Selecting Circuit Breaker for Protecting a Secondary Power Distribution Panel

Electrical and electronic topics for small boats
jimh
Posts: 8498
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Selecting Circuit Breaker for Protecting a Secondary Power Distribution Panel

Postby jimh » Sat May 02, 2020 9:32 am

Typically the 12-Volt power feed to the secondary power distribution panel at the helm console from the battery on a properly designed OEM installation (such as found on Boston Whaler boats) will be done with 8-AWG wiring. The wire size is usually clearly marked on the conductor insulation. The wire size is chosen to be sufficient to handle the current load from the secondary panel, but, more significantly, the wire size is chosen to limit the voltage drop to not more than three-percent at maximum load current. This often results in use of a much larger wire size than if only the current rating for the wire were considered. For more advice on selecting wire size for power distribution, see

Conductor Size for Power Distribution
http://continuouswave.com/whaler/reference/powerConductorSize.html

The purpose of a circuit breaker located close to the battery on the circuit that feeds power to the secondary panel from the primary power source is to shut off electrical current in the event of a wiring fault. The circuit breaker should be chosen so that it has a rating high enough that it would NOT trip off on the normal electrical loads due to use of electrical accessories like lamps, pumps, radio transmitters, SONARs, and so on, but low enough that it will very quickly trip off if there is a dead short at the secondary panel where a very high current could flow.

If 8-AWG wire is used, the wire can handle about 75-Amperes. As long as the circuit breaker rating is below 75-Amperes the wire will be protected. However, the lower the circuit breaker rating, the faster it will operate in case of a dead short high-current fault. If 10-AWG were used to feed power to the secondary panel the maximum current the wire can handle is 55-Amperes, so any circuit breaker less than 50-Amperes would be okay. For these reasons, the rating of the circuit breaker should be in proportion to the total load current expected to be consumed at the Secondary Power Distribution Panel.

A typical total load current expected at the Secondary Power DIstribution Panel on a smaller boat, say an open center console boat like a Boston Whaler MONTAUK 17, might be 25 to 30-Amperes. Note that the total load current does not mean that every circuit will be drawing the maximum current it is fused for at the same time. Typically a load panel can have a total fused current in all its branches that is higher than the expected normal maximum load current because not every device will be operating at full current simultaneously. The total of all circuit protection device ratings on a secondary panel may exceed the panel's rating for maximum current. I don't know the precise scheme or rule used in making this judgement.

User avatar
Phil T
Posts: 1497
Joined: Thu Oct 22, 2015 6:08 pm
Location: NYC then MA, NY, ME,DC,ME, now Kentucky

Re: Selecting Circuit Breaker for Protecting a Secondary Power Distribution Panel

Postby Phil T » Mon May 04, 2020 8:11 am

[There is a similar prior discussion at]

http://continuouswave.com/ubb/Forum6/HTML/002406.html

[which endorsed using a] 25-Ampere circuit breaker.
Member since 2003
1992 Outrage 17, 1992 Evinrude 115

jimh
Posts: 8498
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: Selecting Circuit Breaker for Protecting a Secondary Power Distribution Panel

Postby jimh » Mon May 04, 2020 8:19 am

A further consideration in selecting a circuit breaker to protect a Secondary Power Distribution Panel will be the number of operating cycles in the service life of the circuit breaker. Some circuit breakers are designed to also work as switches, and have a toggle or lever or handle that can be moved to OFF or ON. Circuit breakers of that type are designed to have a service life of many manual cycles of the toggle between OFF and ON.

Other circuit breakers are not designed to be manually operated as a control switch. These types usually have a service life limited in the number of expected times the breaker will trip OFF from a current overload.

A circuit breaker sometimes seen in use is an E-T-A Series 1658. See the datasheet on this product at

https://www.e-t-a.com/products/circuit_protection_devices/thermal_overcurrent_circuit_breakers/p/1658/

Note the maximum current rating available in this series is 30-Amperes. This type circuit breaker is not intended to be used as a switch to control the load. It is only rated for a total of 1,000-cycles of operation.

jimh
Posts: 8498
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Selecting Circuit Breaker for Protecting Single Branch Circuit

Postby jimh » Mon May 04, 2020 1:41 pm

A corollary topic is the selection of a circuit breaker for protecting a single branch circuit from a secondary power distribution panel.

In a properly wired boat the OEM boat builder should be using a minimum wire gauge of 16-AWG for a branch circuit. This is a recommended minimum conductor size according the federal regulations (governing commercial boats) and industry recommendations from the American Boat and Yacht Council (ABYC recommendation for AC and DC wiring, part 11.16.1.1.2) for boats in general. A conductor of 16-AWG can handle a maximum current of 22-Amperes. However, to maintain the voltage drop to three-percent, the maximum current is reduced to 3.7-Amperes for a one-way run of 12-feet maximum. On that basis, an appropriate circuit breaker would be 5-Amperes.

For branch circuit loads with higher load current, a larger conductor would be used. If 14-AWG is used the conductor can handle a maximum of 32-Amperes, but if voltage drop is considered the rating is reduced to 5.9-Amperes.

