This is an example of where some code requirements bump up against
logic. I believe the AC voltage will be quite usable at the
site, just not for code requirements of the AHJ, and it may get
pretty low during high usage times. They make the electrician size
it for 100A minimum for an ADU, even though the loads are pretty
minimal, and then require <5% VD from the main service entrance.
That is the requirement for the certificate of occupancy.
Luckily, off-grid buildings don't have these requirements at all.
I'm going to size it as an off-grid and probably tie it in at some
point in the future.
Thanks for the input.
On Sun, Apr 19, 2020 at 10:20 AM
<[email protected]
<mailto:[email protected]>> wrote:
We had a similar situation, and are now feeding a battery bank
with a pair of 48 V Iota chargers. Iota can run on very low
voltages. We have one Iota on each leg of the the 120/240 split
phase to balance the load. It works.
---
On 2020-04-18 21:25, Ray wrote:
When doing the volt drop calculation, be sure to correct for
temperature, per the note after table 8, chapter 9 of the
NEC. The table values default at 167 deg F (75 C). An
underground cable will be operating closer to earth
temperature. When I run volt drop for a 4/0 copper cable at
60 deg F, I'm seeing that you could go over 60 amps and
maintain a volt drop of 3%.
Not sure what the requirements for the building are, but that
exceeds most of my off grid systems. 60+ amps at a final
voltage of 232 v is almost 14kW. Is there air conditioning?
What are the calculated loads for this building? Is the
inspector actually calling this a problem?
I would do a proof of concept: turn every light on in the
place, along with some typical appliances (hair dryer,
microwave) hit a circular saw to really make the point, with
a DMM plugged in. I found long ago that actual measured volt
drop was not as bad as Chapter 9 first predicts.
Ray Walters
Remote Solar
303 505-8760
On 4/18/20 3:41 PM, Darryl Thayer wrote:
Hi Mac, There are a few questions like is it Al or Cu
wire, What makes it unusable?
for Voltage drop, I use Vd=2KIL/CMA for your case the
unknown is the current. The installation of an inverter
with grid support. Many inverters have this, you want a
multimode inverter such as a Schneider, Outback, SolArk
and I could name at least 4 more. A normal house will
run on 30 amps. So assume 30 amps,
Vd=2x21.6x30x1100/211600 =6.7 volts
So if the main house is at 240 Volts the remote house is
at 233 Volts. The multimode inverter could be set at
grid support beyond 30 Amps. For higher house loads the
inverter can support up to 30 to 50 amps additional.
(SolArk is 50 amp) I would add a balancing transformer at
the end of the line to make the 120 loads balance. (an
Autotransformer) such that 120 volts loads are balanced
and the load is shared between 240-Volt conductors.
Based on my customers' experiences I would use AGM LA
batteries, as I expect low usage of the battery and quick
recharge, but that is the customer's choice. I had one
customer that had 10 years on the float-AGM battery set.
On Fri, Apr 17, 2020 at 8:15 AM Mac Lewis
<[email protected] <mailto:[email protected]>> wrote:
Hi wrenches,
I've got a architect client with a sub-optimal power
situation on one of the projects he is working on.
I'm running through a couple of scenarios that may
work. Its a ADU (accessory dwelling unit) that is
about 1100' from power. The farmer had taken it upon
himself to bury some 4/0 a few years ago to run power
out there. Voltage drop is the big issue and its not
going to pass the AHJ inspection. We could just
ditch the grid and go off-grid, but it seems like a
shame to not tap into the grid resource. Instead of
dealing with the voltage drop, I was thinking it may
make sense to rectify that low AC voltage input and
parallel a smallish battery bank (size is yet to be
determined), then use an off-grid inverter to serve
the loads at good voltage.
I am looking for feedback on this concept. What do
you think?
Also, are there any recommendations for good quality
rectifiers with wide input voltage window?
Thanks in advance for your help
--
Mac Lewis
"Yo solo sé que no sé nada." -Sócrates
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Mac Lewis
"Yo solo sé que no sé nada." -Sócrates
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