Recommended KVA rating?

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larrybartowski

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Question for you. What is the recommended maximum KVA rating of a single motor driven piece of equipment on a 100kVA, 120/240V, single phase, three wire system and why is it considerably lower than the system rating? Any help is appreciated.
 
larrybartowski said:
Question for you. What is the recommended maximum KVA rating of a single motor driven piece of equipment on a 100kVA, 120/240V, single phase, three wire system and why is it considerably lower than the system rating? Any help is appreciated.
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Who says it should be lower?
 
I am not sure of your involvement in the electrical industry since you profile shows PM. With regard to the NEC, the max load load is 100 kva. There may be some engineering decisions that may alter that number. Maybe you could tell us what your teacher's comments are.
 
Motor vs Service Size

Motor vs Service Size

Think about the motor?s starting current and what happens to the voltage as the current flows through the impedance of the power system, the distribution transformer and the service wiring. Ohm's law applies.

Ever had your lights flicker when the garbage disposal starts or the refrigerator kicks on? Those motors were probably a lot less than the maximum rating of the service.

An old engineering rule of thumb is 3:1. If I have a 100HP motor I need a 300 KVA transformer to get it started with minimal voltage drop issues. (1HP is about 1 KVA).

If the customer and equipment can handle a 20% voltage drop you could start a 100 HP motor on a 150 kVA transformer, maybe.

Using a soft start or a VFD can help sometimes.
 
Voltage Drop

Voltage Drop

The probable reason is the allowable voltage drop during motor starting. The 100kVA transformer has impedance. During starting motor current can easily be 6-8 times full load current. This may drop the voltage low enough that the motor controller drops out.

If you have 100 1Hp motors and start them one at a time, you can probably run 100Hp on 100kVA. If you have one 100Hp motor, the starting voltage drop will probably prevent successful operation. If you use a VFD or soft start, you can probably run a 100Hp motor on 100kVA.
 
boboelectric said:
I never heard of a 1 phase/240 motor that big.
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That is what I was thinking

here is the OP

larrybartowski said:
Question for you. What is the recommended maximum KVA rating of a single motor driven piece of equipment on a 100kVA, 120/240V, single phase, three wire system and why is it considerably lower than the system rating? Any help is appreciated.
Click to expand...

I'm not going to say that they don't exist but about the largest single phase motor that you will commonly find is about 16 hp.

add: three phase transformers are just as subject to voltage drop

I have seen larger three phase motors installed on single phase systems with phase conversion equipment however. Soft starts and VFD's will drastically change how much voltage drop occurs during starting.
 
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