what amp fuse to run from my boat battery to switchbox

Bombardment Depository financial institution Fusing

A Battery Final Fuse

Lately I have had a number of questions about battery banking concern fuse sizing and I wanted to requite a brief explanation of how I size these fuses and why they are a good safety characteristic..

*Merely put y'all size the fuse to protect the wire, provided your wire is sufficiently sized.

The ABYC Standards On Bank Fusing:

The ABYC requirement is for a battery depository financial institution fuse to be inside 7 wire inches of the battery bank. In the real world of boats and bombardment compartments this is often difficult to do. If you tin can't get within 7" then the next all-time thing is to exist as shut as possible but the wire should ideally exist in a protective sheath or conduit.

If yous're having trouble coming together the seven" dominion, the Blueish Sea battery terminal fuses shown in this photo are an excellent option. These fuses are excellent but they do add most 2", in height, to a battery post so measure your battery compartment height carefully.

The limiting cistron with the battery terminal fuses is that they only go to 300A then if trying to fuse a bigger engine you may need a Form T or ANL fuse.

If you prefer to cutting to the chase I volition requite you the bottom line minimum banking company fusing I similar to see for any bank that could ever be called upon to start a motor:

Minimum fuse size for small-scale sailboat diesel AUX engines 8HP to 35HP = 250A

Minimum fuse size for medium sailboat diesel AUX engines 35HP to 60HP = 300A

Above 60HP 80% of in-rush every bit minimum fuse size.

If yous follow these guidelines you will not endure "nuisance trips".

SITE PLUG:

As a service to my readers I offer the Blueish Sea battery terminal fuses, shown above, in the MarineHowTo.com web store, and at very competitive prices. Purchasing products through MarineHowTo.com keeps the site running and helps to go along it FREE! The Blueish Ocean fuses are sold as a package, fuse & holder together for just $29.95. My prices vanquish Defender, West Marine and just near anyone out there selling the Bluish Body of water fuses.

Why I Believe In Battery Banking concern Fusing

Hither is a prime case of where battery bank fusing can help save a boat. A plastic wire tie in an engine compartment let continue this possessor. The bombardment cable fell against an engine pulley and began to chafe. When the metal of the engine and the copper of the wire made contact the fuse went POP.

This boat possessor had simply recently installed a 300A bombardment terminal fuse. This owner was very happy he had..

Exceptions to the ABYC Fusing Standard:

The ABYC has an "exception" to the bank fusing rule for cranking motor batteries. This exception however is more than broad based and written to include for large engines which have massive amounts of starting current being drawn. These engines are very expensive to fuse properly hence the "exception" to the rule.. Call back big sport angling boats with a pair of MASSIVE Caterpillars, and these engines are nearly as far away from a small sailboat AUX engine as can be... Small diesel or gas AUX engines on sailboats are oftentimes well served fused, rather than unprotected.

I quote Nigel Calder hither:

"The net result is that nowadays, electric shorts are probably the number-ane crusade of fires on boats. At that place is simply no alibi for not protecting all high-electric current circuits, including the cranking excursion." (From the Nigel Calder Cruising Handbook)

Recently I take had a lot of questions regarding fusing and I will try and reply them as best I can.

Question: What exactly am I protecting?

Over electric current protection (OCP) or over current protection devices (OCPD'due south) are sized to protect the wire non the devices they are powering. This is often misunderstood. You can ever get smaller with OCP, than the wires ampacity rating, but ideally should not exceed the ampacity rating. The OCPD is there to prevent the wire from overheating, melting and starting a fire.

Question: "What if my engine draws more than the ampacity limit the wiring is rated for?"

This is actually not uncommon. Many builders undersized starting wire for many years and got away with information technology due to the short duration starting circuits are loaded for. Today well-nigh builders accept come closer to where they should be. A good example is the original Universal M-25 every bit shipped on Catalina Yachts.

Catalina used to ship the Universal G-25's with 4GA wire. They at present ship that same engine, M-25XPB, with two/0 estimate wire. That is a HUGE difference. If you lot accept small guess wire an upgrade to larger wire can exist a very good investment and your engine will start a lot quicker and the starter will see a lot less voltage drop. Nearly every sailboat I went aboard during the last boat evidence was using 1GA or larger wire with 1/0 and two/0 being the near popular in boats over 30'..

