SW inverter to battery cable calculation [RE-wrenches]

John Berdner jberdner at sma-america.com
Fri Nov 28 12:02:23 PST 2003


Wrenches:
 
I "think" I know the answer Joel's question about where the extra 28
Amps comes from.
I would guess it comes from experimental data.
I will ask J. Wiles and forward his reply.
In the interim I will try to shed some light.
 
I have discussed this issue many times with J. Wiles and the guys at UL
and the rational is as follows:
Normally we take the maximum ac output Watts / inverter efficiency at
full power to get the maximum dc input Watts.
Then we divide the maximum dc input Watts by the lowest battery voltage
to get the maximum battery current.
This current can theoretically be a continuous current so we need to
multiply this number by 1.25 to get the NEC required ampacity for the
battery cables.  
The problem with this approach is that it does not accurately reflect
actual in field measurement of the current flowing in the battery
cables.  
When you make RMS measurements of the current flowing in the battery
cables you usually see a higher number than you would get from the
straight dc calculation.  
The difference is due to the substantial 120 Hz ac ripple you see on
top of the dc current when you measure the vast majority of inverter
chargers. 
I suspect the 28 Amps John Wiles is talking about in his book is the
difference between the calculated dc value and RMS value you get when
you measure it.
This will be addressed in the next release of UL 1741 which says you
have to size the battery cables and over current devices for worst case,
combined dc plus ac ripple, i.e. RMS value, of the current. 
I believe this makes sense because the NEC ampacity tables are based on
RMS current values for calculating cable heating.
 
Best Regards,
 
John Berdner
 
 

>>> allan at positiveenergysolar.com 11/28/2003 10:30:01 AM >>>
Joel,
The upper example has been the one that I have most often seen. It's
used by
Wiles on page E-16 of his PV/NEC book. If you extrapolate these numbers
to
the SW5548, I think you will get 5500 watts divided by 0.85 inverter
efficiency divided by 44 volts = 147.05 x 1.25 NEC = 184 amps battery
cable
rating.
I have no idea where you got the part about adding "28 amps RMS AC
current"
to the DC calculation.
Allan at PosE


----- Original Message ----- 
From: "Joel Davidson" < joeldavidson at earthlink.net >


> Last week I asked someone at Xantrex technical service for the fuse,
> breaker and wire loss calculations for the cable between the SW
inverter
> and the battery, but the person I talked to did not know the
> calculations.
>
> One system designer uses this calculation for the SW4024:
> 4000 watts divided by 22 volts divided by 0.85 inverter efficiency =
> 213.9 (for the 250 amp breaker) and an additional 1.25 NEC = 267
amps
> for the 300 amp fuse.
>
> Another system designer uses this calculation for the SW5548:
> 5500 watts divided by 0.8 inverter efficiency divided by 40 volts =
172
> amps DC plus 28 amps RMS AC current = 200 x 1.25 NEC = 250 amps
battery
> cable rating.
>
> Does anyone know the correct efficiency multipliers for the SW
inverters
> and the calculation to use?






--
[Non-text portions of this message have been removed]

- - - -
To send a message: RE-wrenches at topica.com

Archive of previous messages: http://www.topica.com/lists/RE-wrenches/

List rules & etiquette: http://www.mrsharkey.com/wrenches/etiquete.htm

Check out participant bios: www.mrsharkey.com/wrenches/index.html

Hosted by Home Power magazine

Moderator: michael.welch at homepower.com
--^----------------------------------------------------------------
This email was sent to: michael.welch at homepower.com

EASY UNSUBSCRIBE click here: http://topica.com/u/?bz8Qcs.bz9JC9.bWljaGFl
Or send an email to: RE-wrenches-unsubscribe at topica.com

TOPICA - Start your own email discussion group. FREE!
http://www.topica.com/partner/tag02/create/index2.html
--^----------------------------------------------------------------






More information about the RE-wrenches mailing list