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Old 24-04-2017, 18:19   #1
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Breaker sizing (Amps) in breaker panel

Is there any reason to go with a breaker less than 15A on a DC panel branch cct assuming all wires are 18AWG or larger?

I'm currently debating if I want to have all 15A and 30A or also include some 5A and 10A for ccts with low loads (eg propane solanoid)
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Old 24-04-2017, 18:28   #2
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Re: Breaker sizing (Amps) in breaker panel

I believe you cannot have branch breakers in total more than 150% of the main breaker
All wire not in specifically excepted cases can be smaller than 16 awg
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Old 24-04-2017, 19:14   #3
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Re: Breaker sizing (Amps) in breaker panel

Scott - good point on the 150%. That may push me to the lower Amp breakers...need to check my math.
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Old 24-04-2017, 19:45   #4
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Re: Breaker sizing (Amps) in breaker panel

This chart may help you. / Len

https://www.bluesea.com/resources/1437
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Old 24-04-2017, 20:27   #5
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Re: Breaker sizing (Amps) in breaker panel

Keep in mind that sometimes the leads that come attached to an appliance (anything that uses electricity not just a refrigerator) can be quite small. Hella cabin fans are one of the most obvious examples. I have several 5 amp breakers for this situation although the feeding boat cable maybe say #14.
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Old 25-04-2017, 08:18   #6
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Re: Breaker sizing (Amps) in breaker panel

and the leads on most inline glass fuse holders are thin 16 gauge. What are they thinking..?
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Old 25-04-2017, 08:25   #7
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Re: Breaker sizing (Amps) in breaker panel

Quote:
Originally Posted by geoffr View Post
Is there any reason to go with a breaker less than 15A on a DC panel branch cct assuming all wires are 18AWG or larger?

I'm currently debating if I want to have all 15A and 30A or also include some 5A and 10A for ccts with low loads (eg propane solanoid)
Breakers protect the wire. Breakers do not regulate or limit current, as such. Breakers will protect your wire from being over-current and causing a fire.

A 12 ga wire is safe for 20 amps and should have a 20 amp or smaller breaker.

14 ga wire is safe with 15 amps.

16 ga wire probably needs a 10 amp breaker. I don't have the chart.

18 ga should probably be a 5 amp breaker.

This is what breakers are designed to do; prevent electrical fires. If you have 14 amps going through an 18 ga wire, the wire may get so hot it melts its insulation and/or ignites any flammable material the hot wire is near; wood, books, hull resins, other plastics.

Either re-size your wires to 14 ga (or larger) for the 15 amp breakers; or re-size your breakers to protect your smaller wires.
A short in an 18 gauge wire will quickly burn or melt the wire. A 5 amp breaker will trip before the wire can heat up enough to burn.

I hope this helps.
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Old 25-04-2017, 08:38   #8
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Re: Breaker sizing (Amps) in breaker panel

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Originally Posted by Frankly View Post
Keep in mind that sometimes the leads that come attached to an appliance (anything that uses electricity not just a refrigerator) can be quite small. Hella cabin fans are one of the most obvious examples. I have several 5 amp breakers for this situation although the feeding boat cable maybe say #14.
That's an excellent comment. You don't want the smallest wire to be the fuse.
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Old 25-04-2017, 08:48   #9
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Re: Breaker sizing (Amps) in breaker panel

Um... maybe obvious but some equipment requires 5 amp protection or less. E.g. Seatalk instruments. Although one could use a branch circuit with in-line fuse in those cases, I kind of have an aversion to hidden fuses scattered around the system.
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Old 25-04-2017, 08:58   #10
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Re: Breaker sizing (Amps) in breaker panel

Blue Sea Systems has a really good app on selecting wire sizes and circuit protection. It's based on ABYC standards, and while it's uber-comprehensive, you can be confident in the answers. It's call the Circuit Wizard.

I think that some of the previous information offered in this forum may have been extra conservative and not actually based on the standards. Unfortunately, there are many variables including the quality of the wire you're using, whether you're measuring the gauge in SAE or AWG, whether the wire is in the engine room, and so forth. That's a pretty good reason for using the Circuit Wizard.

