300 AMP vs 350 AMP 1,2,B Switch

[er9]

Was just wondering under what circumstances would a medium size sailboat need a 350 AMP 1,2,B Battery switch vs a 300 AMP switch.

Blue Sea has an 'm' and 'e' line of switches the 'e' line being able to handle 350 AMPS vs 300 for the 'm' line.

it appears the 'e' line of switched handles larger cranking amps so is it just to accommodate a humongous battery bank or alternator delivering huge loads to a larger engine or larger distribution panel (lots of devices running) or mainly a little added over protection?

The 'm' series switches are nice because they have a mini version of the switch.

[noelex 77]

There is not much practical difference in the ratings between the Blue series e and m series battery switches. Blue Seas rate them at 350/600/1200 A and 300/500/900 A (continuous/ intermittent/ cranking).

Some larger boats need even bigger battery switches. The HD series is sometimes needed with a 600/900/1700 rating. Typical high current devices are engine starting, inverters, bow thrusters, anchor windlasses etc.

It is important to check that the equipment fitted is within specification. Marine electrical systems are becoming sophisticated and some boats are pushing the limits of what can be achieved, especially on 12v.

[GordMay]

Originally Posted by noelex 77

... The HD series is sometimes needed with a 600/900/1700 rating...

And, are available with Alternator Field Disconnect (I don't believe the e nor m are).

[noelex 77]

Originally Posted by GordMay

And, are available with Alternator Field Disconnect (I don't believe the e nor m are).

The e series has a large range of options including dual pole models. Some of the models do have alternator field disconnect.

I don’t think the m series has an alternator field disconnect option in the range, presumably because Blue Seas envisioned this range to more commonly used on smaller outboard powered boats.

[Cadence]

Originally Posted by er9

Was just wondering under what circumstances would a medium size sailboat need a 350 AMP 1,2,B Battery switch vs a 300 AMP switch.

Blue Sea has an 'm' and 'e' line of switches the 'e' line being able to handle 350 AMPS vs 300 for the 'm' line.

it appears the 'e' line of switched handles larger cranking amps so is it just to accommodate a humongous battery bank or alternator delivering huge loads to a larger engine or larger distribution panel (lots of devices running) or mainly a little added over protection?

The 'm' series switches are nice because they have a mini version of the switch.

With a 36' boat I would guess your starter amperage wouldn't be a problem with the 300. Chances are you would be paralleling the house and starter battery any way while not attempting to start an the same time.

[smac999]

If all the battery and engine cables are really big. The small Switch is harder to make them all fit.

[Bycrick]

But note that the "rated current" maximum value is ONLY if you use 4/0 wire. They don’t say what the rating is if you use "smaller" wires. It appears that they are counting on the wires to help dissipate heat generated in the switch. Thus I wouldn’t push them too hard.

[er9]

interesting, thanks for the replies. I think im going to go with the 'e' series switches. from researching a bit more on Blue Sea's website it seems they were designed around larger inboard diesels. the 'm' series smaller outboard as Noelex pointed out.

[smac999]

Originally Posted by Bycrick

But note that the "rated current" maximum value is ONLY if you use 4/0 wire. They don’t say what the rating is if you use "smaller" wires. It appears that they are counting on the wires to help dissipate heat generated in the switch. Thus I wouldn’t push them too hard.

well marine 2/0 wire is rated at 280a in an engine room so yes that makes sense that it requires 4/0 for 350a

[Bycrick]

I didn’t mean to imply that the 4/0 requirement wasn’t entirely reasonable. I think it is. My point was that one should read ALL the specifications and hopefully understand what they mean. For example, a 100 amp fuse isn’t rated at 100 amps in an engine room at 130*.

[s/v Jedi]

I use the mini 300A switches for battery disconnects of inverter/chargers.

You can simply look at the fuse (cable) you use: up to 300A fuse the small switch is good to go. I use the 500A remote battery switch at the battery bank which has a 400A fuse and 4/0 cable.

[er9]

Originally Posted by s/v Jedi

I use the mini 300A switches for battery disconnects of inverter/chargers.

You can simply look at the fuse (cable) you use: up to 300A fuse the small switch is good to go. I use the 500A remote battery switch at the battery bank which has a 400A fuse and 4/0 cable.

wowzers! i got a look at a piece of 4/0 cable yesterday for the first time. it looked like a piece of pipe. wouldn't want to have to make a radius of under 4' with that stuff, never mind crimping it.

[Bycrick]

I think some of your fuse values are too high.

You often hear the rule-of-thumb "Fuse for the wire size, not the load." However, that’s technically true only for the maximum fuse size. In another thread, Gord posted a great study that examined the resistance of butt connectors in #14 wire. One of the interesting points was how hot some of them got when the circuit current was well within the maximum that the wire was rated for. And that was for "tiny" wire by comparison.

So my practice is to fuse for the expected load, considering the time-duration of the load, realizing that most fuses will carry 130-150% of their rating for many minutes and maybe 200% for tens of seconds.

My example, the starter fuse on 3-4 cylinder Yanmar engines. Measured starter load was about 240 amps at normal cranking. They’re fused at 175 amps and we’ve never blown a fuse.

[s/v Jedi]

Originally Posted by Bycrick

I think some of your fuse values are too high.

You often hear the rule-of-thumb "Fuse for the wire size, not the load." However, that’s technically true only for the maximum fuse size. In another thread, Gord posted a great study that examined the resistance of butt connectors in #14 wire. One of the interesting points was how hot some of them got when the circuit current was well within the maximum that the wire was rated for. And that was for "tiny" wire by comparison.

So my practice is to fuse for the expected load, considering the time-duration of the load, realizing that most fuses will carry 130-150% of their rating for many minutes and maybe 200% for tens of seconds.

My example, the starter fuse on 3-4 cylinder Yanmar engines. Measured starter load was about 240 amps at normal cranking. They’re fused at 175 amps and we’ve never blown a fuse.

The ampacity of a cable is for continuous use but make sure to check the cable specs, especially temperature rating. I always use 105 deg. C rated but cheap wire can come as low as 60 deg C. which about halves the ampacity iirc. Also, account for de-rating, like when you bundle cables or when used in an engine room etc. I often encounter cabling that does not have enough capacity.

Here is how to do it: take the continuous current from what you connect. Multiply that by 1.25 then look up the ampacity to determine cable size, going to the next bigger gauge after de-rating. Example: 100A, no de-rating needed so minimum gauge is the one that has an ampacity of 100A so that would be 6AWG at 120A. The correct fuse is the corrected ampacity rating so in this case 120A or the next lower value. If that lower value is too low for the load, you need to go for a bigger gauge and do the math again.

Now comes the actual decision: how long is the cable run and how much voltage drop is acceptable? Let’s say the distance is 15’ x 2 (pos + neg cable) = 30’ and we accept 10% voltage drop at 12V... the table shows that you need a minimum of 4 gauge wire instead of 6 gauge.

Now the actual circuit is 4 gauge wire with a 100-120A fuse but when that fuse size does not exist, you can expand the allowable range to 100-160A because the 4 gauge can take 160A max.

Normal startup currents etc. will be absorbed by the cable and the fuse without causing problems.

As shown in this example, most circuits have larger diameter wire than the ampacity rating because the need for limiting voltage drop.