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
Help me size the cable and breaker
- Thread starter Deebee101
- Start date
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
What is the voltage of the pump and is it single phase or 3 phase
curt swartz
Senior Member
- Location
- San Jose, CA
- Occupation
- Electrical Contractor
Wouldn't that be the job of the engineer?
It is a 3-phase pump
Just help
- Location
- Chapel Hill, NC
- Occupation
- Retired Electrical Contractor
You need to know the voltage
It is 415V 3-Phase
Julius Right
Senior Member
- Occupation
- Electrical Engineer Power Station Physical Design Retired
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.
Since you said 415 V rated voltage this is the old British standard, I think.
Julius Right
Senior Member
- Occupation
- Electrical Engineer Power Station Physical Design Retired
By the way, BS7671 does not refer to Fire Pumps only BS EN 809 does, in my opinion.
Julius Right
Senior Member
- Occupation
- Electrical Engineer Power Station Physical Design Retired
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].
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].