Breaker - Battery to Converter

[Carl G]

If I place a resettable circuit breaker between the battery and my Progressive Dynamics PD4045 converter/charger, what size should it be? The converter has an output of 13.6 VDC, 45 amps. The DC portion of the distribution panel is rated as 120 Amps Maximum (max. continuous load on the main or branch circuits not to exceed 80% of the breaker ratings). I don't plan on running anything close to the max DC output.

Thanks,
Carl

[Casita Greg]

It's not so much based on your usage. It's more relative to the current carrying capacity of the wiring that runs from your battery to the converter. Normal load usage does not equate to current draw during such things as a direct short, (like welding for example, which can in fact be done with a normal battery and a pair of jumper cables and a welding rod, in a pinch.) If your feed to the panel is 10 ga. wire, it would limit you to a 30 Amp breaker (resettable or otherwise). If you "up" the wiring size, you can increase the amperage rating, but this too should be done with some thought as to what exactly you are trying to protect. Certain downstream components and equipment may limit your "upsizing" based on potential damage which overcurrent can produce in, and to, these devices. As a rule, if the wiring is 10 gauge, then a 30 Amp would be acceptable. If your wiring is only 12 gauge, then a 20 Amp breaker would be your safe maximum size. Going to a larger conductor, say a stranded 8 gauge feeder would reduce your total circuit resistance, but it is not of any real significance in the few feet of wire run length used in trailers like ours.

[Carl G]

Thanks Greg. To prevent unwanted voltage drop during charging of the battery, I'll be using 4 AWG wire between the converter/charger and the battery since it is a 45 amp charger. I am still new at this, so forgive me if I don't understand. Wouldn't it be necessary to have a circuit breaker with a rating greater than the rated output of the charging unit (45 amps) installed on the postive lead running from the converter charger to the battery? All of the equipment in the trailer running "downstream" from the converter/charger is already protected by fuses located in the converter/charger unit.

[Casita Greg]

Carl, Here's some information that you might find helpful on your circuit breaker question.

44 DC Fuse & Circuit Breaker Types & Installation

[Carl G]

Thank you Greg. This is a great site that does give me insight into voltage requirements and the purpose of the two breakers, one near the converter and one by the battery. But, I am still in need of answer to the question regarding amps. If the converter/charger can product 45 amps going to the battery, would I not need at least a 45 or 50 amp breaker rating, both for the one by the converter and the one in the battery compartment?

Thanks,
Carl

[Matt in SV]

The mains fuse or breaker on the battery is there to protect you from some catastrophic event, so the battery won't overheat and explode, and the wiring won't heat up and melt its insulation off and start a fire.

Practically, you don't want to have to reset it, so it needs to be sized to exceed your expected current draw, but be within the limits of the wiring system's capacity, which depends on wire gauge and length.

I think you'd be fine with 75-100AMP breaker on your #4GA wire. Unless your wiring run is really long (like ten feet or so) you would even be OK with #6 wire. I don't think your charge system is that sensitive to a slight voltage drop. The voltage drop in #6 wire at 45 amps is about .02 volts per foot. The #4 wire is .013 volts per foot. Not a big deal unless your wiring gets into the tens of feet.

If you plan to run an inverter on your battery, the whole picture changes though. You're talking about much higher current capacity and larger wires! If you were charging from a 12V solar system, you might also want large wires to minimize any voltage drop.

Regards,

Matt

[Carl G]

Originally Posted by Matt in SV

The mains fuse or breaker on the battery is there to protect you from some catastrophic event, so the battery won't overheat and explode, and the wiring won't heat up and melt its insulation off and start a fire.

Practically, you don't want to have to reset it, so it needs to be sized to exceed your expected current draw, but be within the limits of the wiring system's capacity, which depends on wire gauge and length.

I think you'd be fine with 75-100AMP breaker on your #4GA wire. Unless your wiring run is really long (like ten feet or so) you would even be OK with #6 wire. I don't think your charge system is that sensitive to a slight voltage drop. The voltage drop in #6 wire at 45 amps is about .02 volts per foot. The #4 wire is .013 volts per foot. Not a big deal unless your wiring gets into the tens of feet.

If you plan to run an inverter on your battery, the whole picture changes though. You're talking about much higher current capacity and larger wires! If you were charging from a 12V solar system, you might also want large wires to minimize any voltage drop.

Regards,

Matt

Thank you Matt. That is more in the neighborhood of what I thought the amps should be. One last thing. I am still in doubt as to whether I need a second breaker on the positive lead just before it enters the converter/charger. I keep running into some sources that say yes and some that say it is not necessary. If y'all think that the converter/charger is protected well enough by the fuses inside, I will be satisfied with just going with the "catastrophic" breaker by the battery. Please let me know.

Thanks everyone! This is the last piece of information I need before ordering all of the additional supplies I need to be set for job!

Carl

[Matt in SV]

Sorry I missed your question. The PD4000 series has a pair of ATC fuses on the charge circuit. The PD4045 has two 30 amp fuses and it also has some kind of electronic current limiting according to the spec sheet. I think you're covered on the converter side!

Regards,

Matt

[Roy in TO]

Barring the specifics of this particular converter or posssible inverters. My lay understanding is that a smaller fuse is used on the "charging" side of the circuit to protect your circuits from something that puts out a lower regulated amperage. OTOH one uses the larger fuse on the battery side because the typical battery we use is capable of producing an unregulated surge of at least 10X the amperage of your charging side. Protecting both your rig and battery from a short circuit.