Q We recently had a problem providing temporary power to numerous pieces of 208V, single-phase equipment of various sizes. Sources of power were a number of 120/208V panels, having different power/current ratings. Some panels were unloaded, some had combinations of loads at 120V, single-phase, 208V, single-phase, and 208V, 3-phase. What is the best and fastest way to calculate the maximum power rating of equipment to feed safely from the loaded and unloaded panels in each case? - L.S.

A A proper approach for providing temporary power to variously rated 208V, single-phase pieces of equipment from existing 208Y/120V panelboards, as L.S. requests "the best and fastest way to calculate...," is as follows:

First, the nameplate current rating (amperes) of the panelboard minus the connected load (on a per-phase basis) yields how many "amps" can be added to each phase. Note: Make sure the panelboard's feeder has the ampacity to match the panel's nameplate rating.

Next, find and record the current rating of each single-phase 208V unit of equipment to be connected. With this data, select a properly rated two-pole breaker (typically 125% of the equipment's rating and round it off to the nearest standard circuit-breaker current rating) for each unit.

Then, using the new breaker's nameplate ratings as "loads," add these loads to the panelboard - up to the point of reaching the panelboard's nameplate current rating. This must be done on a per-phase basis with due attention to balancing the loads among the phases. (This procedure will work for either 3-phase panels or single-phase panels. However, there is no requirement to balance 208V single-phase loads for single-phase panels.)

Continue this process of adding the remaining loads to the remaining panelboards until all temporary loads are served or until panelboards run short, whichever comes first.

The above procedure responds to the question, "What is the best and fastest way to calculate the maximum power...?" Given the "real world" situation of a qualified electrician doing this work, ammeter measurements would be a more accurate method of determining existing loads on panelboards, and a proper way to establish current ratings in case the equipment to be connected is not nameplated. Even more "real world" but probably not "Code world" is the simple approach of adding load until the panelboard's protective device opens, backing off a few amps - and then start at the next panel. - J.F.F.

A The best and fastest way to determine the spare capacity of L.S.'s 208Y/120V panels is to attach a recording ammeter to each of the three incoming line phase conductors on a per-panel basis. This will give the actual existing phase ampere loads of each panel. The recording period should preferably last at least one week and must not be less than one complete normal workday. By comparing these recorded values with the smaller of the panel bus rating, the panel's source side feeder rating or the panel's main breaker rating, L.S. will have the maximum allowable current availability for each panel on a per-phase basis. This is faster and easier than trying to physically survey the nameplates of all items presently connected to each panel and then estimate the duty-cycle loading. With this information, L.S. can attach his temporary single-phase 208V devices such that the total loading on each panel is balanced on a per-phase basis. - F.M.P.

A The best and fastest are sometimes not the way to go. As I understand the question by L.S., it can be reduced to a simpler one: What is the proper way to determine the spare ampacity of an existing 3-phase, 4-wire electrical power panel?

The first step is to determine the existing conditions, including the ampacity of the circuit components feeding the panel, panel ratings, available pole spaces and breakers, and most importantly, the existing maximum continuous load on each phase bus.

For example, consider a branch feeder serving the panel has a capacity of 225A, the panel is rated 225A, 120/208V, 3-phase, 4-wire. There are 36 pole spaces with breakers installed and being used in 27 of them; there are no spare CBs, and the existing phase loads are: A = 188A, B = 157A, and C = 76A. That means there are 13 pole spaces available so you can add no more than four 3-phase loads with 37A balanced full-load current because phase A limits amperage to this value.

If adding single-phase loads, there are 37A available on phase A, 68A available on phase B, and 149A on phase C. For 208V single-phase loads, two of the phase buses share each load. A 10A 208V single-phase load connected the panel on phases A and C would leave spare capacities on 27A on phase A, 68A on phase B, and 139A on phase C. It is better to connect additional single-phase loads to the least loaded phases (in this case, B and C). For the example panel, then a 10A208V single-phase load connected to phases B and C would leave spare bus ampacities of A = 37, B = 58, and C = 139.

It is often better to rearrange the existing CBs to better balance the existing loads and free up more ampacity on the most heavily loaded phase. Moving 30A worth of 120V single-phase load from phase A to phase C would change the spare ampacities to A = 67, B = 68, and C = 119.

Whether the existing panel is defined as "loaded" or "unloaded" makes no difference in the approach. Any existing panel will have a load. The load may be zero, but similar steps are required regardless of the numbers. Always measure the existing phase loads and go from there. - J.H.H.