[BC] AC Voltage Drop

[PeterH]

On Jun 28, 2009, at 3:25 PM, Chris Gebhardt wrote:

> They planted a new pole & 3x new transformers.  (We moved to 208  
> 3phase).
>

Broadcasters or anyone else with a significant infrastructure should:

1) seek 120/208 Y or 277/480 Y services, and

2) develop any 120, 120/240 or 240 from their service, using customer- 
owned "dry-type" distribution transformers.

> One thing the engineer who led our project mentioned was that there
> isn't a lot of demand for 240 Delta these days, and the equipment  
> in use
> for such supplies is typically quite a bit older and not as well
> maintained.   Sort of "forgotten infrastructure" I guess.

120/240 Æ , which can supply 120 single-phase loads over only two of  
the three incoming phases, 120/240 single-phase loads over the same,  
or 240 single-phase loads over any two of the incoming phases is the  
most common three-phase service.

120/208 Y has an advantage that all three incoming phases can provide  
120 single-phase, while any two plus the neutral can provide 120/208  
single-phase is the second most common three-phase service.

277/480 Y has an advantage that all three incoming phases can provide  
277 single-phase (for, say, gas-discharge lighting) is the second  
most common three-phase service. 277 volts is seldom useful  
otherwise, although some manufacturers make certain machinery for 277  
single-phase.

Some modern transmitter manufacturers go even farther, demanding true  
three-phase in the form of a set of three transformers, rather than  
the more modern "T Transformer" method (two transformers of  
dissimilar capacities within a single can in a "Scott T" connection,  
but providing three-phase transformation, with or without a neutral)  
with capacities up to 333 KVA.

The NEC establishes rules for use within a premises.

Electric utilities are generally exempt from the NEC.

Which certainly explains why we often see service drops which are  
very undersized for the customer's load.

Apparently, the utilities assume a low duty cycle, such as 20  
percent, as a first cut appraisal of a customer.