Generator & Battery Bank Loading

I've heard it commonly said that 80% is the maximum loading on generators and battery banks. However, I'm not seeing this in the NEC (Arts. 110,300,445,480,700,and 701). Is maximum loading capacity a manufacturer standard? I've got fluorescent & HID lighting loads to back up on both sources.
(The generator is 30 years old.) --- Thanks

 

[busbar]

beware restrike charactersitcs and cold/wet stacking

ethical/legal implication of succesfully operating system

life safety outside NFPA 70

 

[peebee]

I'm guessing the 80% max loading is coming from the 80% power factor rating of most UPS and generator systems. On transformers, usually you just worry about kVA, as transformers can handle a load at 100%PF. However, most gensets and UPS's cannot handle a load at their full kVA rating with a 100% PF.

Also, you mention that you have lighting that you are trying to back up with both sources. If you are trying to support a UPS from a generator, be careful. UPS systems often have a leading power factor, which can mess badly with the generator governor. Some newer UPS designs avoid this. However, with some generator and UPS combinations, the total UPS load on the generator cannot exceed 50 or 60%.

 

[brupp]

I've never heard of an 80% loading limit on generators, but when sizing a new battery it is generally limited to 80% loading because of reduced capacity at the end of life. IEEE specifies that an 80% aging factor should be used. If your battery or generator is in a critical (for example life sustaining) service there are all kinds of good reasons to apply additional margin, but if they are just used for typical plant operation purposes I can't imagine any reason to derate a generator from the information given.

 

if you are going by the max power rating of the genset, as specified by the MANUFACTURER, this is normally tested at 110 % on a resistive load before final release to the customer. (if operating at a high altitude a de-rating may also be necessiary). The sump, block, radiator top, cab top, air in, air out and alternator teperatures are all monitored and the test will continue untill all values are settled. 100% on a resistive load should be a walk in the park.
however, operating at 100% loading will make heavy demands on engine oil longivity and fuel economy.
IF YOU ARE GOING BY THE POWER RATING SPECIFIED ON THE ALTERNATOR CASING it may be a wise idea to run at slightly less than 100% as the engine power output may be rated at the same value as the alternator output ( 1 horse power is approx 750 Watts). as the efficiancy of the alternater is typically 80% you may overload the engine. hope this helps.

 

[jbartos]

Suggestion: The 80% design margin is coming from manufacturers' catalogs, IEEE Standards, Engineering Handbooks, and some textbooks. It essentially leads to the increased lifecycle of the electrical equipment applied to, which appears to be a reasonable idea.

 

Normally consideration is given to avoiding loading generators to their maximum because to avoid oveheating thereby lengthning thrie working life. I suppose 80% is still a bit high. Lets make it 75%.

 

[jbartos]

Suggestion: It just may pay off to be a prudent investor.

 

[peterb]

What type of generator are we talking about here? Note that utility generators are normally operated at 100% of rating on base load.

 

[r19h72]

Battery:
The rated capacity is linked to a given discharge-time, eg 5h, 10h, 20h. The longer the discharge-time, the higher is the rated capacity of the same battery.
In general it's not wise to take out more than 80% of the available capacity of a lead-acid-battery, because of the risk of deep-discharge. Deep-discharge will damage the battery.
The NiCd-battery is more robust against deep-discharge.

 

[jbartos]

Suggestion to peterb (Electrical) Jul 3, 2002 marked ///\\What type of generator are we talking about here? Note that
///regulated\\utility generators are normally operated at 100% of rating on base load.
///The "supervised" deregulated utility may somewhat change the loading pattern.\\

 

[peterb]

jbartos -
I believe that the principle of economic dispatch will continue to govern the reality of power system operations. A utility will operate its most efficient units at base load before bringing on lower ranked units to meet system demand. This will result in the more efficient units being operated continuously at their design loading.
Of course, small systems may have different objectives. An example would be a cogeneration plant whose mission is to optimize thermal loads rather than electrical, or which has a mandate to supply in-house loads only, without exporting power to an interconnected utility grid. Either of these scenarios could result in the generator being operated at less than rated load.

 

[peebee]

Where is the most efficient point on the motor load curve? At 100% loading or somewhere below?

 

[jbartos]

Suggestion to peterb (Electrical) Jul 4, 2002 marked ///\\jbartos -
I believe that the principle of economic dispatch will continue to govern the reality of power system operations.
///Yes, this is true. However, what the principle of economic dispatch includes appears to be changing. The contractually committed power has to be delivered or the penalty is imposed. This means that the power generating plant may need to have some spare capacity available for cover the load fluctuations to avoid penalties. One way to be prepared for the load fluctuations is to have a generator running at less than 100% (or 110% or so, if a temporary and short peak is encountered).\\

 

[jbartos]

Suggestion to Peebee July 4, 2002: Actually, the ordinary three-phase induction motor efficiency tends to increase beyond the 100% loading before it peaks and starts decreasing. However, the efficiency variations around 100% or rated loading is very small, e.g. 1% or so.

 

[peebee]

I should have been more clear, where is the peak on the diesel motor loading-vs-efficiency curve for an engine-generator set? At 100% load or somewhere below? If the goal is to minimize variable costs, the genset would want to run at the point of peak efficiency. Can the gensets be designed for optimum performance at different points on the curve depending on application?