Generally on a boat a branch circuit load would consist of just a one or two permanently wired devices. An appropriate method to develop the rating for a branch circuit is to consider the devices that will be connected. For example, cabin interior lighting may consist of two incandescent bulbs in small lamp fixtures, each drawing 2.5-Amperes. Such a branch circuit could be current protected at 5-Amperes.

Spc337
Posts: 95
Joined: Thu Aug 10, 2017 4:44 am

Re: Selecting Circuit Breaker for Protecting a Secondary Power Distribution Panel

Postby Spc337 » Thu Oct 01, 2020 12:17 am

I’m using a [Blue Sea System] ST Blade Fuse Block under the console. I can use either 10-AWG or 8-AWG wire from the battery to the fuse block.

Should I put a fuse near the battery for the positive wire?

Is that fuse unnecessary since all loads will have their own fuse 20-Ampere or lower within the block?
Boston Whaler 1979 V-22 Outrage

jimh
Posts: 8498
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: Selecting Circuit Breaker for Protecting a Secondary Power Distribution Panel

Postby jimh » Thu Oct 01, 2020 8:50 am

If the secondary power distribution panel is a very short distance from the battery, and if the length of the conductor to the secondary panel from the primary power distribution point (which in this case sounds like it will the battery terminal post) is short, say 12 to 18-inches or no longer than it would be to reach a circuit breaker, then perhaps the main fuse for the secondary panel could be omitted, but omitting it would leave the secondary panel itself at risk of a short and being melted by heat.

Normally the only unfused load is the engine starter motor. This is because the peak current involved in normal operation is very high, perhaps 400-Amperes, and the conductor itself is very large (4-AWG or larger) and not easily damaged.

If a fuse is used, the fuse should be right at the battery terminal (or right at the main battery disconnect switch—assuming there is one (and there should be). Using a fuse is better than omitting the fuse.

Spc337
Posts: 95
Joined: Thu Aug 10, 2017 4:44 am

Re: Selecting Circuit Breaker for Protecting a Secondary Power Distribution Panel

Postby Spc337 » Thu Oct 01, 2020 10:04 am

If [I have correctly] read earlier posts, then an inline blade fuse of 75-Amperes should work [as an over-current protection for the circuit from the primary power distribution bus to feed a secondary power distribution panel]. [On my boat's wiring] nothing was fused previously.

How did Boston Whaler [use circuit breakers to protect a secondary power distribution panel] in the wiring grom the factory?

The battery on my Boston Whaler boat is located in the stern with a main switch.
Boston Whaler 1979 V-22 Outrage

jimh
Posts: 8498
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: Selecting Circuit Breaker for Protecting a Secondary Power Distribution Panel

Postby jimh » Sun Nov 08, 2020 8:53 am

Boston Whaler boats that had a secondary power distribution panel located at some distance away from the primary power bus used a circuit breaker to protect the feed to the secondary bus from the primary bus. Typically the circuit breaker rating would be about 30- to 50-Amperes. The circuit breaker was located close to the battery and was housed in a plastic enclosure that was fastened to the inwale of the cockpit.

The circuit breaker typically used would be similar to the ETA circuit breaker I mentioned in an earlier replay and gave a hyperlink to.

Even for boats made a late as 1992, the secondary power distribution panel on a Boston Whaler boat was rather minimalist.

ImageA Boston Whaler secondary power distribution panel as seen in a 1992 REVENGE 22 boat.

jimh
Posts: 8498
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: Selecting Circuit Breaker for Protecting a Secondary Power Distribution Panel

Postby jimh » Sun Nov 08, 2020 9:00 am

Spc337 wrote:If [I have correctly] read earlier posts, then an inline blade fuse of 75-Amperes should work ...

Finding a blade fuse of 75-Amperes may be difficult.

The ATO or ATC blade fuses are generally only available in a range of 0.5 to 40-Amperes.

The only fuse that I find readily available at a current rating of 75-Amperes is not a blade fuse and not an inline fuse. It is an MRBF TERMINAL FUSE. Compare at

https://www.bluesea.com/products/5180/MRBF_Terminal_Fuse_-_75A

For wiring directly to a battery post, the fuse must be mounted on a MRBF TERMINAL FUSE BLOCK. Compare at

https://www.bluesea.com/products/5191/M ... 30_to_300A

You cannot just "dial up" any value of current for a fuse or circuit breaker. They are only made is certain values of maximum current.

jimh
Posts: 8498
Joined: Fri Oct 09, 2015 12:25 pm
Location: Michigan, Lower Peninsula
Contact:

Re: Selecting Circuit Breaker for Protecting a Secondary Power Distribution Panel

Postby jimh » Sun Nov 08, 2020 9:11 am

There are some higher-current blade fuses in the Maxi-fuse line that can be installed using an in-line fuse holder. The problem is the fuse holder is only rated at 48-Amperes maximum current. Compare at:

https://www.bluesea.com/products/5068/MAXI_In-Line_Fuse_Holder

For Maxi-fuses for circuit greater than 48-Amperes, a Maxi-fuse block must be used. See

https://www.bluesea.com/products/5006100/MAXI_Fuse_Block