Choosing wire size

Hey Guys,

I'm installing one of these freezers on the FB of our Mainship. https://www.westmarine.com/buy/isotherm--top-loaded-refrigerator-boxes--P011020542?recordNum=17

I'm trying to figure out the correct wire size, the circuit run is about 30 feet (it's closer to 25 but I'm allowing room for error). The amp draw listed is 1.8 amps, but I know the start up load is much higher than that. Any advice?

Thanks,
Doug

https://www.westmarine.com/WestAdvisor/Marine-Wire-Size-And-Ampacity

http://www.marinewireandcable.com/p/voltage-drop-calculator.html

There are two basic criterion for sizing wire: 1) the current must be within the maximum current carrying limitations (before the insulation starts melting) and 2) to maintain acceptable voltage drop.

So using 60' round trip distance and setting a 3% drop criterion, my voltage calculator says 14 gauge which can carry up to 35 amps. But use a much smaller fuse or breaker, say 10 amps near the battery or at the DC panel.

David

Thanks David, it's about 30 feet round trip. Does that drop the size?

Not that it matters though, 14 gauge is pretty small. Would that allow for start up voltage? 1.8 amps seems so little for a freezer to me.

16 AWG would be adequate for the 2 amp draw, but I have no idea about the start-up load. I would use whatever Isotherm recommends.

I just did a quick look to see if I could see what Isotherm recommends. For that 4.1 cu model they recommend 10 gauge wire with a max run of 19 feet. This is a general recommendation for all the their units it looks like and your model has less of a draw than some of the larger ones. I would try calling Isotherm and asking them if there isn't anything more specific in the information that comes with the unit. If they still have the 10 gauge, 19' recommendation, I would probably go 8 gauge to hit your 25'.

but I know the start up load is much higher than that.


Size the wire for that. the last thing you want is too much of a voltage drop on the wires when trying to start the compressor.

Assuming a Danfoss/Secop BD35 compressor and 12 volt power, Danfoss/Secop recommends 10 gauge up to 20 feet and 8 gauge to 33 feet battery to compressor. They also recommend a connection directly to the battery.

http://files.danfoss.com/TechnicalI...N0210-20-40_0300-20_08-2011_dehcei100b602.pdf

I learned this fighting with a deck freezer that never worked right. I am still not sure I have won the battle.

Always go heavier size wire when sizing for items that have a surge on start up. I would go 10 gauge. Nobody ever said, "wished I had run smaller gauge wire for this circuit.

Ted

#12 good enough, and #10 excellent for 3% voltage drop. Since refers have a tendency to work poorly on low voltage, I would choose #10.

If your run is that long, and it's a shorter run to the battery or a main buss, consider running a line to a fused connection at that closer connection point. You can maintain the shutoff function provided by the panel's "freezer" breaker by using that breaker to power a relay, or powering the thermostat circuit, if it can be divorced from the compressor, from the panel breaker.

Keep in mind that the Danfoss/SECOP control modules are quite sensitive to voltage drop, so whenever you're powering refrigeration equipment, it's best to provide oversized conductors with the shortest run practical, and a minimum of clean, tight, low resistance connections. If you're powering your refrigeration from a main panel and you have a lot of other loads connected to that panel, you can run 2/0 between that panel and your refrigeration equipment, and STILL experience low voltage due to the other connected loads. So look at the SYSTEM, not just the wire!

Maerin said:

If your run is that long, and it's a shorter run to the battery or a main buss, consider running a line to a fused connection at that closer connection point. You can maintain the shutoff function provided by the panel's "freezer" breaker by using that breaker to power a relay, or powering the thermostat circuit, if it can be divorced from the compressor, from the panel breaker.

Keep in mind that the Danfoss/SECOP control modules are quite sensitive to voltage drop, so whenever you're powering refrigeration equipment, it's best to provide oversized conductors with the shortest run practical, and a minimum of clean, tight, low resistance connections. If you're powering your refrigeration from a main panel and you have a lot of other loads connected to that panel, you can run 2/0 between that panel and your refrigeration equipment, and STILL experience low voltage due to the other connected loads. So look at the SYSTEM, not just the wire!

