Cable sizing
Cable sizing
(OP)
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.
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.






RE: Cable sizing
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
RE: Cable sizing
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
RE: Cable sizing
RE: Cable sizing
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.
RE: Cable sizing
RE: Cable sizing
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
RE: Cable sizing
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.
RE: Cable sizing
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/resources/sites...
RE: Cable sizing
RE: Cable sizing
RE: Cable sizing
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.
RE: Cable sizing