1-Phase VAV on 3-Ph load

[Pierre C Belarge]

What is the actual load of the individual VAV boxes? How many of them are there? How far apart are the VAV boxes? You may be adding extra cost to the job, as you well know the tap distance is limited.

It seems to me you are overengineering the job.

 

[sceepe]

By utilizing an actual 60A/3P rated breaker and not (3) 20A/1P tied together I'm able to then divide the sum total of all the VAV by 831 instead of 277V. Thus I'm able to add more load.

yipee!!! You are saving a little bit of #12 wire. Lets assume you have 9 4KW VAV box to feed. Do you actually think it is going to be cheaper to run 3#4 and #8 gnd in a 1" conduit to each VAV box. The alternative is 2#12 and #12gnd in a 1/2" conduit. Oh yeah your way needs 1 one 60A, 3p, 480V breaker and 9 30a, fused disconnects instead of 9, 1P, 277V breakers.

Your way vs normal way
1" Conduit vs 1/2" conduit
3, #4 conductors vs 2, #12 conductors
1, #8 ground vs 1 #12 grd
30A, fused disconnect vs not needed
1, 60A, 3p, 480V breaker vs 9, 20A, 1p, 277V breakers

IMO your way is more confusing for the electrician who has to balance the loads across the phases and more expensive to the owner. But if thats how you want to do it, more power to you.

Lastly, to be accurate, you should be comparing 3, 1P, 60A breakers to 1, 3P, 60A breaker. If you make that comparision, you will find that you can not "add more load" with the 3 pole breaker than the 3, single pole breakers. Lets take the 9, 4kw vav boxes. If you were to put 3 Vav boxes on a single pole 60A breaker, your load would be 12kw per phase. If you put 9 4kw VAV boxes on a 3 pole breaker you still have 12 KW per phase. By the way 831 / 3 = 277. IMO a lack of understanding this fundamental concept is dangerous.

 

[necnotevenclose]

If I'm understanding this correctly it seems to me that in the field electricians would rather see equipment combined when they are the same voltage and phases. Is that correct?

What if I had a grouping of 480V/3P VAV's with fan and heat and then one or two straglers that I could add to that load to save using an additional breaker? Would that be reasonable to do? Or should I add an additional breaker?

 

[fizzy]

If I'm understanding this correctly it seems to me that in the field electricians would rather see equipment combined when they are the same voltage and phases. Is that correct?

It's all about what the requirements of the load are and what the customer wants and is willing to spend.

Getting required power source from point A to B can be done a number of diffrent ways.

Considerations to be considered should meet present needs of device and possible future needs.

A field electrician should be able to receive the print and the materials and put it together to code and job specs. It doesn't always work that way though.

 

[iwire]

If I'm understanding this correctly it seems to me that in the field electricians would rather see equipment combined when they are the same voltage and phases. Is that correct?

I will get back to you later when I have more time.