Single largest load (motor or feeder) in a Switchgear or MCC 3

[NickParker]

I watched a webinar in YouTube, which says
The maximum single load that can be connected to low-voltage switchgear is 1/3 the incoming breaker's amps; for example, if the incoming breaker is 4000A, the maximum single load that can be attached is 4000/3=1333A. If one load is more than 1333A, another MCC or switchboard must be used to connect it (which is better in his opinion)

I wonder, What is the justification behind this? Is it because of coordination between the Incoming breaker and that particular largest outgoing feeder?

 

[waross]

As a general statement it is false.
For specific applications,

Motors; the breaker may be 250% of the motor current.
For MCCs, the vertical bus bars (where the load is connected) may be 1/3 the ampacity of the main bus.​

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

 

[che12345]

Mr. NickParker (Electrical)(OP)1 Sep 23 14:07
1. As a general statement, it is invalid.
2. Consider a motor rated 1330A. Direct-on-line starting say 6 x 1330A =7980A . This starting current is 7980A/4000A = 1.995 say 2. Most ACB tripping at 2 x setting would trip at about 25s. This delay time about 25s would be able to handle the DOL starting.
Note: 1. 1330A at 400V = 800kW approx., is not often started DOL.
2. The starting current at 6x is unlikely > 25s.
2.1 The Inst. setting may be set at say 3x i.e. 3x4kA=12kA.
3. This heavy load 1330A may be connected to the main busbar rated 4kA, preferred close to the main incoming.
4. Starting with Star delta or VFD would have starting current lower than DOL.
Che Kuan Yau (Singapore)

 

[NickParker]

Note: 1. 1330A at 400V = 800kW approx., is not often started DOL.

I have not seen motors beyond 200-250kW connected to low voltage for a voltage range of (380 to 480Volts)
This raises another question of maximum motor rating that could be connected to LV switchgear?

 

[che12345]

Mr. NickParker (Electrical)(OP)2 Sep 23 07:40
"...I have not seen motors beyond 200-250kW connected to low voltage for a voltage range of (380 to 480Volts).This raises another question of maximum motor rating that could be connected to LV switchgear?"
1. You are right that LV 800kW motors are not widely used. Not to mention with DOL starting.
In most cases higher voltage say 6.6kV would be preferred,
2. However, there are motors rated up to 1000kW at 690V, 970A approx. , 3000rpm by motor manufacturers in Europe. In this case, the current would exceed the usual contact size. Therefore, a 2kA ACB would be chosen for AC3 operation, function as a contactor. Attention: 1000kW motors and ACBs are not designed for high frequency/number of switching.
3. In marine application it is very common 600kW , 480V motor with DOL starting used for bowl thrusters.
4. There is no limitation on the maximum motor rating that could be connected to LV switchgear. It depends on the ACB and busbar rating , capacity of the power source, and the local authority limitation.
5. There are ACB and board with busbar rated 6300A on the market.
Che Kuan Yau (Singapore)

 

[waross]

I have not seen motors beyond 200-250kW connected to low voltage for a voltage range of (380 to 480Volts)
This raises another question of maximum motor rating that could be connected to LV switchgear?

A rule of thumb is that when the motor Hp exceeds the voltage, consider a higher voltage.
This is mostly an economical consideration.
In individual cases and after consideration, a motor may be connected to a system with a voltage less than the motor HP.

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