<div dir="auto">Kirk:</div><div dir="auto"><br></div><div dir="auto">First of all, I apologize for the typo in the subject line. I fixed it. I take my writing seriously but sometimes my devices fight me with autocorrect. </div><div dir="auto"><br></div><div dir="auto">You make good point regarding a scenario with a battery combiner. Relying on multiple BMSs for OCP would be a bad choice. </div><div dir="auto"><br></div><div dir="auto">Fortunately some cabinets come with breakers and those should be sufficient. <br clear="all"><div dir="auto"><div dir="ltr" class="gmail_signature" data-smartmail="gmail_signature"><br>William Miller<br>Miller Solar.com<br>805-438-5600<br><a href="http://www.millersolar.com">www.millersolar.com</a><br></div></div></div><div><br></div><div><br><div class="gmail_quote gmail_quote_container"><div dir="ltr" class="gmail_attr">On Tue, Jul 15, 2025 at 9:50 AM Kirk Herander <kirkh@vermont.solar> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;padding-left:1ex;border-left-color:rgb(204,204,204)"><div dir="ltr"><div dir="ltr"><div>Hello William-</div><div dir="ltr"><br></div><div dir="ltr">I agree with your approach.</div><div dir="ltr"><br></div><div dir="ltr">I never historically have OCPD'ed lead acids at both ends of the battery cable run, but do now, esp. with high kwh LFP, where a DC combiner is in use.</div><div dir="ltr"><br></div><div dir="ltr">For instance, I had to design a 2000 ADC bus @ a nominal 55 vdc, for a 500 kwh Blue Ion LFP bank. To create enough space for cables and fuses, I started with -2- 2000A Midnite combiners, using one for positive and one for negative.</div><div dir="ltr"><br></div><div dir="ltr">Class T fuses are bolted directly to the DC bus (properly sized copper from <a href="http://copper.org" target="_blank">copper.org</a> charts), and the BI BMS breaker protects at each respective battery cabinet.</div><div dir="ltr"><br></div><div dir="ltr">Same thing on the DC to inverter cables, class T's bolted to the bus, to its respective outback Radian, where the standard 175A breaker resides.</div><div dir="ltr"><br></div><div dir="ltr">I would never again do a building power system of any type or size without both ends of a power cable (point of use & point of origin) being protected, A best practice IMO.</div><div dir="ltr"><div><br></div><div><br></div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">On Tue, Jul 15, 2025 at 1:13 AM William Miller via RE-wrenches <<a href="mailto:re-wrenches@lists.re-wrenches.org" target="_blank">re-wrenches@lists.re-wrenches.org</a>> wrote:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left-width:1px;border-left-style:solid;padding-left:1ex;border-left-color:rgb(204,204,204)"><div dir="auto">Friends:</div><div dir="auto"><br></div><div dir="auto">When I design a lead-acid battery system I decide if I need over-current protection right at the battery or not. My logic is: if the leads are short, don’t go through a wall and are well protected, I can rely on OCPD in the BOS cabinets. If any of these criteria is a no I put appropriately rated class T fuses right at the battery. </div><div dir="auto"><br></div><div dir="auto">I’m thinking that since BMS devices have over-current protection baked-in, fusing at the battery would not be necessary for BMS equipped batteries in any case. Do you agree with that premise?<br clear="all"><br clear="all"><div dir="auto"><div dir="ltr" class="gmail_signature"><br>William Miller<br>Miller Solar.com<br>805-438-5600<br><a href="http://www.millersolar.com" target="_blank">www.millersolar.com</a><br></div></div></div>
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</blockquote></div><div><br clear="all"></div><div><br></div><span class="gmail_signature_prefix">-- </span><br><div dir="ltr" class="gmail_signature"><div dir="ltr"><p></p><p class="MsoNormal" style="font-family:Arial;font-size:16px;margin:0in 0in 0.0001pt;color:rgb(0,0,0)"><a name="m_-4690279726919280965_m_-7249494732929429864_m_790650293411317218_SignatureSanitizer__MailAutoSig" style="font-size:10pt;font-family:Arial"><b style="font-family:Arial"><img width="96" height="72" src="https://ci3.googleusercontent.com/mail-sig/AIorK4zIGox3hmaGW8StlnANQvRdXBrTEgpee6ZmV_8h-pfWcEl7lwlqfhSqd2PgPh89cgzwpzfUzuuIGxKE" style="font-family: Arial;"><br></b></a></p><p class="MsoNormal" style="font-family:Arial;font-size:16px;margin:0in 0in 0.0001pt;color:rgb(0,0,0)"><a name="m_-4690279726919280965_m_-7249494732929429864_m_790650293411317218_SignatureSanitizer__MailAutoSig" style="font-size:10pt;font-family:Arial"><b style="font-family:Arial">Kirk Herander / </b></a><b style="font-size:10pt;font-family:Arial"><a href="mailto:kirkh@vermont.solar" title="Send an email to kirkh@vermont.solar; Send an email to kirkh@vermont.solar" style="font-family:Arial;color:blue" target="_blank">kirkh@vermont.solar</a></b></p><p class="MsoNormal" style="font-family:Arial;font-size:16px;margin:0in 0in 0.0001pt;color:rgb(0,0,0)"><span style="font-size:10pt;font-family:Arial"><b style="font-family:Arial">Owner|Principal, VT Solar, LLC</b></span></p><p class="MsoNormal" style="font-family:Arial;font-size:16px;margin:0in 0in 0.0001pt;color:rgb(0,0,0)"><span style="font-size:10pt;font-family:Arial"><b style="font-family:Arial"><i style="font-family:Arial">Celebrating our 34<sup style="font-family:Arial">st</sup> Anniversary 1991-2025!!</i></b></span></p><p class="MsoNormal" style="font-family:Arial;font-size:16px;margin:0in 0in 0.0001pt;color:rgb(0,0,0)"><span style="font-size:10pt;font-family:Arial">dba Vermont Solar Engineering</span><br></p><p class="MsoNormal" style="font-family:Arial;font-size:16px;margin:0in 0in 0.0001pt;color:rgb(0,0,0)"><span style="font-size:10pt;font-family:Arial">802.559.1225</span></p></div></div>
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