where's my VD

[KROC2004]

Built 480v 3wire system. Length from distribution to disco is 1050'. A 70 amp 3/p is feeding #2 XHHW. Before installing pipe sent RFI in regards to what I thought was a design flaw. Per my calculation I came up with about 455v at Disco which is feeding a 45kva XFMER in turn supplying a com room. My RFI was shot down so I painfully installed system. After heating up breaker to disco I was gloating in Glory to the meter reading 455v Phase to Phase. Before calling engineer to talk trash I decided to recheck using different meter, to my humbling surprise the second meter should 479v Phase to Phase and 275 to ground. First meter was 4 year old fluke second 1 weak old IDEAL.

1. What kind of deviation do meters have.
2. If voltage is OK what happen to my calculation?
3. What will adding load do to voltage?
4. Test was done in morning in winter will extreme hot summer come in to play down the road?
5.If voltage dose change do to any factor, will added resistance to XFMER be long term problem ?

 

[iwire]

1. What kind of deviation do meters have.

Your meters manuals will have the accuracy that can be expected.

That seems like a large difference to me.

2. If voltage is OK what happen to my calculation?
3. What will adding load do to voltage?

If the only load is the meter itself there will be no voltage drop.

Once you add load the voltage drop.

How did you do a calculation without knowing the load?

 

[KROC2004]

load

load

Sorry the XFMER is feeding 150 amp 120/208 pnl for data/com room.

 

[hardworkingstiff]

What was the load at the time you took your voltage readings?

 

[dereckbc]

First calculations are just that, calculations. As Bob stated the voltage drop is entirely dependent on load current. So if there is no load, there is no voltage drop.

I have done hundreds of load measurements over 30 years and discovered some time ago all the load calculations are too liberal with the copper, in particular those based on ?K? of a conductor. Every formula published using ?K? list copper as 12.9, when in reality it is closer to 10.5 when measured and averaged out. Personally I use 11.1 for ?K? and it has never let me down (still liberal), not to mention save a few dollars when voltage drop has been a concern.

 

[ptonsparky]

I hate to even ask, but was the battery good on the Fluke? It does make a difference.

 

[KROC2004]

The 120\208 pnl is supplying a data com room, the schedule only shows 1 30 208 recptacel for ups unit on data rack, 2 dedicated 20 recp also on data rack. 2 more cir split between four convince recp. 2 120v cir for future CCTV rack. I did not design system, I just want the system to work properly. The Data com room is in any airport buildout and is the only line of cmounaction for all special system with existing terminals I assume it is very important.

 

[KROC2004]

The 120\208 pnl is supplying a data com room, the schedule only shows 1 30 208v receptacle for ups unit on data rack, 2 dedicated 20 receptacle also on data rack. 2 more 20amp circuit split between four convince receptacle. 2 120v cir for future CCTV rack. I did not design system, I just want the system to work properly. The Data com room is in any airport build-out and is the only line of communication for all special system with existing terminals I assume it is very important.