Question: "Won't the starters inrush electric current accident my fuse?"

Offset, what exactly is "inrush current"? Inrush current is the very cursory fasten in current that the starter undergoes to get the motor to begin turning over from a stopped country. The inrush elapsing is usually almost 200ms to 250ms long, and not long enough to blow a properly sized fuse. ANL, MRBF or Course T fuses are not sized for the inrush, they are sized to the wire they are protecting.

This video beneath shows the accented superlative inrush as captured by a Fluke 376 meter. The engine is an older 2QM20 Yanmar. The accented peak current draw, mayhap 2/tenth of a second, is 316 amps yet this motor is protected by a fuse rated well below the inrush. It has never blown nor will information technology at this inrush chapters. The average starter load during the elapsing is closer to 150A.

In-Blitz Yanmar Motor:

Trip Delay Bend

Question: If the starter tin can draw more the fuse rating so why doesn't it accident?

Reverse to popular belief ANL, Class T or marine rated battery fuses (MRBF's) exercise not accident or trip at face value unless the elapsing is long enough.

Every bit you can see below a 200A ANL tin can support 500% of it'due south rating for about .7 seconds, longer than average inrush by more than double. It tin back up over 300% for 1 2d and 200% for as long as five seconds. You can push 150% through for up to 500 seconds.

Even if you lot figured an inrush that, by freak run a risk, lasted for ane second, which should non happen, a 200 amp ANL can supply 600A which is almost double what the pinnacle inrush is for the 2QM20 in the video above then in that scenario y'all'd still be fine.

Just as a signal of reference a 1987 Universal M-25 draws roughly 225A +/- depending upon temp and other factors. This is the in-rush load though. All engines are slightly different. Some draw more than and some draw less but the in-blitz is nonetheless a very short duration on all of them. None of this changes the fact that y'all are even so protecting the battery cable with the fuse and not the battery or other systems. The fuse is yet sized to protect the battery cablevision and this in-rush data is only a reference point..

200 AMP ANL Seconds vs. Amps

.7 Seconds =1000A
1 Second = 600A
five Seconds = 400A
500 Seconds =300A

NOTE: Photo taken of original image by Blue Sea Systems

What Type of Over Current Protection For My Bank?

Question: What type of over electric current protection for my bank/banks?

Generally speaking larger banks should exist protected with the following types;

These fuses are available from Blue Sea and others. They should yet be "Ignition Protected" fuses if installed on a gasoline boat and Blue Body of water is the just i I know of offering ANL fuses with ignition or spark protection for ANL's. Class T fuses are not IP rated however they are fully encased in a metal body. Class T fuses have merely non been tested for IP rather than do not meet IP. In speaking with Blueish Sea systems I was told they have no documented cases of an IP alienation on any Grade T fuses.

AIC stand for Amperage Interrupt Electric current and Grade T, ANL and MRBF fuses all have AIC suitable for decent sized firm banks. If you have a large bank of Odyssey or Lithium batteries then a Class T would be best bet as the AIC rating of Grade T fuses is almost twenty,000 amps. ANL fuse AIC is 6000A and MRBF is 10,000A.

The concern with AIC is that some breakers can literally weld shut before tripping, if the bank has enough short circuit behind it. For decent sized battery banks you ideally want an AIC rated fuse or breaker of 5000A AIC or greater. AIC is a greater concern for breakers but fuses are also AIC rated and can fail dangerously when subjected to shorting amperage greater than their AIC rating. The actual ABYC requirement for batteries is that any depository financial institution over 1100 CCA needs 5000 AIC rated protection or greater. Even two parallel group 27 batteries tin supply more than 1100 cold cranking amps...

Why Does The AIC Rating Matter?

Question: Why does the AIC rating matter?

Here is an example of what tin can happen to a cheap ANL fuse. These are not ignition protection (IP) nor AIC rated fuses. I searched and could non observe any sort of data for them other than "Made in Cathay".

The fuses were continued directly to a LiFePO4 lithium battery banking company and and so the circuit was shorted. The fuse trip was and then violent, it literally blew the windows out of the fuses. This is an Dangerous failure mode for a fuse. It failed for both AIC and ignition protection safety.

