calculating single phase loads on three phase service

[gregpack]

Hi, I'm looking at a rehab project, a small special use building (carwash) with a 240V 400 amp 3 phase service. We're trying to avoid upgrading service and this appears to be very tight. Before I consult the electrical contractor I wanted to get a rough idea of our real needs. Currently, if I add up all 3 phase motor loads for the proposed new equipment the total is 295 amps. Left in the mix are the single phase items found in every small commercial building (lighting, HVAC, etc). My question: when evaluating single phase loads on three phase service, is an "amp an amp"? For example: if my 110V lamp fixture (continuous use)uses 1.3 amps, do I add that number to the total load, or is there a formula used to reduce that number for load calculations?

Also, does anyone have any recommend reading on the subject, something easy to understand that might assist me in learning more about this topic?

 

[charlie b]

Re: calculating single phase loads on three phase service

To start with, no, an amp is not an amp. This is not an easy subject to explain to a person not already familiar with the terms and phrases. I'll take a quick stab at it, but you really are going to have to get some professional assistance.

The thing that is the same, regardless of whether single phase or three phase, is "apparent power." It is expressed in units of "volt-amps," or "VA." For any single phase load, you multiply the rated voltage times the rated current, and you get its VA. For any three phase load, you multiply the rated voltage times the rated current, and then multiply by 1.732 (i.e., the square root of three), to get its VA. Then you add all the VA. Finally, you divide the total VA by the system's line-to-line voltage (which I believe you said was 240 volts), and then divide that result by 1.732. You now have the rated current.

But there are several twists to this story. The first is that you don't use value for amps that you find on the nameplate of a motor. Instead, you look for the horsepower rating, and then you look up the amps in the National Electrical Code. Table 430.248 is for single phase motors, and Table 430.250 is for three phase motors.

The next twist is that you have to add (i.e., to the total VA) 25% of the VA associated with the largest motor. That is essentially to allow for the higher starting current that all motors experience during their initial starting period.

This process might give you a more accurate picture of your needs. But it may still not be a complete picture. There may be more "twists" to the story that I cannot name, without knowing more about your system. Your electrical contractor will have to see what you have, before telling you what your actual options might be.