Comm. calc

[barbeer]

Any ideas why a design professional would have a calc that looks like:

67251va/208x2.74=118amps

?

I have seen:

67251va/208x1.732=186.8

I will admit calculations are a weaker point of mine but I have never seen that.

 

[mdshunk]

Is the transformer bank built with only 2 pots?

 

[peter d]

Is the transformer bank built with only 2 pots?

Open delta?

 

[mdshunk]

Open delta?

It does say 208, which has me messed up. There's got to be a reason, but I can't figure it out right now.

 

[bob]

67251va/208x2.74=118amps
67251va/208x1.732=186.8

Perhaps the engineer made a typo and entered 2.74 rather than 1.74 or 1.73.

 

[Engineer83]

1.73 x a very good power factor?

I would have to agree that it could be a typo. Do you know what type of transformer is/was used?




(added question)

 

[barbeer]

I guessed typo, this is just a simple comm. load calc on a set of plans. 208v 3ph 200amp service.

 

[drbond24]

What is the load being fed by this service?

 

[barbeer]

A 1400 sq. ft. mani/pedi shop!

 

[drbond24]

Ordinarily, the equation would look like this:

P = sqrt(3)*V*I*cos(theta)

where cos(theta) is the power factor.

In this case, if you assume that the power factor is 0.95 and work the equation with the given power and voltage, what you get is a 0.6 multiplier. In other words, instead of the above equation the designer used:

0.6*P = sqrt(3)*V*I*0.95

Looks like a load diversity factor to me. He/she figured that only 60% of the equipment would be operating at any one time.

 

[barbeer]

Looks like a load diversity factor to me. He/she figured that only 60% of the equipment would be operating at any one time.

Would you normally even show this math on a simple set of plans that electricians and inspectors(non-engineer types:smile: ) would be using in the field?

 

[drbond24]

Would you normally even show this math on a simple set of plans that electricians and inspectors(non-engineer types:smile: ) would be using in the field?

I personally would not, no. I would have just put 118 amps and left it at that.

 

[Smart $]

67251va/208x2.74=118amps
67251va/208x1.732=186.8

Just an FYI...

These calculations are written improperly. They should be in either of the forms below:

VA/(V?√3) = A, or
VA/V/√3 = A​

 

[drbond24]

Just an FYI...


These calculations are written improperly. They should be in either of the forms below:

VA/(V?√3) = A, or
VA/V/√3 = A
​

How do you get a square root symbol to show up?

 

[Smart $]

How do you get a square root symbol to show up?

Copy it from Character Map (Windows OS's). Rather than visually finding it in the "map", I use the advanced view and Search on "sq".

 

[jerm]

Copy it from Character Map (Windows OS's). Rather than visually finding it in the "map", I use the advanced view and Search on "sq".

There's a way to do it with just pressing keys too, but I don't remember it off hand. I think you hold down ctrl (or alt?) and press a four digit number. Anyone remember that method?

 

[billsnuff]

http://tlt.psu.edu/suggestions/international/accents/charmap.html

try this link or Control + V