Can DC breakers be oversized?

[Tom Bradford]

Hi.

As outlined below I'm upgrading/improving a small solar set-up powering a remote wireless relay system.

Presently it has nothing in the way of breakers between the panels and the controller or the controller and the battery, but I understand they are a good thing to have there.

It's a small set-up, with three panels in parallel putting out (in theory) 13.82 amps at 17.6v. Our supplier's price list has DC breakers for 10, 15, 20, 50 Amps and upwards all at 150v, and all at the same price.

I think I'm right to say a 15 Amp breaker would do between the panels and the controller but we'd need a 20 Amp one between the controller (MPPT) and the battery as it would be putting out 13v float at >18 Amps, but as they're the same price is there any penalty to having oversized breakers - say 2 x 50 Amp ones - to allow for future expansion?

[Mike90250]

When you have more than 2 panels in parallel, you need to use a fuse or breaker for each panel, before they are combined. The PV panels should have a spec on their sticker, for what their fuse should be.
Generally, this amperage is below a common breaker value, and so simple automotive type fuses can be used in most 12V systems at this location.
Charge controllers often have internal protection fuses, so check it's manual, to see if a fuse is recommended between the controller and PV panels. This is where I have a circuit breaker, as it's an
easy way to connect and disconnect the panels for any reason. Your 15A breaker should be OK here.

What I don't understand is your 18A Float.

between the controller (MPPT) and the battery as it would be putting out 13v float at >18 Amps

If you are expecting >18A at float, you need to figure how much will be in the circuit at ABSORB charge rates, which are often quite a bit more than float amps.

[Tom Bradford]

When you have more than 2 panels in parallel, you need to use a fuse or breaker for each panel, before they are combined. The PV panels should have a spec on their sticker, for what their fuse should be.
Generally, this amperage is below a common breaker value, and so simple automotive type fuses can be used in most 12V systems at this location.
Charge controllers often have internal protection fuses, so check it's manual, to see if a fuse is recommended between the controller and PV panels. This is where I have a circuit breaker, as it's an
easy way to connect and disconnect the panels for any reason. Your 15A breaker should be OK here.

Aren't fuses and breakers different things? A fuse is a protection against an overlead whilst a breaker is 'simply' a switch should you wish to isolate the panel array from the controller?

Alas the more I'm finding out about this subject the more I'm finding I don't know.

What I don't understand is your 18A Float.
If you are expecting >18A at float, you need to figure how much will be in the circuit at ABSORB charge rates, which are often quite a bit more than float amps.

See what I mean? I guess that should have been 'absorb' rather than 'float'. I believed MPPT controllers work their magic by using all the power coming from the panels at a lower voltage but higher amperage. Hence the amps between controller and battery will be higher than amps between panels and controller.

If the switch from PWM Controller to MPPT doesn't make up the 'current' shortfall in our system we might have to add another panel so while the 15A breaker might be OK now it might need to be bigger later. So I'm wondering whether it's OK to start out with overlarge breakers - say 30A or even 50A, as they're all the same price - or whether there is a power penalty with an overlarge breaker which makes it preferrable to get as small a one as possible.

[Sunking]

Aren't fuses and breakers different things? A fuse is a protection against an overlead whilst a breaker is 'simply' a switch should you wish to isolate the panel array from the controller?

No technically they are both Over Current Protection Devices, and either can be used in place of each other.

[Sunking]

I think I'm right to say a 15 Amp breaker would do between the panels and the controller but we'd need a 20 Amp one between the controller (MPPT) and the battery as it would be putting out 13v float at >18 Amps, but as they're the same price is there any penalty to having oversized breakers - say 2 x 50 Amp ones - to allow for future expansion?

I think you are a little mixed up. When a battery is in float mode it is fully charged up and not taking any current, or I should say very little current.

Fuses, breakers, or other types of Over Current Protection Devices are to do one thing only, protect the cable they are connected too and sized accordingly. For example a 20 amp breaker must have a minimum 12 AWG copper conductor. It can be larger, but no smaller.

[Tom Bradford]

No technically they are both Over Current Protection Devices, and either can be used in place of each other.

In my world, which only has a tentative toe in the solar world, a fuse is a once-only device that blows under certain conditions and has to be replaced before normal service can be resumed, while a breaker is a manual switch for 'OFF' and 'ON'. However this thread suggests to me that as well as being a manual switch a breaker is a kind of multi-use fuse that blows where a fuse would but can just be switched on again to restore the circuit.