And remember that you're dealing with both the ampacity of the wire (it's ability to get rid of heat due to current flow) and the voltage drop (which is generally what gets you on a 12V boat circuit).

Here's one example:

Voltage: 12V
Max current: 45A
Length of conductor: 10'
Allowed voltage drop: 10%
Insulation temp rating: 105 deg C
Not in engine room.
2-3 wires in the bundle
Duration: 30 minutes

Answer: 8 AWG

14 gauge would work from a voltage drop standpoint, but 8 gauge was needed due to ampacity.

I don't know that I would take the time to calculate every circuit on the boat, but I find it very handy to at least verify that you're doing the right thing.

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Old 25-04-2017, 09:03   #11
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Re: Breaker sizing (Amps) in breaker panel

Quote:
Originally Posted by geoffr View Post
Is there any reason to go with a breaker less than 15A on a DC panel branch cct assuming all wires are 18AWG or larger?

I'm currently debating if I want to have all 15A and 30A or also include some 5A and 10A for ccts with low loads (eg propane solanoid)
I think there is confusion between conductor ratings and overcurrent protection. You need to be concerned about overcurrent protection.


Small Conductors, the overcurrent protection shall not exceed ...
18 AWG Copper. 7 amperes, provided all the following conditions are met:
(1) Continuous loads do not exceed 5.6 amperes.
(2) Overcurrent protection is provided by Branch-circuit-rated circuit breakers listed and marked for use with 18 AWG copper wire.

16 AWG Copper. 10 amperes, provided all the following conditions are met:
(1) Continuous loads do not exceed 8 amperes.
(2) Overcurrent protection is provided by Branch-circuit-rated circuit breakers listed and marked for use with 16 AWG copper wire.

14 AWG Copper. 15 amperes

12 AWG Copper. 20 amperes

10 AWG Copper. 30 amperes

So the 18 ga wire may have up to a 7 amp breaker.
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Old 25-04-2017, 10:01   #12
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Re: Breaker sizing (Amps) in breaker panel

It seems we are looking at different data... for a single 18AWG wire with 105C insulation I read the chart below as 20A. The wire is not in the engine compartment.

https://www.bluesea.com/support/refe...e_Sizing_Chart

Although it's true it's not free air...so perhaps the right side is safer - 3conductors - so 12A.

Having said that I think 5A breaker makes sense for other reasons. (150% as noted by Scott)
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Old 25-04-2017, 10:10   #13
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Re: Breaker sizing (Amps) in breaker panel

Your insulation may be treasured at 105C but your termination may only be rated for 75C.
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Old 25-04-2017, 10:48   #14
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Re: Breaker sizing (Amps) in breaker panel

Quote:
Originally Posted by masonc View Post
Your insulation may be treasured at 105C but your termination may only be rated for 75C.
I have found some as low as 65C, not an easy spec to find thou.

In very general terms you want the breaker sizes large enough to not nuisance trip under the load but smaller then the ampacity of the wire. On a personal level I like to use a smaller size to better match equipment ratings as an additional failsafe. The last panel I built for a family member was mostly 5 and 10 amp breakers for instance (lights etc).

Also you can use breakers for equipment protection, some things you still want fuses thou. When we do some of the military projects they require all circuit protection be via breaker on a central panel unless a special fuse is required by the equipment. There are a number of high end boat builders that do the same with things like 2.5 amp and 3 amp breakers for electronics.
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Old 25-04-2017, 10:48   #15
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Re: Breaker sizing (Amps) in breaker panel

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Originally Posted by sailortype View Post
and the leads on most inline glass fuse holders are thin 16 gauge. What are they thinking..?
They are thinking you will have 1 amp fuses. I have a 5 amp breaker to a number of low-draw electronics called "electronics". It feeds a four fuse sub-panel with 1 amp fuses. The wires from the breaker are 14 ga. and 16 ga. outbound.
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