Incoming Current calculation for MCCB & power cable

Hi guys,

I'd like to what is a proper calculation for the incoming current sizing for the MCCB & power cabling ,for example, if I have 10 motors (DOL + Star Delta). Each motor for example running 20 Amps (3 phase, 415V).

Does that means my require current for incoming MCCB & power cable shall be;

General calculation:
10 motors x 20 Amps x 1.3 (Safety factor)
= 260 Amps?

Let's say we just neglect other small instrument power in the panel.

Thank you, guys.

Even better - use a Schneider MCCB with a 5E protection relay, a Schneider intrrfece and extract via Modbus RTU.

Bob,

Thanks for your reply, i need to know the calculation method i did if it's correct. Enlighten me if I'm wrong.

flyers said:

Hi guys,

I'd like to what is a proper calculation for the incoming current sizing for the MCCB & power cabling ,for example, if I have 10 motors (DOL + Star Delta). Each motor for example running 20 Amps (3 phase, 415V).

Does that means my require current for incoming MCCB & power cable shall be;

General calculation:
10 motors x 20 Amps x 1.3 (Safety factor)
= 260 Amps?

Let's say we just neglect other small instrument power in the panel. Thank you, guys.

Click to expand...

Yes this is a good starting point. At least you have some idea of what you are getting into. Gets more complicated from this point
Some other considerations in the next steps of calcs
Ambient temperature, derating factors such as
number wires in conduit, motors that start simultaneously
motors that may be deducted (non coincidental) ie two motors where only one will run at any time ie air conditioning vs heating.
The list goes on

Dan Bentler

I suggest you get someone who has a sound working knowledge of your local electrical standards / regulations to help you with this.

Apart from just your working load, your MCCB and individual branch breakers need to have the correct current breaking capacity rating to ensure they will be able to operate in the case of a dead short. Too low an Isc rating and the breaker will either explode or weld closed.

This is no trivial task for an industrial installation and requires a understanding of where the distribution network transformer is located, its rating, cabling length, type and diameter to the installation.

For smaller installations (domestic etc) some supply authorities set an "assumed", conservative rating of 6kA phase to earth and 10kA phase to neutral.

I've got an MCCB at the moment that we're busy investigating where it appears the sub-circuits do not have sufficiently rated breakers - a couple have exploded recently when short circuits have occurred.

There is no calculation with my suggested method - just a Modbus RTU read from the MCCB.

As Per NEC motor FLC + FLC + 125% of largest motor FLC. But that is NEC

http://www.kolumbus.fi/sepala/Cable calculation.htm

There You can see principle, what ambient temperature meaning for cable size.

BobB said:

There is no calculation with my suggested method - just a Modbus RTU read from the MCCB.

Click to expand...

That might give you your steady state current draw, but will not tell you whether you need a breaker with interrupting capacity of 6kA, 10kA, 25kA etc.

If you've got a 300kVA transformer on site with short cables to your MCCB you could have up to 25kA prospective fault current. A 6kA rated breaker is going to go BOOM.

Your breakers MUST be rated to interrupt the prospective fault current at that point in the circuit.

I think the OP should seek professional advice from someone familiar with his local electrical regulations.

calcs are correct, or use 1.25 as factor.
then see what is temp and construction method (in ground, on wall etc.
then temp of cable
length is also needed.
then the way of fusing in box
short circuit safety etc.

Hi guys,

Thanks for all the input, here the client does not want to provide the calculated amp of 260Amp, because their point is the pumps are not power up and run at the same time. Even of the pumps are running at the same time, they will not be running at full load. Most probably 70% of the load.

They are providing 200Amp x 0.7 = 140Amp or the most 200Amp.

Looks like we really have to calculate in details and show them.

flyers said:

Hi guys,

Thanks for all the input, here the client does not want to provide the calculated amp of 260Amp, because their point is the pumps are not power up and run at the same time. Even of the pumps are running at the same time, they will not be running at full load. Most probably 70% of the load.

They are providing 200Amp x 0.7 = 140Amp or the most 200Amp.

Looks like we really have to calculate in details and show them.

Click to expand...

Uh OH
I hear alarm bells in my head all under the phrase
"it costs too much"

It is a problem when customer knows nothing about electrical and asks you for your expertise and fall over when they see the cost estimate. It may all work out and after a couple sit downs and explanations the job gets done and all are happy.
On other hand,,,,

The customer may have a point in that all pumps will not be needed - the part I find bothersome is "even if all are needed then they are at 70% capacity".

I said previously
motors that may be deducted (non coincidental) ie two motors where only one will run at any time ie air conditioning vs heating.

Dan Bentler

The other option is to use mechanically and electrically interlocked contactors to prevent two loads from being on at the same time "if they never should be". When people say this, I always ask "so what happens when an operator switches both motors into manual by accident, or a contactor welds closed?"

However using interlocked contactors probably going to cost more than the slight cable upsizing required and in this situation would not be worth it. If the upstream Switchboard / supply is not physically capable of supplying you with your required 260A then yes, it might be good option. I've had to do this on a little water supply pump station before to accomodate an air blower without needing to get the supply transformer upsized. It worked out cheaper to install the interlocked contactors and wasn't a problem since the blower would never need to be operated while the pumps were running.

What sort of distance are we talking between the point of supply and your new MCC? Your cable cross sectional area will of course also be dependent on whether it's 10m or 1000m.

I'd ask the client to supply their calculations in writing, so if they insist you install a particular size cable, you've got evidence that it's their ****-up when you start getting nuisance trips of the MCCB or motor thermal overloads tripping because the motors don't have sufficient voltage and are pulling higher current. I bet you right now the guy making all the fuss does NOT understand all the aspects of sizing this cable / MCCB correctly and will probably get very nervous if you ask to see his working.