Standard practices question regarding transformer load to parent

[DeanSoftware]

Greetings, we are the makers of DraftLogic Electrical--automated building electrical systems design software.

We recently consulted with Mike Holt on various NEC code questions to ensure that DraftLogic Electrical would be NEC compliant. Mike has been very helpful and we have incorporated his valuable suggestions into our product design.

We are also seeking advice from the industry regarding common practice on a couple of calculation preferences & Mike suggested we use the forums to get some input, thus the reason for this posting. We would appreciate hearing back from you about a couple of matters.

First, the necessary background & an explanation of a term we will use:

DraftLogic Electrical automatically sizes transformers, protection, and conductors based on connected loads. It also completely automatically creates a single line diagram of the complete power tree, including fault levels?this is just one of the many reports DraftLogic Electrical generates. We refer to the branch circuit panels, feeder devices, and service devices collectively as ?the power tree?.

On to the first question! When carrying load up the power tree do you carry load through transformers as actual wattage used or full load based on the transformer?s size?

For example, let?s say we have a load of 23 kVA on the secondary side of a transformer. Based on that, we would select a 30 kVA transformer to accommodate the load. Would you carry the 23 kVA load up through the power tree or the full 30 kVA of the transformer?s capacity? Please also advise why you use the load that you do.

The second question is about the number of versions you typically go through in a project. How many times would you estimate that you rework each of the panel schedules, single line, and motor/heater schedule on an average project? The rework can be a result of designer decisions, designer errors, client direction change, or in response to other consultant changes.

We look forward to your valued response. Please feel free to respond to this thread or private message me to get my email or phone number if you would prefer to respond that way.

Kindest regards,
Dean Whitford, COO
DraftLogic

 

[charlie b]

I use a set of spreadsheets, or more recently another type of software, to record the loads assigned to each breaker on each panel schedule. Both types of software add the actual assigned load to each panel further upstream. The load on a transformer, and not the rating of the transformer, is what is assigned to the upstream panel. I size the primary and secondary conductors for the full rating of the transformer (plus 25%), but the actual load that the transformer will see, in operation, will be far less than the calculated load. So there is no reason to assign its full rating as the load on the upstream panel.

As to reworking the design, I would echo the words of a famous producer of musical theater, who said that a musical is never finished, but rather is at some point abandoned. So the single line, the panel schedules, and the equipment/motor schedules are "living documents" all the way from the early schematic design phase to the final issuance for construction. But if you are asking how often, for example, the mechanical engineer changes a pump size enough to cause me to change the branch circuit breaker and conductor ratings, that happens a half dozen time or so, on each project. If you are asking how often we need to add or remove a panel from the single line, once we make it through the design development phase that very seldom happens at all.

 

[rcwilson]

We use excel spreadsheets and carry both the rating of the transformer or MCC as well as the connected load and the operating load, and sometimes carry multiple operating scenarios (Example: plant in standby, plant in startup mode, tie breaker open, tie breaker closed, etc.). The operating load is the most important. We carry the equipment size to meet some clients' requirements that upstream equipment must not be loaded past XX% with downstream devices at full capacity.

From initial design using estimated loads through final "Issued For Construction" on through final "abandonment" as Charlie says, averages 8-10 versions.