Portable Generator For Sale

Emergency generator for small business - single or triple phase?

We are a small business that has 15 refrigerators protecting our stock - 9 commercial refrigerators 115Volt/60 Hertz/1Phase and 6 residential refrigerators - 20 amp. We are in the process of installing an emergency generator to protect only our refrigerators and 2 UPS servers. I'm working with two electricians who are bidding on the project. One is telling me that we need a three phase generator (30KVA). The other electrician is recommending a single phase 14KVA. I'm confused - since all of our refrigerators are single phase as are the two UPS servers, and each of the refrigerators and servers are on dedicated circuits. Do you think we need a triple phase generator? Thank you in advance for any help you can offer. All the best, Paige

Public Comments

1. Do you have 3 phase power now? If you do, then a 3 phase generator is the way to go. (all of your loads being single phase doesn't mean you don't have 3 phase power). I'd need to know the exact power used by each refrigerator to determine which quote is best. Plus see the actual wording of each quote. I suspect one of the quotes is to provide back up for everything. With that much motor load, a few lights won't make much difference, and may avoid extensive rewiring. .
2. If you are sure you do not have any 3 phase equipment that is going to use the emergency power then there is no real reason to install a 3 phase generator. Aside from that there is something bad wrong with one of the electrician`s calculations on your kVA requirements. If I were you I would consult the refrigerator and UPS server manuals, or factory reps if necessary, and find out the individual Volt Amp requirements for each piece of equipment you plan to put on the emergency power system. Total the individual VA requirements up and add about 40% (or what ever you think necessary) to the total for future equipment additions and so on. Now see if that total is less than 14 kVA. If so you can feel safe going with the 14 kVA generator which I am sure is considerably more economical than the 30 kVA generator assuming both units are new and under about the same warranty. If you go with the 3 phase system for whatever reason you will need to place equal loads on each of the 3 phases and that probably means your present distribution system will have to be modified. Make sure the folks that does the installation are licensed and bonded. Be sure and remember that the advice you get from us is free and sometimes advice is worth about what you pay for. We try but sometimes we don`t know enough details about the project and sometimes we just don`t know enough period.
3. It is always a good idea to get more quotes. With just two there is always likely to be confusion. A three phase generator at this size may or may not be cheaper than a same power single phase. A contractor may also have access to a particular machine at a good price, so the actual calculations of power required may not be the total story. The end price, and what else you get are therefore important. Some points to consider are what rearrangement of the cabling to suit either system, whether the changeover is automatic, parts availability, battery charging (float), whether the exhaust is properly taken outside to a suitable height with no chance of rain getting back down it, whether there is a day tank and a storage tank, ventilation and cooling of the generator, compliance with local regulations about exhaust emissions and fuel storage and so on all come into it. A diesel engine that is too big may run inefficiently, and may also suffer from glazing of the bores due to waxes etc in the fuel. Fuel consumption/efficiency is a very important issue once you have it installed. You will probably have to run it regularly to get the cobwebs out. From what you have said it is difficult to determine the load. You need to find out the ratings, add them all up. The generator has to drive everything together in this case. When a power failure occurs the generator comes on line in a few seconds, so the refrigerators need some sort of time delay before they come on. Refrigerators don't take well to being turned off then on again a few seconds later. Additionally, if they all come on together there will be a bigger surge to accommodate. Random set "turn on" timers for each unit are a good idea, so they come on at different times with a delay up to a minute or two. Some may have this feature already, otherwise you will rely on the "Klixon" which is an overload switch inside the fridge. At a blind guess the domestic units are 300W and the commercial maybe 1KW. The servers maybe 1KW too. A stab in the dark is almost 14KVA. This leaves no reserve for expansion, and the labour to rearrange cabling for a "standby" bus as well as a "mains" bus might make it cheaper to power the lot instead, and the 30KVA doesn't look too bad. Rearranging the existing single phase building wiring to three phase could also be an issue, so in the end it is the price. It will have a relatively light load though, on average, because the refrigerators cycle on and off. It could be as low as 5KVA on average. Depends on the fuel quality then. You could ask the manufacturer/supplier of the diesel about that. It is important that these guys know what they are doing, and that you have some sort of insurance or bond from them, as with any other contractor work. Get more quotes if you can, and ask the contractors about all these issues.