Cable sizing 1

[bee79]

Dear All,

I have to determine the size of the lighting panel based on the total lighting fixtures connected under each circuit. My calculation is based on the data below:

Total load connected = 3kW
Lighting panel rated voltage = 400V 3Ph+N
Outgoing feeders for lightings = 230V 1Ph+n
Outgoing feeder rating 16A MCB

Continuous current rating of the panel = 3/(1.732*0.4*0.95)
= 4.56A

Adding 20% spare capacity = 4.56*1.2
= 5.472A

I'll have to choose at least 32A MCCB (thermal magnetic type) for the lighting panel incomer in order to be selective with outgoing feeders which are 16A MCBs. However, based on vendor data, incomer MCCB rating must be at least 100A to obtain total discrimination with the load side MCB. If I chooose 100A MCCB as the minimum, my cable size will be also has to be based on the 100A. But I'll be oversizing the cable since the cable will never carry the continous current which is close to 100A due to the small load. If I choose a 32A MCCB I can achieve partial discrimination only.

I'm not sure which way to follow now. Please share your experience on this problem.

 

[waross]

It depends!
What are the implications of the main breaker tripping on a branch circuit fault?
Is life safety or possible property damage a possible issue?
Are these lights mandated by fire codes?
How long will the feeder be and what will be the added cost of the larger cable?
How much are you willing to pay for complete co-ordination?
Are there any breakers available with a more friendly tripping curve? (Either branch circuit breakers or main breakers.)

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ShooterBooter]

As Bill wrote; It depends.

Total discrimination means that no matter how high short circuit current you have (up to max rated s.c. current of the MCCB/MCB) the downstream device will always trip before the upstream.
There is seldom need for total discrimination; if you know the value of the highest short circuit current available then you coordinate the devices to this value.

So we need to have some more information:
- Maximum available short circuit current at the outgoing MCB's?
- Characteristic of the MCB?

Regards,

-SB

 

[ScottyUK]

Why do you want an MCCB incomer instead of a standard switch-disconnector? Generally they're only used when the available fault level at the board is very high and you need the MCCB to give an enhanced breaking capacity for the MCBs. Most modern MCBs are good for 10kA, but with the MCCB assisting they can claim an enhanced rating of perhaps 25kA or so. What are you feeding this board from?

 

[bee79]

Hi all,

The mcb is c-type for the lighting feeders. Lighting panel kA rating is 10. Main DB which is feeding the lighting panel is 20kA.Feeder from this DB is MCCB. My initial design for the lighting panel is based on fused switch disconnector as the incomer. However my lead engineer told me to change he disconnector to MCCB due to the project requirement. I'll have difficulty in coordinating two MCCBs which are in series in addition to the mcb on outgoing feeders. However as shooter pointed out, perhaps I can calculate the actual short circuit current available on the lighting panel for each cases and choose a proper MCCB so that it will be economical for the cable sizing as well.

 

[ScottyUK]

Can you argue for a moulded case switch at the DB incomer - which is effectively an MCCB without tripping capability? Then you only need to coordinate the MCCB at the source with the MCBs at the lighting board? Sometimes project requirements don't always stand up to logical analysis - the people who wrote the specification couldn't consider every possible scenario and often a concession can be given where the proposed deviation is reasonable.

 

[Alajji]

Hi,
To ensure discrimination of CB on individual circuit , the rating of the main CB is to be at least twice that of the largest circuit breaker controlling an outgoing circuit.

also you need to check the melting time for the cable and the capability of the cable during the short circuit.
generally, during the short circuit the CB shall beak the circuit before the cable burn!

best regards

 

[ShooterBooter]

bee79;
I think both 10kA and 20kA are a bit too even and round to be the actual short circuit current. I suppose that these are each panels rated s.c.c.?
You really need to calculate the actual s.c.c. in the lighting panel.

Alajji;
To ensure discrimination of CB on individual circuit , the rating of the main CB is to be at least twice that of the largest circuit breaker controlling an outgoing circuit.

No, things aren't as simpel as that. With fuses it will work but not with MCCB's or MCB's.

 

[7anoter4]

In my opinion, you have to check if you actually need a total discrimination. You have to calculate what is the presumable short-circuit current at the end of each supplied circuit.
If, for instance, the maximum short-circuit current will be at the 50m length 3*4 mm^2 copper cable, the short-circuit current will be not more than 0.4 kA and 32 A MCCB will be o.k.-in my opinion.
See:

http://www2.schneider-electric.com/..._US/Discrimination tables MCCB to iKQ MCB.pdf

 

[ScottyUK]

That would only apply if you can guarantee that a cable fault will only occur far from the distribution board. If a fault occurs close in to the distribution board where the fault level is still high then the MCB would be in trouble.

 

[7anoter4]

I agree with you. If a fault occurs in a cable both circuit breakers open in the same time. However, it is the same as if the fault is located in the panel and it happens seldom, in my opinion.

 

[ScottyUK]

I agree it's a rare event, but it's still a code violation if the fault level immediately downstream of the MCB exceeds the breaking capacity of the MCB. The easiest solution when it's a new board is to provide an MCCB incomer to the board which has been type tested with the MCB as a series combination to give an enhanced breaking capacity of the MCB. If the OP is lucky then the MCCB at the switchgear and at the distribution board will be from the same manufacturer and he could dispense with the DB incomer and just use a switch.

Schneider publish tables showing the enhanced breaking capability for their MCB range when used with their MCCB products: look at the 'Cascading' section starting on page 95. I'm sure the other big manufacturers publish something similar. Unfortunately equivalent test data where manufacturers differ doesn't seem to exist, e.g Schneider MCB with an ABB MCCB, which is a very common combination where I work.

 

[7anoter4]

Thank you, Scotty.