Running this same test with Bluish Body of water Systems ANL fuses I could not go a unmarried fuse to neglect in an unsafe manner. The quality and brand of fuse yous choose matters. Not all fuses are created equal.

And then which fuses are safe for bombardment banking company fusing:

ANL

Class T

MRBF

How Do I Decide My Wires Ampacity Rating?

Question: How do I determine the correct size fuse for my wire?

This is Tabular array VI from the ABYC E-11 Electrical Standard (click information technology to make it larger). It is for single conductor wires not bundled or sheathed together. The table is organized by jacket temperature rating.

UL1426 Marine wire should by and large exist 105C rated. Manufacturers like Pacer, Ancor, Berkshire, Cobra and others all build marine bombardment cable to UL1426 standards and nearly whatsoever chandler will have it.

The ABYC standard does permit you to get to 150% of the Table Six ampacity rating, if necessary, only if y'all do this always round down to the next size fuse rather than upwardly. My personal preference is to size the wire correctly so that use of the "150% rule" is non necessary.

And then, if yous were using 2/0 wire, and information technology was outside the engine space, then you could use a fuse up to 330A @ 100% of the ampacity rating.

If you needed to go bigger with your fusing you lot could use the 150% dominion and apply 330A X 150% = 495A fuse, or rounded down to the next commercially available size.

What do manufacturers suggest?

Here are some minimum manufacturer suggestions for battery/starter cablevision from Westerbeke & Universal.

Universal / Westerbeke BATTERY CABLE REQUIREMENTS (wire length round trip)

2GA = 8'
1GA = 10'
1/0 = 14'
ii/0 = 18'
iii/0 = 22'
4/0 = 28'

It does non take much to have 10' of wire length even with batteries merely a few anxiety from the engine. Keep in mind these numbers are wire length along the conductor, not an "every bit the crow flies" altitude.

Data Table Courtesy ABYC E-eleven

This Vessel is Protected With A 300A ANL Fuse:

This is another prime example of "inrush" vs. average starting current and what will blow the fuse and what will non.

In this screen capture we tin see that the starter drew slightly over 640A during the peak inrush. It should be noted that this iv cylinder diesel motor has been started in excess of 1000 times with a 300A ANL.

When sizing for starting loads e'er use the largest fuse you can for the wire, so every bit to eliminate any chance of "nuisance trips".

Starting Load At 20F

In this screen we can come across the "average" starting load was 286A. This was measured on a day when the air temp was 20F and the batteries in the bilge were at 32F. As such these currents were slightly college than they usually would be on a typical minor sailboat AUX motor.

Question: "How does a 300A fuse deal with an average starting current of 286A and non trip? Don't you lot desire to always exist nether 80% of the fuses rated trip point?"

The underground is in the duration. This engine starts, from a loaded starter to unloaded, in just 0.75 seconds, as evidenced by the; "Time 765mS".

If yous look back at the trip delay curves for an ANL you'll see why this 300A ANL fuse has never diddled despite over 1000+ starts on this engine since the installation.

Exceptions To The "Rule" vs. Common Sense

I hear it stated over and over and over that the ABYC standards make an exception for starting motor circuits, and this is 100% true. The reason information technology is true is to accommodate HUGE engines that tin can not easily or only be protected with over current protection. The vast bulk of marine engines in the world today can hands be protected with over electric current protection. If you own a massive yacht, with massive engines, please put your starting motor conductors in a protected conduit if they tin can not be fused.

ABYC E-11

"eleven.10.1.i.one Overcurrent Protection Device Location:

Ungrounded conductors shall be provided with overcurrent protection within a altitude of 7 inches (178mm) of the betoken at which the conductor is connected to the source of power measured along the conductor.

1. Cranking motor conductors."

On engines sub 300HP +/- in that location is no adept excuse for non fusing all battery banks on-lath including starting circuits. If your start depository financial institution can not be fused so please accept the time to protect the wire in a conduit for your own safety.

What do we know about this fire on a xv' Boston Whaler?

1- Electrical fire
2- Single Group 24 battery
3- Multiple children under the age of 9 on-lath seconds before the burn!
3- NO OVER CURRENT PROTECTION!!!!!

Use the ABYC exception, or common sense? You decide.....

MAY-2006

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Source: https://www.pbase.com/mainecruising/battery_fusing

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