If this is the case presumably the answer to my question is that you need appropriately sized breakers, and that one too large could cause damage to the system by not responding correctly.

Yes?

[Sunking]

If this is the case presumably the answer to my question is that you need appropriately sized breakers, and that one too large could cause damage to the system by not responding correctly.

As I stated the purpose of a OCPD is to protect the wiring. If you oversize the OCPD, you must oversize the wire. On the flip side you can oversize the wire, but you do not have to oversize the OCPD.

[Mike90250]

Breakers CAN function as a switch, but their main purpose is to cease conducting when their limit is exceeded. I use the PV breaker in my system as a switch, but I will have to purchase a spare pretty soon, even DC rated breakers, when used as a switch, cause arcing damage to the contacts, and that can impede their main function as Circuit Protection.

[Sunking]

Breakers CAN function as a switch, but their main purpose is to cease conducting when their limit is exceeded. I use the PV breaker in my system as a switch, but I will have to purchase a spare pretty soon, even DC rated breakers, when used as a switch, cause arcing damage to the contacts, and that can impede their main function as Circuit Protection.

Thanks for bringing that up Mike i was about to say using a breaker as a switch does damage with each operation when there is a load. They are not made for a switch.

[SunEagle]

Thanks for gringing that up Mike i was about to say using a breaker as a switch does damage with each operation when there is a load. They are not made for a switch.

There switch duty rated breakers that can handle multiple operations but even they will wear out and not do their job of over-current protection when they are needed. It is best to install both an over current device and a switch to manually open the circuit.

[Tom Bradford]

OK, so once more round the mulberry bush.

A breaker is a switch except when it's a OCPD when it's a fuse. So even when I have a breaker I need a switch.

My boat has a big red knob called a battery isolator which I use when I've leaving her for a while to disconnect the battery from all the electrics and prevent drainage. Is that the sort of switch I want in addition to the breakers that are also fuses or do I need something else because it's just called a battery isolator instead of a SBI40-RCW so doesn't count?

[inetdog]

OK, so once more round the mulberry bush.

A breaker is a switch except when it's a OCPD when it's a fuse. So even when I have a breaker I need a switch.

My boat has a big red knob called a battery isolator which I use when I've leaving her for a while to disconnect the battery from all the electrics and prevent drainage. Is that the sort of switch I want in addition to the breakers that are also fuses or do I need something else because it's just called a battery isolator instead of a SBI40-RCW so doesn't count?

A switch is just a switch, although some other things can serve as switches in addition to other things they do.

A breaker is a thing that opens a circuit automatically when the current goes too high and allows the circuit to be restored again by flipping a handle of pushing a button.
A breaker that has a handle can also be a switch. A breaker that resets with a push button cannot be used as a switch because there is no way to turn it off manually.
A fuse is a tiny piece of wire inside a package and when the current goes too high it melts and opens the circuit. It then has to be replaced. You can open a circuit manually by removing the fuse, but since that involves touching it and also sparks and arcs it would be a bad idea.

The battery isolator you talk about is probably just a switch. To provide protection to the wiring and the battery against a short circuit, you also need a fuse. For very high current DC it is often hard to find a proper circuit breaker, so a fuse is typically used. A fuse has to go into a fuse holder, which is often attached directly to one terminal of the battery.

How's that for a start?

[SunEagle]

OK, so once more round the mulberry bush.

A breaker is a switch except when it's a OCPD when it's a fuse. So even when I have a breaker I need a switch.

My boat has a big red knob called a battery isolator which I use when I've leaving her for a while to disconnect the battery from all the electrics and prevent drainage. Is that the sort of switch I want in addition to the breakers that are also fuses or do I need something else because it's just called a battery isolator instead of a SBI40-RCW so doesn't count?

The battery isolator would be considered a switch. They are usually beefy enough to disconnect the circuit even under load so it will have a rating with a high number of operations.

Use the battery isolator and an over-current protection device like a fuse or circuit breaker and you will be ok.

[Tom Bradford]

The battery isolator would be considered a switch. They are usually beefy enough to disconnect the circuit even under load so it will have a rating with a high number of operations.

Use the battery isolator and an over-current protection device like a fuse or circuit breaker and you will be ok.

Thanks.