What is the best service entrance design for a 4000 amp, 480 volt, 3 ph load for a plant that has about 500 amps of conventional building load, 500 amps of mechanical load, and 2000 amps of process equipment loads through motor control centers? Run through one main OCP, divide into three service disconnects, or ask the utility to supply transformers for two separate services?
Looks like you have some home work to do, and not knowing the type of industry, demand and future growth requirements most of the responses you might get here will all be speculative at most, but I do agree with others that having a primary metered service can be beneficial as if there will be future growth you can have the POCO size their primary lines and metering for this allot easier then if they metered after the utility own transforming and they are usually more agreeable to this then if you try to get them to oversize the service if it is metered after the transformer, this is because they would be absorbing the losses of the transformer which can be significant when transformers are not loaded close to their rating.
Another benefit of primary metered service is the customer can have better control over voltage drops as it can be hard to get a POCO to do anything about VD problems after the fact.
But this brings up another question that will need to be asked, how much actual load will be required for each transformer, as with a primary metered system you will want your transformers sized close to the actual loads rather then sizing them larger for future growth as this puts the transformer losses on the customers bill not the utility's, also keep in mind that if the operation of the plant is not 16 or 24 hours a day the customer can expect to be metered for the transformer loss at the times the plant if not running at full capacity, while were not talking a big amount but it can add up over a years time, this all should be figured in when comparing which way to go, there are some great transformer people on here who might be able to give a better idea of what your looking at then I can, but like I said if you size your transformers for the load demand you are expecting now, then future transformers can be added for new loads as the plants expands if it ever does since you already have the future primary line sizing done.
With the sizing of the 4k amps you stated above lets say 1ka for 280/120 loads, and 3ka for the 480/277 volt loads if the primary is a 13.8kv then your looking at about around 110 amps on the primary, if we want to build in futures then lets say we shoot for a 150 amp primary which will give you a capacity of 3.6MW available for future growth or even a 125 amp primary will give you just under 3MW, with you existing loads stated in your post equaling to 94 amps on a 13.8kv primary which is about 1.3MWs so even a 125 amp primary will give you the room to double your loads at a later time so at that time you can add transformers for these future loads this way you keep your transformers sized more closely with their loads.
The idea is to not cause you customer to pay to magnetize a lightly loaded transformer, if the transformer is on the utility's side of the meter then the utility will have to absorb these cost which they will in one way or another pass them back down to the customer in higher utility cost per kwh + peek demand and PF losses if so applies.
