208/230 single phase on 277/480 systems

[kwired]

That seems to make sense on an xray machine. So then design it for that higher spec, and use the 80% rule, and you should be fine.

That kind of equipment they usually tell you what kind of voltage tolerance they will allow for their equipment. It is up to designer and or installer to determine if other equipment will effect voltage when it is in operation.

 

[desert designs]

happy monday.


so, I talked to my esteemed PE buddy about the xrays...

good information from someone who has done alot of 'em.

1) He said plugging in 50% of the peak demand is good for your VA calculations at the panels
2) power factor.....doesn't mean anything for the task at hand
3) since these units have their own disconnect with OCP, you need another breaker of the same rating at the panel.
4) luckily voltage drop calc.s for the feeder sizes at various disctances are provided by GE.
5) 517.73 Rating of Supply Conductors and Overcurrent Protection. gives us everything we need for our system calculations....50% largest, 25% next largest, 10% of the remaining units....
6) the biggie, provide a note that the required 22K AIC panel for the mammo units is series rated by the manufacturer for the incoming fault current at the distribution panel.

All my stuff gets signed for by a civil PE......it'll be interesting to see if this one gets kicked back though for an electrical PE....we'll see


GOOD TO GO!

know the code.
steve

 

[charlie b]

" Isn't your explanation of the sum of loads and vectors, pardon the phrase, nitpicky and theoretical. . .

Given that a ?nit? is the larvae stage of certain insects, if one does not pick the nits, then the adult insects will return someday to attack you.

. . . when it's common practice to just add up the watts as VA and sum it up, add 125% where appropriate and call it a day ?

Watts and VA are not the same for motor loads, certain types of lighting loads, welders, and a few other things. And the value of VA for such loads is always higher than the value of watts. Therefore, to simply add them as though they were the same is to make a non-conservative error.

 

[charlie b]

You must also add their reactive power values (i.e., VARs), and then use vector addition of the watts and the VARs to get the total apparent power (i.e., VA).

Watts is scalar, not vector.

Watts and VARS can be represented as two legs of a right triangle, and VA makes the third side (i.e., the hypotenuse). Vector addition is an available method for adding the two legs to get the third leg. There are other methods, but they amount to the same thing.

 

[charlie b]

All my stuff gets signed for by a civil PE......it'll be interesting to see if this one gets kicked back though for an electrical PE....we'll see

I don't know anything about your arrangement with the civil PE. But I will say that if you have a completed design, and if you hand it to an electrical PE with the request that that person review it, sign it, and apply the PE seal, then you are asking that person to commit a violation of professional ethics, and perhaps as well a violation of the law. If you want the electrical PE to seal a document on your behalf, you must get that person involved earlier in the design process. Perhaps I am picking nits again, but the seal of a PE asserts that that person did the work personally, or that that person supervised the work. A PE cannot make that assertion if the work was all done before it was ever shown to, or discussed with, the PE.

 

[desert designs]

believe me mr. beck, I feel bad about saying nitpick, I regret it, and had to go for a long hike yesterday, and talk to my EE PE counselor this morning to feel whole again. I'm glad I signed up and got knocked around. It's good for me. Running head on into those xray specifications quelled my giddiness as well.

caveat to the students that read this board: This is NOT the way this should be done. Charlie is abolutely correct in that an engineer who signs for engineering work is lawfully responsible to be familiar with the discipline and be involved in the design.

In my neck of the real world though, and at many other firms the secretary signs the PE of record's signature on the plans, with his blessing of course. again, DO NOT try this at home. I have 3 safety nets: my PE friends, EE, CE, and SE, plan check and contractor. None replace professional studied knowledge. The way fees are these days though in a very competitive market that was flush with work 6 years ago, you have to be lean and mean.