Ray, One thing I didn't understand is how you can pair a 400 amp class-T fuse with a 4/0 cable. The 400 fuse curve looks like the continuous rating is 350A in the graph supplied by Allan. Which is well above the 4/0 ampacity of 250A (copper). Anyone want to explain?
thanks, Troy Harvey --------------------- Principal Engineer Heliocentric 801-453-9434 tahar...@heliocentric.org On Apr 5, 2013, at 10:12 PM, Ray Walters <r...@solarray.com> wrote: > I just finished a rewire and we kept the Ananda power center, Allan :-) > Back when we used class T fuses more they were always over sized relative to > Heinemann breakers. > 400 amp class T fuse = 250 amp breaker= 4/0 cable > 200 amp class T fuse = 175 amp breaker= 2/0 cable > > I still think class T fuses are superior in some ways to breakers: they have > better interrupt capacity and trip faster in a short circuit condition. > > However, If you spend too much time thinking about and looking at the trip > curves, you'll break your brain and special order some weird fuses and > breakers. (anybody need a 350 amp class T fuse?) > Just follow the inverter manufacturer's recommendations and all will be well. > On the other hand, William has brought up a topic I have harped on for years: > Having the OCPD in the cabinet doesn't protect the majority of the circuit. > Class T fuses at the battery terminals do, but they're not rated for the > corrosive environment. I have thrown a few class T fuses away that had acid > eating away at the ends. > I don't have the solution, but I will continue to point out that this is a > real problem. Dropping a wrench across the battery terminals can lead to a > spectacular failure that not only can cause a fire, but might even cause a > battery explosion, yet NEC offers no protection. > We use insulated wrenches from experience, and hope for the best. > R.Ray Walters > CTO, Solarray, Inc > Nabcep Certified PV Installer, > Licensed Master Electrician > Solar Design Engineer > 303 505-8760 > On 4/5/2013 8:29 PM, William Miller wrote: >> Friends: >> >> Good topic. Some questions: >> >> 1. Most manufacturer's present an installation guide that shows one OCPD in >> the battery circuit and that is in the BOS cabinet. This means the battery >> leads are unprotected. Do we need an OPCD at the battery terminals? >> >> 2. Class T fuses are generally recommended for this application. The data >> shows them as "fast acting." Is this a problem? Will they act too fast and >> open during normal surge loads? >> >> Thanks in advance! >> >> William Miller >> >> >> >> >> >>> Troy, >>> >>> Overcurrent device size is matched to the conductor size. The inverse time >>> constant nature of an overcurrent device can typically handle the surge >>> currents as long as conductor sizing has truly been done correctly for the >>> conductor. Circuit breakers are preferred to fuses because they can be >>> reset. >>> >>> There has been volumes written on this issue. The constant current at >>> lowest battery voltage should be used, plus the ac ripple content on the >>> battery circuit. This is usually a much larger conductor than your average >>> designer will plan for. The best thing is to look at Midnight, Outback, and >>> Schneider and see what size overcurrent devices they require for their >>> products. That will give you a good clue as to how to size the conductor >>> and overcurrent device. >>> >>> Bill. >>> >>> From: re-wrenches-boun...@lists.re-wrenches.org >>> [mailto:re-wrenches-boun...@lists.re-wrenches.org] On Behalf Of Troy Harvey >>> Sent: Friday, April 05, 2013 3:38 PM >>> To: RE-wrenches >>> Subject: [RE-wrenches] Fuse sizing in battery circuits >>> >>> I've got a question about battery string fusing. Typically we size the wire >>> from the batteries to the inverter based on continuous rating procedures >>> (max power/efficiency)*125%. >>> >>> However a 6kW inverter, can peak at 12kW for 5-10 seconds, doubling the >>> source current. That is no big deal for the wire, because it is a short >>> time frame... little heat will be generated. However, in fusing the >>> sub-strings, you need to account for that peak surge current so you don't >>> blow fuses all the time. But if you put a 500-1000 amp fuse on a 4/0 wire, >>> above the max surge draw of the inverter, the wire will be under-protected >>> for its ampacity rating. Any thoughts on the catch-22? >>> >> >> >> _______________________________________________ >> List sponsored by Home Power magazine >> >> List Address: RE-wrenches@lists.re-wrenches.org >> >> Change email address & settings: >> http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org >> >> List-Archive: >> http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org >> >> List rules & etiquette: >> www.re-wrenches.org/etiquette.htm >> >> Check out participant bios: >> www.members.re-wrenches.org >> > > _______________________________________________ > List sponsored by Home Power magazine > > List Address: RE-wrenches@lists.re-wrenches.org > > Change email address & settings: > http://lists.re-wrenches.org/options.cgi/re-wrenches-re-wrenches.org > > List-Archive: > http://lists.re-wrenches.org/pipermail/re-wrenches-re-wrenches.org > > List rules & etiquette: > www.re-wrenches.org/etiquette.htm > > Check out participant bios: > www.members.re-wrenches.org >
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