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re: pv to house = 500' ... is it do-able?
28 nov 2002
bill kaszeta / photovoltaic resources  wrote:

>there are 3 basic designs for which to estimate the costs...

>1. inverter with remote array (48-vdc recommended nominal voltage),
>2. 120 vac system with remote batteries and inverter,
>3. 120/240 vac system (depends on size of loads)with remote batteries 
>    and inverter (one-half the current if loads can be balanced).

one might add:

4. remote grid-tied 240 v inverter with no batteries. 
 
>minimum wire size is based on code requirements of ampacity
>and allowable voltage drop (5% per usa nec 310-15).

section 310-15 of the 2002 nec has 3 pages of text, but none of them
seem to mention 5%. section 310-15(a)(1), fpn no. 1 says

  ampacities provided by this section do not take voltage drop into
  consideration. see 210.19(a), fpn no. 4, for branch circuits and
  215.2(d), fpn no. 2 for feeders.

210.19(a)(1), fpn no. 4 says

  conductors for branch circuits as defined in article 100, sized to
  prevent a voltage drop exceeding 3 percent at the farthest outlet...
  where the maximium total voltage drop on both feeders and branch
  circuits to the farthest outlet does not exceed 5 percent, provide
  reasonable efficiency of operation. 

there is no section 215.2(d)... 215.2(a)(4), fpn no. 2 says 

  conductors for feeders circuits as defined in article 100, sized to
  prevent a voltage drop exceeding 3 percent at the farthest outlet...
  where the maximium total voltage drop on both feeders and branch
  circuits to the farthest outlet does not exceed 5 percent, provide
  reasonable efficiency of operation. 

it looks like these are just recommendations vs requirements.

ampacity is a requirement, but section 310.15(c) provides an alternative
formula for use under engineering supervision: 

  i = sqrt((tc-(ta+dtd)/(rdc(1+yc)rca), where 

      tc = conductor temp (c)
      ta = ambient temp (c)
      dtd = dielectric loss temp rise
      rdc = dc resistance of conductor at temp tc (in ohm/cm?)
      yc = ac resistance resulting from skin effect and proximity effect
      rca = effective thermal resistance from conductor to ambient (c-cm/w)

ignoring the ac bits, it looks like this is just ohm's law for heatflow:
dt = prca, where dt is the conductor-ambient temp diff and p is the heat
power and rca is the conductor-ambient thermal resistance. p = i^2rdc,
so dt = i^2rdcrca, and i = sqrt((tc-ta)/(rdcrca))...

i wonder how this formula works. pa soil is about 12 c, and table 310.16(a)
says uf12/2 can carry 1.08x25 = 27 a max at 25 c. assuming tc = 60 c and
1.93 ohms/1000', ie rdc = 0.000063. with ta = 12 and dtd=yc=0 and rca = 60
(from nec appendix b), i = sqrt((60-12)/(0.000063x60)) = 112 a, half the
itt book's bare-wire fusing current of 235 a :-) but as bill says, this is
probably uneconomical. well, it might be, with a solar rebate of $5 per
panel peak watt and no wire rebate and solar panels that only cost $3.75. 

nick




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