Load List - Peak Load estimation 1

[NickParker]

The Peak load is estimated based on (100% x Continuous Loads) + (30% x Intermittent Loads) + (10% x Standby Loads). Typically this is how load list is estimated in Oil & Gas Industry. Say If I have 10kW load as continuous loads, 75kW Motor as Intermittent load and No standby loads, the peak load comes around (100% x 10kW) + (30% x 75kW) -> 32.kW

Should I size my transformer based on this 32.kW? here the single motor even if it operates for 1 hour, the transformer sized based on this peak load could be overloaded.

 

[waross]

When common sense shows that a specific installation does not fit with the typical way of doing things, go with common sense.
30% intermittent loads assumes enough small motors or loads to justify a diversity factor.
There is no diversity with one motor.
Under North American codes, you would need 75KW / .9 PF x 125% = 104 KVA + 10 KW / PF = >114 KVA transformer.
Under North American codes, transformer and feeder sizing is based on the sum of the motor currents (hence KVA, not KW) plus 125% of the largest motor.
A diversity factor may be used only if it can be justified.
Back to common sense: How will you start this motor on a 32 KVA transformer. 10 KW base load leaves 22 KVA for motor starting.
If the motor starting current is 6 times running current, the starting current may represent a 2200% overload. Think, short circuit on a transformer with an imp voltage of 4.4%
In practical terms, the current will drop as the voltage drops, but it is unlikely that the motor would start on a 32 KVA transformer.
There would not be enough voltage to hold in the contactor. The contactor will go into machine gun mode, rapidly closing and opening under starting current. With automatic start, the contactor may be destroyed in a few minutes or less. I have seen this a couple of times when an overloaded generator tries to start one more motor.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[prdpks2000]

Dear Mr. Nick/Waross

please correct if any thing wrong in this calculation.

Pradeep

 

[waross]

I would use the following order plus judgment.
Plant base load in KVA
Plus
125% of largest motor in KVA
Plus
Additional plant loads with a load factor or diversity factor in KVA
Plus
Additional motor loads with a load factor or diversity factor in KVA

Judgment: do any additional loads or conditions of loading exceed the load factor value?
Two or more loads or two or more motors that always run together should be considered as one load when developing a load or demand factor.
Any motors that always run with the largest motor such as lube pumps, cooling fans, in-feed conveyors and out-feed conveyors should be added to the largest motor at 100%
Notes:
Adding KVA.
Normally the PF of a load must be considered when combining KVA values due to differing phase angles.
When developing a load sheet, KVA values are often added directly without regard to phase angle. The slight discrepancy adds a small aafety factor.
Exceptions: If a large load is known to operate at a very low power factor.
Do a quick check by combining the large load with the balance of the load, taking the balance of the load to be at an assumed high power factor.
If the result indicates that it may be possible to reduce the main transformer and/or sizes, then do rigorous calculations.
Motors with PF capacitors connected. Use the corrected current values for KVA.
Leading power factor loads. Overexcited synchronous motors, synchronous condensors, bulk capacitors banks. Rigorous calculations are warranted.

Motor starting:
Often motor starting issues are ignored. Transformers are capable of considerable short time overloads. However when a large motor is a significant portion of the load on a transformer, it is good practice to do a voltage drop calculation. In some installations, a transformer size may be increased to avoid objectionable voltage sag when the large motor starts.
In other installations, the owner may be happy to live with the voltage sag once or twice a day rather than pay the price for a larger transformer bank.



--------------------
Ohm's law
Not just a good idea;
It's the LAW!