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I'm installing the freezer in the summer kitchen which is up on the boat deck aft of the flybridge. Very difficult run down to the main panel, and likely longer than my planned run from the upper helm.

Under the upper helm there are two buss bars, one ground and one positive, that I am going to wire to. I think (with Mainships it is sometimes hard to tell) that they are wired directly to main buss off of the battery. Either way, they are serviced by very big gauge cables, like 0 or 00. There is not that much pulling off of them at the upper helm that would draw a lot of current, so I feel good about that. I'm going to put an in line fuse right after the buss under the helm then run the wires under the FB seats and back to the Summer kitchen. I'm planning on putting two more buss bars, or maybe a fuse panel under there in case I need DC at the summer kitchen in the future for something else.

After reading all of your threads (thank you for the advice) I'm leaning toward going over sized, like 8 or maybe even 6. Not much cost difference and plenty of room for the wire.

Sounds like the new plan is a good one. Big enough wire up to the bridge which is close. I would suggest an actual fuse block rather than inline. It's a bit more money, yes, but utility is better.

I hate inline fuse holders as they can often be hidden after a time, labelling useless and usually no easy way of testing for good or bad.

Anyway your shot. Overall plan sound fine.

This thing is DC, so volt drop calcs are different from 120vac stuff.

I looked at the specs, it is not clear if this thing has any startup surge like a rotating compressor. Not sure what process it uses. 1.8A at 12Vdc is under 25W. Does not sound like much cooling capacity!! My 120Vac fridge runs at 1.6A, about 200W.

Ski in NC said:

This thing is DC, so volt drop calcs are different from 120vac stuff.

I looked at the specs, it is not clear if this thing has any startup surge like a rotating compressor. Not sure what process it uses. 1.8A at 12Vdc is under 25W. Does not sound like much cooling capacity!! My 120Vac fridge runs at 1.6A, about 200W.

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Yes, I'm surprised and somewhat skeptical of its stated draw, that just doesn't seem like enough to me. Which is another reason, I guess, to oversize the wire. I have plenty of AH to handle a much higher load than it's rated for.

My house bank is 4 6V golf carts. We typically cruise in the summer in the Bahamas and run the gen set about 6 to 8 hours a day for A/C, so plenty of time to recharge batteries.

Specs say it will go down to 0 degrees while a lot of the other boxes in this class say 10 or 14 degrees. Most of the time we plan to use it as a fridge in the 40 degree range but it would be nice to have the freeze capability when we want it.

That said, I doubt it hits 0 on a hot summer day, but I figure if it's rated at zero it should hold 10 to 15 pretty well. Whereas if it was rated at 14 degrees it may struggle to freeze.

That's my thought process, such as it is.

I think his unit uses the Danfoss BD35 [maybe a BD50] The stated current draw may be low at 2A. There are some adjustments a person can make to speed or slow cooling which has an effect on the current draw.

Mine , BD35, was set at the fridge builder and draws about 3.5A at 12Vdc. It can draw ~ 7.5A if set for full tilt. Yes, I have measured mine at full tilt. So his would/should be similar.

If the stated 1.8A is correct then his builder set the compressor for a slower run speed so less cooling per hour but it will run longer each on/off cycle. Apparently it is actually more efficient, power, to run longer but slower drawing less current.

Brings up a point that I forgot about. Dougcole should be considering sizing the wire for the max. draw that unit can pull which I believe if he digs into it will be around 7.5A. Even though the speed setting will keep it around 2A the wire should be sized for max. draw.

Most standard refr. compr. will draw 4 to 6 times their rated so at 7.5A a 14 wire will be too light. His latest proposal should be fine going with a 6AWG bridge feed off the o or oo wire that apparently is used. The odds of other heavy loads are slim although not none. If I am even close the Vdrop at startup will easily be handled.

These Danfoss units are actually little 3 ph units with a varialble speed inverter to run the actual motor so the inrush is typically less than we would see in an industrial unit although even that is changing.

DOUG I strongly suggest that you contact, phone if need be, the builder and find out for sure what the total specs are and find out specifically which compressor is used. Also find out what their suggestion is for wire size. most builders already have that although it may not be in a website. If you know which compressor is used then you may be able to go to the compressor mfgr. and ask them ir the fridge builder cannot.

