Generator classes and sizing

Diesel gensets · silent canopy sets · temporary site power

A generator is sized on starting load, not running load, and then oversized twice — once for operating headroom and once because a diesel set loses output in UAE summer heat. Skip either step and the set trips when the largest motor starts, usually on the first hot afternoon. This page sets out the classes and the sizing method; the owner of the set quotes the rate.

The classes

UAE rental fleets step through standard sizes — 10, 20, 30, 45, 60, 80, 100, 125, 150, 200, 250, 300, 400, 500, 650 and 800 kVA. Your calculated requirement will land between two of them; the set you hire is the next one up.

Prime, standby and continuous — the rating matters more than the number

The same physical set carries more than one rating. Prime is what it delivers continuously under a varying load, and it is the only rating to size site power on. Standby is an emergency rating for a set that runs occasionally, and it is a higher number for the same machine — a set advertised at 500 kVA standby may be roughly 455 kVA prime. Continuous is a lower rating again for constant unvarying load. If a quotation does not say which rating the kVA figure is, the figure does not mean anything yet.

How to size it — the method

1. Sum the running loads in kW that will genuinely be on at the same time. Not everything on the site inventory runs together.
2. Add the starting surge of the largest motor. A motor drawing X kW running pulls roughly three times that at start, so add about 2 × X kW on top of the running total. Only the largest motor — two large motors starting simultaneously is a sequencing problem to solve on site, not a reason to hire a much bigger set.
3. Convert to kVA at a 0.8 power factor: kVA = peak kW ÷ 0.8.
4. Add 25% operating headroom — × 1.25.
5. Add 25% for UAE heat. Diesel output derates by roughly 1% per °C above 25 °C, so at 45 °C ambient the set is delivering around 20% less than its rated output. × 1.25.
6. Round up to the next standard size from the list above, on the prime rating.

Steps 4 and 5 together are a single factor of 1.5625 applied to the kVA figure.

The method on three typical load pictures

Illustrative load lists, not a specification for your site. Run your own figures through the generator sizing calculator, then have the result checked by a qualified electrical engineer or the supplier's engineer before you commit.

Load factor — and why oversizing is not free

Load factor is the average load the set actually carries divided by its rated output. Target a steady 70–80% of prime rating. A set running long hours at very light load — below roughly 30% — wet-stacks: unburnt fuel accumulates in the exhaust, the engine glazes, and the set runs progressively worse and dirtier. That is why the answer to an uncertain load is not simply the biggest set available. If the load profile is genuinely lumpy — heavy for a few hours, nearly idle the rest — two smaller sets, or a set plus a load bank arrangement, is the engineering answer.

What sits around the set

• Distribution. The genset is not the installation. Cabling, distribution boards, RCD protection and an earth electrode all have to be specified, and none of it is automatically included.
• Cable runs. Voltage drop over a long run can leave equipment undervolted even with a correctly sized set. The distance from the set to the load matters as much as the kVA.
• Fuel. Almost always excluded. Base tank capacity sets the refuelling interval, and the interval sets whether you need a bowser visit daily or weekly. Anything with an external tank needs bunding and spill containment.
• Noise. Residential proximity and night work force a canopied set regardless of what the load needs. Noise is rated in dB(A) at a stated distance — usually 7 m — so compare like with like.
• Changeover. If the set is backing up or bridging a permanent supply, the changeover arrangement — manual or automatic — is part of the scope and has to be agreed.
• Air and dust. The set needs unobstructed airflow for its radiator. Boxing a canopied set into a corner for tidiness is a common cause of high-temperature shutdowns in summer.

UAE practice

• Supply standard: 230 V single phase and 400 V three phase at 50 Hz. Equipment imported for another market may not match.
• Civil Defence. Permitted sites require approved equipment with its documentation. This is a gate on the set being allowed to run, not a formality to sort out afterwards.
• Temporary authority supply. A builder's supply from the electricity authority is the alternative to hired power on a long build; the comparison is application lead time against hire duration, so make it early.
• Ambient design point. Size for the summer peak, not for the day you place the order. A set that is comfortable in February and marginal in August is an undersized set.
• VAT at 5% applies to equipment hire.

Permit routes, certificate validity periods and working-hour rules differ by emirate, by authority and by free zone, and they change. Treat everything above as the questions to ask, not as a current legal position — confirm the requirement with the authority, the site and the equipment owner before the machine is booked.

What to state in the enquiry

• Required prime kVA, and say prime explicitly.
• The load list you sized it from, including the largest motor.
• Single or three phase, and the voltage.
• Running pattern — hours per day, days per week, continuous or intermittent.
• Canopied or open, and any noise limit that applies at the boundary.
• Whether distribution, cabling and earthing are in scope.
• Fuel basis and whether a bowser can reach the standing position.
• Dates, emirate and site, and whether Civil Defence documentation is required.

Next

• Generators on the marketplace — what is listed right now, and the specification points that move a UAE quotation.
• Generator sizing calculator — the method above, applied to your own load list.
• Civil Defence approved generator rental in the UAE

Other class references

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