[RE-wrenches] single string of batts: must the interconnects be inverter size cables?

[Exeltech]

As an inverter manufacturer .. I (we) agree with Phil.  He's absolutely on target with his reply.


Dan
Sr. Engineer


--- On Fri, 2/19/10, Phil Undercuffler <solarphil at gmail.com> wrote:

From: Phil Undercuffler <solarphil at gmail.com>
Subject: Re: [RE-wrenches] single string of batts: must the interconnects be inverter size cables?
To: "RE-wrenches" <re-wrenches at lists.re-wrenches.org>
Date: Friday, February 19, 2010, 1:28 PM

The interconnects are part of the circuit.  Typical flexible cables used with batteries are THW rated (75C column), so in free air 2/0 interconnects would be good for 265 amps, before any temperature derates.  Therefore, you'd be safe from a NEC point of view.  However, if the inverter manufacturer has recommended 4/0 cables, then that recommendation would apply to all cables in the circuit.  Battery based inverters are asked to surge many times their rated capacity -- a 24V 4kW inverter surging to 2x capacity (pretty normal circumstances) is going to draw over 360 amps (8,000 / 22), even if only for a short period of time.  You don't want your interconnects to be the weak link in the system, causing shutdowns due to undervoltage unnecessarily.  


I'd use 4/0.

Phil Undercuffler
Conergy






On Fri, Feb 19, 2010 at 12:01 PM, Mick Abraham <mick at abrahamsolar.com> wrote:

Example: 12 two volt battery cells in one 24 volt string. 4kW inverter/charger has 250 amp DC breaker size and 4/0 cables...as the inverter folks would want.

So...do the "cell to cell" interconnect cables have to also be 4/0? In the example above, 2/0 interconnects would suffice from the standpoint of basic safety...and even from the voltage drop standpoint...right?




Inverter company people may be best qualified to answer this, but any replies will be appreciated. While I'm at it: do the inverter folks request oversize cables because they want more capacitance on the DC input...or what?




Jolliness,

Mick Abraham, Proprietor
www.abrahamsolar.com

Voice: 970-731-4675






      
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