RT Firefly said:

https://www.westmarine.com/WestAdvisor/Marine-Wire-Size-And-Ampacity

Marine Wire and Cable: Voltage Drop Calculator

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Is it just me or is the person who put this chart together color blind. Looks like the colors for 1 and 14 are swapped.

The specs from the West Marine website seem to say the average amps are 1.8/hour. That would make sense given the typical duty cycle. Since the Isotherm says 10 gauge up to 19'. I would use 8 gauge and call it good at 25'.

You should be good to go with the #8. Any inrush will be a split second on these units and will be taken care of.

I will make the point that too many of these ads/sellers average the current out telling us that that is the draw and that is simply WRONG.

There are two values that should be attended to:
---of course the average as that is a guide to the power use that bears importantly on battery use.
---the other is the actual running and max. amps which is the determinant for the wire and fuse/C.B.
When determining wire size the second is the most important and too darn many of them make that hard to find.

An idea, if you have various gauge wires for a house for free, which I do, test it with the device to see if the gauge works. Then buy the multistrand boat wire that matches that gauge.

Simplest is to purchase a big spool of 10ga and use it for most everything.

Higher amperage ,double the wiring.

The amp draw seems low to me. My Novacool fridge and freezer units use 5 amps each, and the duty cycle is at least 80% in the summer.

8 AWG wire for a fridge???? Really?

You'll be fine with 10 or even 12 AWG.

Go look at the factory runs on your boat for things. You're probably not going to find any or very little 8 AWG in the mix. Look and see what a DC fridge is wired with on a boat. 10AWG is the largest you'll see.

Even physically connecting 8 AWG is a PITA. Standard crimpers are generally sized 10 AWG and up. Larger than that and you are getting into a ratcheting crimper that most boaters just don't carry on board.

My rule of thumb is any motor load should use 12 wire minimum unless higher than 15 amp draw or longer runs of wire. Personally, I think one is tripping over a dollar to save a dime by considering anything less the #12.

Conall

Conall63 said:

My rule of thumb is any motor load should use 12 wire minimum unless higher than 15 amp draw or longer runs of wire. Personally, I think one is tripping over a dollar to save a dime by considering anything less the #12.

Conall

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When wiring LED light that are drawing something less than .5A even 12AWG is a total overkill. Everything is just a matter of how much the load is and using a 12 or 14 AWG wire for little draw is just a waste. Money is one thing but also routing and connecting a 10 or 8 AWG wire is much more difficult.

L.

I agree with you Lou on things like LED lights, but I said "motor loads" in my post should have minimum of #12 wire.

Conall

12 vdc and 120 vac not exactly the same regarding current and wattage ideas and wire length voltage drops.

12 gauge seems right but if the wire gets long then go with 10, certainly not 8 or 6 gauge.
If wire is short then like 14 is ok. Running amps of only 1 or 2 even with the start up surge you wont need some of these huge gauges people are throwing out there.

The point of using one size wire is price.

Hard to pay for 25 ft of skinny wire when a 100 ft spool of better wire costs about the same.

Most boats have ongoing improvements.

Why guess when there are hundreds of wire size calculators and charts with the stroke of a keyboard?

All the old timers I worked with in the marine trades grossly unDer sized wiring runs...just look at the wiring on older boats.

Too many here are eager to upside because it's cheap, even when not really needed, and it not adding capability or redundancy.

Last of all, many of the installation guides seem to correspond with the wiring charts so I usually follow them....if not I use the larger ga suggested of the two.

sdowney717 said:

Running amps of only 1 or 2 even with the start up surge you wont need some of these huge gauges people are throwing out there.

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You have to consider start current as the wire itself will drop voltage from that current. If the drop is appreciable then the voltage drop will be proportionally large as well. V = R*I.

The last thing an electric motor rated for 12V DC wants is to have too low of voltage when the current demand is highest. Its a sure fire way to kill the life of the motor.

I would suggest is for the OP to determine what the start current is and then determine the appropriate wire gauge needed rather than trying to guess which of all the wild range of guesses is the right one.

A DC motor with brushes is very different from an AC motor.

Feed it lower voltage and it simply puts out less power, no overheating,no problem.

Oversized wiring does weigh more than light wire but cruising at 3 HP per ton its not a big deal.