Sizing Electrical Equipment: Maximum Running vs. Peak Loading

[ThePunisher]

Hi all,

I would like to place this Query(s), though this might be an old-long discussion.

I am currently looking into an electrical load list to size "new" MCC and another to assess "existing" MCC.

The electrical load lists have a summary of three loading categories, Continuous, Intermittent and Peak. The MCCs (and switchgears, transformers) are sized based on PEAK loading.

The Maximum "Running" loading is based on (100% x Continuous Loads) + (30% x Intermittent Loads), where 30% is the factor of intermittency and is variable based on engineering judgment.

The PEAK loading, on the other hand, is based on (100% x Continuous Loads) + (30% x Intermittent Loads) + (10% x Standby Loads), where again, 30% is the factor of intermittency and 10% is the standby factor used, both variable based on engineering judgment.

I would like to question the 10% standby load added into the peak loading, which in turn is used for sizing the MCC bus rating. If these standby loads are only used for operational reliability and are only switch on if the other motor is off (like in the case of motors A and B, or MOVs used to open/close only...not throttling), is the "standby" loads added to the peak make any sense. Even if, say the "B" motor is started first before "A" motor is switched off, this loading will only take less than 10 seconds to start and overall less than 20 seconds for both running, can this be a good justification to say make them part of equipment sizing.

The problem is that when I have significant "standby" loads in one MCC, their 10% contribution to the peak load can be substantial. This becomes critical when evaluating loading of an existing MCC wherein taking 10% standby would make the result almost borderline.

Can anyone here help me out in case, I am missing something or misunderstood the concept.

Thank you in advance.

 

[dpc]

I've never used that load breakdown when sizing equipment. I think this will be influenced by the type of system and types of loads. How long do peak loads run? And what do you use for "continuous load"? Few motors actually run at full load amps.

For most industrial processes, I've found taking 70% to 80% of the connected load is generally a good estimate of the peak demand.

But, there isn't that much cost penalty for oversizing the main bus or even the feeder. The tougher decision comes when calculating the required transformer size, or how much additional load the transformer can take.

From the school of hard knocks: No one ever got fired for oversizing something.

 

[ElecEE8]

You have a correct understanding! The above calculation is most applicable for oil&gas and petrochemical load assessment when we are going to sizing equipment i.e. switchgear, transformer..
For transformer, some of client specifications also require additional of 25% for future growth of load and for future expansion. means they consider peak loading plus 25% for future load for transformer sizing..

 

[xnuke]

Most motors are oversized by the engineers I work with for a couple of reasons: the best efficiency point (BEP) of the connected load is at about 80-85% of full speed (e.g., centrifugal pumps, fans) and the machine ratings are selected so they operate near the BEP, and the motors are sized for 100% because they are afraid to undersize them. This means most of my MCC connected load takes around 80% current when running. When you look at standby loads on the same MCC and put a percentage on that, the total load on the MCC will be somewhat less than 80% of the fully connected load, like dpc said. Obviously this may be different for other kinds of loads like torque motors.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[stevesummers]

Hi,
ELECEE8 is correct that it is mainly used in Oil and Gas - developed by Shell I believe. The idea is to use the intermittent and standby, instead of simple diversity factors. It’s generally a fairly robust system, but does tend to oversize things a bit.

The idea with standby loads, is that they may have 2 loads running at the same time for a shortish period, whilst they swap services over, or have two running at 50% load for some reason.

In your particular case, it would very much depend on what the standby loads are. I have always argued you need to use a bit of engineering judgement on load lists, and if you can argue / demonstrate that the loads will never run together most Client engineers will usually accept it.