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How to Decide Where to Locate Electrical Generators

Here are some key considerations for how to decide whether to place generators inside a building vs outside in enclosures with sub-base fuel tanks.

By James Coe  

Many property owners are challenged with the question of where to locate their electrical generators. The difficulties are understandable: Generators —whether they’re emergency systems, legally required standby systems, or optional standby systems — are needy beasts that require a fuel supply, a combustion air supply and exhaust pathway, and a cooling air supply and exhaust pathway. They are connected via conductors and conduits to electrical power systems via automatic transfer switches or switchgear. They have extensive controls connectivity and alarms. They are generally noisy, and they generate problematic combustion emissions.  

These characteristics complicate decisions about location. Property owners do not want generator operation to be so loud as to disrupt the function of the building, nor do they want the combustion exhaust gases to enter the building via outside air intakes. It’s also possible that the exhaust from the generator’s warmer cooling air reduces the rated performance of other generators or any HVAC equipment – another scenario to avoid.  

To further complicate matters, the cost per kilowatt-hour (kwh) of power from a generator is usually higher than that supplied from the electrical utility, so most owners want to reduce generator run time. Quite often, the air permits for the generator installation will limit their run time when utility power is available. 

A key step in addressing these issues is a fundamental decision: to locate the generator in an owner-constructed building or outside the building in vendor-supplied housing. Each option offers advantages and disadvantages: 

Generators Inside a Building 


It is more comfortable to perform maintenance (and may lead to better maintenance) and repairs inside a building than in a cramped enclosure. 

The generator’s condition and status can easily be observed visually. 


The installation will be more expensive. Furthermore, the one-of-a-kind layout could lead to design or construction errors. Generator enclosures are a standard and vetted design that have been used extensively in manufacturing.  

Providing adequate cooling air flow into the building and around the generators is more challenging. 

Day tanks inside a building are limited to no more than 660 gallons (total), resulting in bulk fuel storage being required. If the owner is a data center operator seeking Uptime Institute Tier III or IV certification, the fuel system will have to be concurrently maintainable (maintained while the site is powered by generators) and fault-tolerant. This generally leads to concurrently maintainable double valving and redundant tanks. 

A fuel polishing system may also be necessary with bulk storage of fuel.  

Dispersing the generator exhaust fumes is more expensive and challenging when in a building since the fumes are usually routed horizontally out of the building instead of out or up. Consequently, the fumes can be more easily drawn into a building’s outside air intakes. 

Required clearances around the generators will require additional square footage.  

A generator failure incident, such as a thrown rod or a fire, is more likely to impact an adjacent generator. 

Generators in Enclosures with Sub-Base Fuel Tanks 


Installation will be less expensive since this is the most common way generators are purchased by data center hyperscale and colocation customers. The trend has increased generator industry competition and innovation. 

Quite often, no fuel bulk storage is required since the fuel required per generator can be stored underneath each generator. With no bulk storage, Uptime Institute Tier III and Tier IV certification is greatly simplified. 

Required clearances around the generator can be achieved by opening enclosure doors/panels without requiring an extension of the building footprint. 

Fuel polishing is usually done by the fuel supply vendor during regular re-fill visits. 

Generators in enclosures can be stacked if there is limited site area available. 

Dispersing the generator exhaust fumes is easier since the fumes can be directed into the cooling exhaust air stream or straight up. 


During a hot summer or cold winter, a cramped enclosure is less comfortable for performing maintenance (and may lead to poor maintenance) and repairs than inside a building.  

It can be difficult to get 120,000 CFM air supply into a generator enclosure with a sand louver (in the desert), and a snow prevention scheme (cold climates) and may necessitate a remote radiator. 

The answer to where an owner should locate electrical generators on a building site will vary for each unique situation. However, as the comparative length of the plusses and minuses categories above suggest, the design and construction risks are reduced by utilizing exterior generators in enclosures with sub-base fuel tanks. Computational fluid dynamics (CFD) studies on exteriors can mitigate the risk of entraining combustion air into the building intakes or reducing adjacent generator capacity or HVAC cooling capacity.  

In short: Detailed site analysis is a time-consuming, but very necessary, step. Power through it. 

James Coe is a senior principal at Syska Hennessy and the director of its critical facilities practice. 

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  posted on 4/20/2022   Article Use Policy

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