[RE-wrenches] Sunny Island DC wire size

Wed Feb 13 15:05:50 PST 2013

Alan:

In my experience, the worst case currents occur during battery charging following a deep discharge.
(You should see a generator exhaust pipe at night when charging hard - such a pretty shade of yellow/orange)

The SI has parameters you can set to limit the maximum charge rates which should solve your concern.

This is found in SI Parameter #222.01 under the Battery Settings menu
#222 Battery Charge Mode
No. __Name Description__Value Explanation ___Default value
01__ BatChrgCurMax__Charging current of the battery10 A ... 1200 A__1,200 A

I am not sure if this value is the sum of the SI currents or if this is actually the value the inverters read off the external current shunt needed for DC coupled systems.
I think the SI uses the current shunt value (if present) but I am not sure - Check with SMA Support to find out.
If it is the former then you would need to consider the Radian charge current and subtract that off your maximum allowed value then enter that "net" value into the SI's.

As I mentioned in my earlier posts the breaker in the SI was selected to match the short duration current versus time trip curves.
Since the SI can do very substantial surges (up to 12 kVA per inverter) the short duration tripping characteristics were key.
As I recall the breaker was actually 2 internally parallel AMS 125 (non-standard part from Eaton).
See curve 252 (DC 25x) data on page 8 of the following http://www.heinemann-electric.com/acrobat/ams-cata.pdf
Not sure what breaker is used in current SI production but the characteristics should be similar to the above.

In normal operation the maximum continuous current will be the inverter rating divided by minimum battery voltage.
I ASS U ME this is where the 125 Amp recommendation for a SI5000 comes from (5000W/40Vdc).
Never mind that pesky 1.25 * the continuous current NEC thingy.

If you are using an external OCPD be sure to use a L...O...N...G time delay type and compare the short duration trip characteristics of your OCPD to that of the SI's breaker.
If your OCPD will trip "sooner" than the SI's breaker you may see nuisance  tripping of your OCPD during high surge events.
There might be a parameter to limit the maximum discharge (surge) current but I could not find it in my admittedly quick skim of the SI manual.
If you do not foresee very high surge loads (motor starting) this will not be an issue.
48 kVA seems like one heck of a surge but, then again,  I have not seen the customer's home :)

Best Regards,

John Berdner
General Manager, North America

[cid:image001.jpg at 01CE09F2.47122470]

SolarEdge Technologies, Inc.
3347 Gateway Boulevard, Fremont CA 94538 USA  (*Please note of our new address.)
T: 510.498.3200, X 747
M: 530.277.4894

From: re-wrenches-bounces at lists.re-wrenches.org [mailto:re-wrenches-bounces at lists.re-wrenches.org] On Behalf Of Allan Sindelar
Sent: Wednesday, February 13, 2013 1:41 PM
To: RE-wrenches
Subject: Re: [RE-wrenches] Sunny Island DC wire size

John (and other Wrenches),
I read with great interest this post about Sunny Island breaker size. We are about to install a large semi-off grid system for a wealthy and eccentric customer. The system has 17kW of PV, and four SI5048 inverter/chargers, along with an Outback Radian serving as an inverter only to some separate protected circuits. The system has utility backup power available as well as a backup 30kW generator, but for a complicated reason the system can't be set to feed back into the grid; the SIs' sell function will be disabled.

The four SIs are set up as a pair on each 120V phase, as SMA allows. After consultation with SMA tech support, I have designed each SI to be on a 125A dc breaker. This is done to keep the total protected ampacity of the dc battery cabling and bussing to 850 amps (125 A x 4 = 500 A, plus 175 A x 2 for the Radian, all passing through a common 1,000 A shunt to allow accurate SOC metering). It's hard enough to address battery cabling and shunting with 850 amps; with 175 A dc breakers the total would be 1,050 amps, and with 250 A dc breakers the total would be 1,350 amps.

Given that the loads supplied are residential, primarily electronic and resistive, and thus few if any large surges, and given that the inverters on each phase are paralleled, how much should I worry about the 125a dc breaker size? Are there SI programming settings that I should know about to better protect against nuisance tripping?

Thank you,
Allan
Allan Sindelar
Allan at positiveenergysolar.com<mailto:Allan at positiveenergysolar.com>
NABCEP Certified Photovoltaic Installer
NABCEP Certified Technical Sales Professional
New Mexico EE98J Journeyman Electrician
Founder and Chief Technology Officer
Positive Energy, Inc.
3209 Richards Lane (note new address)
Santa Fe, New Mexico 87507
505 424-1112
www.positiveenergysolar.com<http://www.positiveenergysolar.com/>
On 2/4/2013 1:29 PM, John Berdner wrote:

Ryan:

Not sure what is in the manual now but...The lugs in the SI 5048U are dual 2/0.

We had to use parallel AWG #1 because we kind of painted ourselves into a corner.

The breaker in the SI ended up being 250 Amps, the conduit fitting was only 1.5" and the wire bending space was too small for a 3/0.

The only way to get enough Ampacity in a 1.5" conduit and that wire bending space was to go with dual #1's.

The 250 Amp breaker was used just to get enough current for the very short duration surges without tripping the breaker (12kW per SI).

If not for this a 200 A breaker would have been fine.

Note that the actual full power ampacity required for normal operation is 5kW @ 40 Vdc = 125 Amps continuous.

It is the high surge loads that require more current.

I run mine with a single 2/0 and we have never even come close to hitting full power.

Worst case current is typically while charging not when inverting and I set the current limit so that I do not exceed ampacity of 2/0.

In my experience it takes >= 14kW generator to drive a pair of SI's to full power while charging

Best Regards,

John Berdner

General Manager, North America

SolarEdge Technologies, Inc.

3347 Gateway Boulevard, Fremont CA 94538 USA  (*Please note of our new address.)

T: 510.498.3200, X 747

M: 530.277.4894

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