Your questions answered: Low- and medium-voltage electrical systems

Several questions about low- and medium-voltage electrical systems were left unanswered; read answers to these questions here

By Jane Baikadanova, PE December 17, 2020

There are several reasons to choose one voltage level over another for electrical transmission. Some of the questions left unanswered in the Dec. 9, 2020, webcast “Critical power: Low- and medium-voltage electrical systems” are answered here by Jane Baikadanova, PE.


  • Kenneth Kutsmeda, PE, LEED AP, engineering manager, Jacobs, Philadelphia
  • Jane Baikadanova, PE, associate principal/senior electrical engineer, Page, Washington, D.C.

Is it important to have two different power sources for switchboard/switchgear?

Jane Baikadanova: It is based on the project’s requirements; some hospitals and data centers require N+1 configuration.

Perhaps this is an amateur question, but what distinguishes switchgear from switchboards?

Jane Baikadanova: Short circuit testing, size, clearance requirements, cost, reliability, maximum voltage/amperage ratings, breakers used and interrupting ratings.

I have seen some engineers mention that ductwork can be routed over dry-type transformer since it does not fall under the same clearance requirements as panelboards. Is that correct?

Jane Baikadanova: Yes.

How do we determine if the generator needs to follow which NFPA 70: National Electrical Code rating? Is there a list some place or do government agencies specify the requirement?

Jane Baikadanova: Please refer to the definition in each section, it will elaborate what is required. Some of the legally required will be determined by government agencies, standby will also be determined by the client. Article 700 is the most straightforward — requirements are based on electrical codes and standards.

Any advantage to using a 3-pole automatic transfer switch?

Jane Baikadanova: It is cheaper, and smaller in size.

For low-voltage systems (600 volts and below). what is the highest available fault for North American market? Are most of the installations 65 kA, 100 kA or other? Is there any data on the installed base?

Jane Baikadanova: UL 891: Standards for Switchboards covers switchboards for use on circuits having available short-circuit currents of not more than 200,000 amperes.

What is the minimum depth clearance for 75 kVA 480- to 208-volt transformer that is placed in front of the CMU?

Jane Baikadanova: 3 feet 6 inches.

Jane: Is there a standard that can be used as a guideline for load pickup/shedding for sizing the genset?

Jane Baikadanova: I use generator manufacturer’s provided software for sizing gensets; they have options for load pickup/shedding. I think using software to size your genset is the most cost-effective way to do so.

Jane: When should both an automatic and a manual transfer switch be required for generators?

Jane Baikadanova: Typically, we see ATS when we have an on-site generator, and MTS is typically provided for temporary roll-in generators. If you have both types (one on-site, another cam-lock connection for roll-in), you may want to consider both types — MTS and ATS.

Jane: When is an automatic transfer switch required to be service-entrance rated?

Jane Baikadanova: When the entire load of the building needs to be switched to emergency generator in case of the power loss.

Author Bio: Yevgeniya (Jane) Baikadanova, PE, associate principal/senior electrical engineer, Page, Washington, D.C.