Cable Size reduction near the consumer. 2

[joji90]

I have a power cable running from the MCC room to an analyzer cabinet, we calculated the required size and pulled 10mm2 cable(10-P-1142/1143). however after the switch , its reduced to 2.5mm2 since this portion of the cable was existing. this section of the cable ( 10-P-2101/2017) is only 5 mts. Can someone tell me if this would be an issue wrt the tripping of the breakers in the MCC room or could there be any other issue?
Thanks in advance

 

[RRaghunath]

The arrangement mentioned in your post is standard and widely practiced to my knowledge.
The reason for using large cross section is to restrict voltage drop to within acceptable limits and the reason for small cross section cable near the user end is to match with the available terminals in the cabinet/switch.
The practice is to replace the existing terminal block with one that can accept large cross section cable. When this is not possible, provide a JB to connect large cross section wire one side and small size on the other side.
To conclude, this can no way be a reason for trip.

 

[che12345]

Dear Mr. joji90 (Petroleum)(OP)20 May 22 14:06
"... I have a power cable running from the MCC room to an analyzer cabinet, .... pulled 10mm2 cable. #1. however after the switch , its reduced to 2.5mm2 since this portion of the cable was existing. #2. this section of the cable is only 5 mts. #3. Can someone tell me if this would be an issue wrt the tripping of the breakers in the MCC room or could there be any other issue? ..."
#1. From your dwg, I take it that the power flow is from MDB-1-01 F21 (F22) with cable size 10mm², up to the switch. " after the switch , its reduced to 2.5mm2 since this portion of the cable was existing." The switch should be replaced by a breaker or fuses rated for 2.5 mm² conductor. Attention: A switch does NOT protects the conductor.
#2. With the switch replaced by a breaker/fuses, it would protect the 2.5 mm² conductor irrespective of the length. FYI: When there is a change of conductor size, a protective device (e.g. breaker or fuse but not switch) shall be provided to protect the smaller conductor.
3. The breaker which is of a much higher rating, would NOT protect the 2.5mm² conductor. The small breaker rated for 2.5mm2 conductor would likely co-ordinate with the much higher rated breaker. Therefore, in general; no co-ordination issue.
Che Kuan Yau (Singapore)

 

[joji90]

@Rraghunath, thanks for your reply. my question was not whether it would cause a trip rather will the existing breakers inside MDB-I-01(designed for the 10mm2 cable) provide enough protection incase of any short-circuit in the 2.5mm2 section of cable ?

@che12345, thankyou, we have breakers inside MDB-I-01 which is rated for the power consumption of 1400w . Just wondering if it really makes sense(costwise) to install new safety switches(SS-10-CP-1030/1031) and change 10-P-2101/2107 downstream of the switch also to 10mm2 type( around 4 mts each). the existing switch cannot take in 10mm2 cable.

 

[RRaghunath]

"will the existing breakers inside MDB-I-01(designed for the 10mm2 cable) provide enough protection incase of any short-circuit in the 2.5mm2 section of cable?"
It depends on what breaker / settings of trip unit in that breaker. You have a three core cable to the load. 1400W three phase load is not an issue for 2.5sqmm wires. If your breaker is set to protect 1400W load, it can certainly protect 2.5sqmm wires.
The information available in your post is inadequate to provide more specific response.

 

[che12345]

Dear Mr. joji90 (Petroleum)(OP)23 May 22 14:44
"...#1. @Rraghunath, .... my question was not whether it would cause a trip rather will the existing breakers inside MDB-I-01(designed for the 10mm2 cable) provide enough protection incase of any short-circuit in the 2.5mm2 section of cable ? #2. @che12345, .... we have breakers inside MDB-I-01 which is rated for the power consumption of 1400w . Just wondering if it really makes sense(costwise) to install new safety switches(SS-10-CP-1030/1031) and change 10-P-2101/2107 downstream of the switch also to 10mm2 type( around 4 mts each). the existing switch cannot take in 10mm2 cable...."
#1. The breaker (rated current ?) designed for 10mmsq cable is unlikely to protect the 2.5 mmsq cable under [over-load or short circuit]..
#2. I may not understand/misinterpret your question. I try to answer it:
Note: the basic requirement is " whenever there is a change in the conductor size, a OCPD shall be installed at its origin to protect (over-current and short-circuit) of the smaller conductor..
In this case, suggest to installed a new small OCPD* rated for 2.5mmsq conductor inside the MDB-1-01 . The supply cable is to tap off from the outgoing side of the 10mmsq breaker. The tapping off wire to this new small breaker* rated for 2.5mmsq cable is allowed to be of smaller size (rated for 2.5mmsq cable inside the cubicle) [with length < 3m]. After this new small breaker* rated for 2.5mmsq cable, the cable can be 2.5mmsq up the next external switch and to the load etc.
Che Kuan Yau (Singapore)

 

[joji90]

Dear Che12345,

due to the reasons stated above , i connected the power cable directly to the JB( got rid of the Switch after client said that can be done)

Thanks for the response.

Regards,
Joji