UPS Input and Output Cable Size

Cable size is related to the rating of the circuit protection device(s), which hasn't/haven't been mentioned. Neither has the voltage nor the country of installation, nor the method of installation of the cable nor the ambient temperature to be expected! In all cases, follow the electrical installation codes applicable to the country of installation; in the UK it is British Standard 7671.

If in doubt, consult a qualified local electrician.

This is actually a 240V single phase installation. The load is 4kVA at pf 0.8. It gives a load current of 13.3A. Ambient temp is 24 degrees Celcius. The circuit breakers are at 16A. The length of cable from UPS to load is 3 metres. The length of cable (expected) from mains to UPS is 10 metres. The cable is to be run through trunking installed on the wall. I also wanted to know as a rule of thumb if the input cable to a UPS caries a lower load (current) than that from the UPS to the load (higher current).

What would make you think that the UPS is providing a higher current to the load? If anything the current load utilized by the UPS would be higher due to the charging circuit for the battery bank(s). The load side will only reflect what is being used by...well...the load itself. An exception would be a highly efficient UPS with an internal SDS.

Since your power factor is so low (.8) I assume you are powering a number of PC's or such with a number of SMPS powered devices. I would recommend a PFC bank as well to improve efficiency. 240V single phase? What system drives this? How is the rest of the facility balanced off of this phase?

Are you sure. VA is volts x amps so at 4kVA at 240V thats 16.6A.

You probably need to find out about pF - its very important.

At 5kVA and more likely 230V your current is just short of 22A. To assume that the UPS would never get fully loaded is very dangerous. Forget the 4kVA.

The important cable depends on upstream protection, the type of cable and how it has been installed.

Time to pay for an electrical engineer.

If you are installing this and signing of an installation cert by signing section 1 you are accepting the responsibilities and possible liabilities of the designer.

Totally agree with one of the earlier responses - look to the I.E.E. regs.

Your understanding of how the UPS works is flawed.

Keeping in mind the protections required to make the setup safe, the Input cabling needs to be Larger than or the same size as the Output cabling.

The reason for this is that After a discharge the Input will actually have a higher current than the Output while the batteries are recharging, ongoing there will be a trickle charge to float the batteries at the optimum voltage. Therefore Input will always be slightly higher than output.

If you have a bypass circuit for maintenance, then this circuit will have to support the Load.

All the cabling and system design need to be rated for the Maximum Rated Loading of the UPS, not Your load. This is because of the simple case that once the UPS is there, someone will try to take advantage of any spare capacity.

As someone else pointed out, you should get an Electrician or Electrical Engineer to design and install this for you, it is beyond your understanding at this point, so learn from it and get qualified people to do the work.

Regards,
Sapper

Thanks everyone.