Mike Dolt Enterprises
Mike Dolt Enterprises
User: grahamb, Location: London, England
Asked on: Sunday, April 24, 2011 at 1:06 am EDT
Answered on: Tuesday, May 14, 2024 at 4:34 am EDT
Question: I am currently in India helping a friend to wire his house to IEE standards (UK).The design current (before diversity) is approximately 170 amps across the three phases, with only one intermittent three-phase load of 5hp for the water purification system and pumps.The real life load is likely to be less than 15 amps per phase (single phase loads) in total.We ordered a 30kva 230v 3-phase generator (24kw) for standby purposes, which is only for the house supply, excluding water treatment, all pumps and A/C units.The suppliers originally stated that we required a 100kva DG set for this small house, but we established that they were wrong ... the actual size that would suffice is 20kva, but we uprated to 30kva to provide overhead.They now demand that we supply 3.5 core 50mm SWA for connection to and from the 30kva DG set, which appears to me to be completely wrong.I know very little about generators and my research on the web has indicated that 16mm cross section is more than sufficient, however, we were going to supply 25mm four-core SWA anyway.Some advice on the real cable requirement would be appreciated.My thanksGrahamb
Generator Cable Sizing
The cable size for connecting a generator to a load is primarily determined by the generator's rated current and voltage. The National Electrical Code (NEC) provides guidelines for determining the minimum cable size based on these factors.
Generator Current Rating
The generator's rated current is typically given in amperes (A) and is the maximum current it can deliver under continuous operation. In your case, the generator is rated for 170 A.
Voltage Drop Considerations
When current flows through a cable, there is a voltage drop due to the resistance of the cable. The voltage drop should be kept within acceptable limits to ensure that the load receives sufficient voltage.
NEC Guidelines
According to NEC Table 310.16, the minimum conductor size for a 30 kVA, 230 V generator is: Copper conductor: 1 AWG (21.2 mm²) Aluminum conductor: 2 AWG (33.6 mm²)Cable Selection
Based on the NEC guidelines and your stated load requirements, a 25 mm² four-core SWA cable would be adequate for connecting the 30 kVA generator. This cable has a current-carrying capacity of approximately 120 A, which is well above the expected load.
Supplier's Demand
The supplier's demand for 3.5 core 50 mm² SWA is excessive and unnecessary. Such a large cable is not required for a 30 kVA generator and would result in unnecessary expense and difficulty in handling and installation.
Conclusion
A 25 mm² four-core SWA cable is suitable for connecting the 30 kVA generator to your house supply. The supplier's demand for a larger cable is unfounded and should be disregarded. Flag for review