Help calulating load on 3 phase transformer

[jbreeland]

Hi,

I'm designing a control panel for a new assembly line. We have some 240V equipment, all singe phase except for one unit, which is 3 phase. We decided it'd be best to use a 480/240 3 phase transformer and feed all our single phase and our one 3 phase pieces of equipment with the one transformer instead of having multiple transformers floating around the assembly line.

However, since our single phase equipment is split between different legs of the 3 phase secondary side, with the addition of the 3 phase unit obviously tied to all 3 phases, I'm not sure how to calculate what the actual load would be in order to properly fuse the secondary side of the transformer and also size the wire coming into the distribution block. I was told that since it's 3 phase output, but for certain parts we are only using 1 phase, that I can't just added up all the sums, and that loads need to be moderately balanced between different phases.

I'm attaching screenshots of my AutoCAD Electrical drawing. Some notes in case the font is too small......

• 240V 1 Phase - Hot Melt Pump 1 = 35A fuses (Fed from secondary phases L1-L2)

• 240V 1 Phase - Hot Melt Pump 2 = 35A fuses (Fed from secondary phases L1-L3)

• 240V 1 Phase - Hot Melt Control Unit = 20A fuses (Fed from secondary phases L2-L3)

• 240V 1 Phase - Nordson Pump = 12A fuses(Fed from secondary phases L2-L3)

• 240V 3 Phase - Nordson Control Unit = 15A fuses (Fed from secondary phases L1-L2-L3)

I appreciate any advice or help you guys can offer for this.

EDIT: The screenshots are really small. Not sure why they got shrunk. Added more detail to compensate.

 

[charlie b]

Calculate the load in units of VA, and convert to amps at the very end. Ignore the fuse sizes. You need to know the amps that each load will draw. Using the fuse sizes will give you a conservatively high answer, and you may not wish to spend the extra money for the larger transformer that it may wind up giving you.

• For each single phase load, multiply the load amps times 240.
• For each 3 phase load, multiply the load amps times 240, then multiply the result by the square root of 3 (approximately 1.732).
• Add the results of these calculations. That will give you the total VA load for your system. Select a transformer that has a VA rating at least this high.
• Divide the VA rating of the transformer by 240, and then divide by 1.732. You now have the minimum amps needed for the secondary side. Select fuses that are at least this size. You can go as high as 125% of this value.

Welcome to the forum.

 

[Saturn_Europa]

http://forums.mikeholt.com/showthread.php?t=179198&p=1769959#post1769959

Some additional reading. Smart$ does a very good job of answering a very similar question.

 

[Smart $]

You should not calculate your load using the fuse ratings, as their ratings are typically greater than the load. You need to use the watt or ampere rating (converted to kVA) on the nameplate of the equipment.

 

[jbreeland]

A problem I have is 2 of the units (the 2 NORDSONs) are very old and we have no documentation on them. Also, the original manufacturer doesn't make them any more. The 3 HOT MELT units have documentation provided to us already, but no FLA information and we don't physically have the units as they have been ordered but not delivered yet. I've reached out to the product owner to see if they can provide me FLA information for those.

What do you do in a situation like with my NORDSON units where there is no documentation and no nameplates on the equipment?

Thanks again for helping me through this. I only had one AC power systems class in college probably 5 years ago and I haven't done anything like this since then.

 

[Smart $]

If all you have are fuse ratings and not the nameplate load value, using the fuse ratings will do, as it will always yield a conservative load calculation.