Help me size the cable and breaker

D

Deebee101

Member
Location
Ghana
Occupation
Engineer
The equipment is a 5.5kW Fire Pump. The cables would be run in a PVC conduit. The cable run is about 15 meters. The location is a tropical coastal west african city let's say Monrovia. The cable is a multicore pvc insulated cable. What is the appropriate breaker size and cable size in square millimeter? We use the British system
 
In my opinion, since we refer usually to National Electric Code of United State of America , Cannadian Electric Code or British Standard BS7671 you have to study first the ELECTRICITY DISTRIBUTION CODE OF LIBERIA which is less known here.
Since you said 415 V rated voltage this is the old British standard, I think.
 
By the way, BS7671 does not refer to Fire Pumps only BS EN 809 does, in my opinion.
 
The usually data of a 5.5 kW 50 hz 415 V induction motor are:
Rated current 11.4 A cosfi=0.78 eff.0.86 Istart/In=6.5
Let's say the cable run underground in pvc conduit.
The current-carry capacity of a cable are Table 4D3A for copper conductor and Table 4H2A for aluminium in BS7671. Table 4H2A start only from 16 mm^2 for aluminium conductor. So, I would prefer
IEC Standard 60364-5-52 for 1.5 copper conductor or 2.5 aluminium where in Table A.52-4 (52-C3)
Is 13 A for 1.5 copper and 13.5 A for 2.5 aluminium[3 loaded conductors in conduit].
For such a thin conductor we may neglect skin effect, proximity effect and even the reactance.
In this case the resistance of 1.5 mm^2 copper conductor at 70oC will be:
R=1/1.5/58*1.04*(234.5+70)/(234.5+20)=0.0143 ohm/m
Voltage drop for rated current and 15 m length will be:
DVr=sqrt(3)*11.4*0.0143*15=4.24 V[1%] and for start DVs=4.24*6.5=27.56V[6.6%]
Then, the rated current is 11.4 A and start current is 11.4*6.5=74.1 A .
The maximum short-circuit temperature for a conductor pvc insulated is 160oC in 1 sec.
Calculated according NEC/2023 article 240.92 the maximum short-circuit current if the clearing time is maximum 1 sec[copper conductor]:
I=1000*A/0.5067*sqrt(0.0297/t*log10((T2+234)/(T1+234))
A=1.5 mm^2;T2=160o C;T1=70o C;t=1 sec
Ish=171.2 A
It depends on Fire Pump national standard if 1 sec for clearing is correct. There are some national standards stating t=∞[never clearing].
 
Julius Right said:
The usually data of a 5.5 kW 50 hz 415 V induction motor are:
Rated current 11.4 A cosfi=0.78 eff.0.86 Istart/In=6.5
Let's say the cable run underground in pvc conduit.
The current-carry capacity of a cable are Table 4D3A for copper conductor and Table 4H2A for aluminium in BS7671. Table 4H2A start only from 16 mm^2 for aluminium conductor. So, I would prefer
IEC Standard 60364-5-52 for 1.5 copper conductor or 2.5 aluminium where in Table A.52-4 (52-C3)
Is 13 A for 1.5 copper and 13.5 A for 2.5 aluminium[3 loaded conductors in conduit].
For such a thin conductor we may neglect skin effect, proximity effect and even the reactance.
In this case the resistance of 1.5 mm^2 copper conductor at 70oC will be:
R=1/1.5/58*1.04*(234.5+70)/(234.5+20)=0.0143 ohm/m
Voltage drop for rated current and 15 m length will be:
DVr=sqrt(3)*11.4*0.0143*15=4.24 V[1%] and for start DVs=4.24*6.5=27.56V[6.6%]
Then, the rated current is 11.4 A and start current is 11.4*6.5=74.1 A .
The maximum short-circuit temperature for a conductor pvc insulated is 160oC in 1 sec.
Calculated according NEC/2023 article 240.92 the maximum short-circuit current if the clearing time is maximum 1 sec[copper conductor]:
I=1000*A/0.5067*sqrt(0.0297/t*log10((T2+234)/(T1+234))
A=1.5 mm^2;T2=160o C;T1=70o C;t=1 sec
Ish=171.2 A
It depends on Fire Pump national standard if 1 sec for clearing is correct. There are some national standards stating t=∞[never clearing].
Click to expand...
Thanks
 
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