a building or fence, or by debris such as leaves.

Take care when positioning to prevent anything that might get too close to the exhaust and catch fire, such as overhanging trees, bill boards, signage.

Can the generator fail to start?

All emergency standby diesel generators (and some prime generators) will require an auxiliary supply to power both the battery charger and jacket water heater, referred to as the Block Heater.
Battery chargers are there to keep the batteries charged when the generator is not running.
Block heaters keep the engine at an ambient temperature to ensure it starts and supports load, every time.
Ensure the auxiliary supply between the ATS/AMF and the generator control panel is never tampered with or accidentally disconnected.
NOTE: – Battery chargers require anything from 5 Amps up to 25 amps and a jacket water heater up to 50 Amps sometimes much larger than your standard 13 Amps plug socket.
Preferably place this on a separate Distribution Board Supply.

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

Diesel Generators in acoustic enclosures are called silent generators.
Generally speaking, ‘super silent’ generators are containerised, ‘Walk Inside’.
Acoustic canopied generators are generally 80dBA @ 1 metre which equated to [email protected] 7 metres.
Containised Generators range begin from 80 dB(A) at 1m down to very low sound levels.
Our bespoke generators can be built with sound levels as low as
60-50 dB(A) at 1m= 45dBA/[email protected] 7metres and long duration fuel tanks of up to 7 days

For more information on acoustic sound levels and dBA please visit our page dBA Noise Level Generators

Sizing a Diesel Generator Useful Conversion Explanations:

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.

PF = Power Factor

Eff = Alternator Efficiency

KWm = Engine kW Mechanical,

kWe = kW Electrical).

FFC = Free Field conditions

Lumens to Watts = on our app

Watts to Lumens= on our app

Check out our free power calculator app

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