Wire Gauge importance

[islander20]

I am in the process of installing a 15A macerator pump on my boat and getting to the step of wiring it in. I am getting to the wiring step and realized my fuses and switches are all for a max 5A draw. If I I upgrade one switch and fuse to 20 amp service and use 10 gauge wire to get the pump hooked up to the circuit board will I be ok? Some of the wires in the circuit board look like the may be a bit under 10 gauge.

Really appreciate the help

[bstreep]

Here's the chart. REMEMBER, it's ROUND TRIP, not one way:

Amps and Wire Gauge - 12V Circuit

[Ex-Calif]

You need the third part of the equation and that is length of the run.

Also this presumes the supply to the bus you are drawing from is adequate to supply all the loads from the bus.

[islander20]

Thanks for the site, very informative. I will only need 10 gauge wire to cover the 15 foot distance, I was just worried that some of the wires in the circuit board wouldn't be 10 gauge. Will it be ok since they travel such short distances?

[osirissail]

Originally Posted by islander20

Thanks for the site, very informative. I will only need 10 gauge wire to cover the 15 foot distance, I was just worried that some of the wires in the circuit board wouldn't be 10 gauge. Will it be ok since they travel such short distances?

Basically, NO. The feed wire from your circuit breaker main panel supply bus to the new circuit breaker also needs to be 10ga wire. In essence the whole run from the supply bus bar or post to the circuit breaker to the pump and back to the negative bus bar/post all needs to be at least 10 gauge.

And hopefully you are using Marine grade tinned stranded wire and not any "house/home" type wire like from Home Depot or such places. Marine grade wire has more strands than "home/house" wire as normally the wire used in Marine wiring is subject to more flexing as the boat moves and bounces and pounds its way through the seas.

[CarinaPDX]

I disagree. Basically there are two issues with selecting a wire gauge: voltage drop and maximum current. The tables referred to determine wire gauge for a specified voltage drop (usually 1-3%). Since 12V devices take about ten times the current as 120V devices of the same power consumption, 12V systems need large cables to keep the voltage drops to a minimum. The other issue is related to the heating of the wire created by the current, and whether the insulation can withstand that heating. This is why the thermal rating of the insulation is a variable in the table, as is whether the cable is routed through the (hot) engine compartment. In this (15A) case, for a very short run, the voltage drop is negligible if you used 12ga or even 14ga wire. And as far as heating, consider that the 20A circuits in your house are 14ga - it is not a problem.

Greg

[CarinaPDX]

I hasten to add, that if doing this from scratch, of course you would use the same or larger cable as you move back up the distribution system. And that is best practice. But as long as, say a one foot length, is done with 14ga it is not going to cause problems.

Greg

[islander20]

Thanks for all the replies, didn't want to start a fire in my baby. Of course if I was doing it from scratch I would use the proper gauge wire throughout, was just trying to avoid retiring the entire circuit board if I could.

Thanks again!

[motion30]

when in doubt go with the larger size wire

[hellosailor]

What "circuit board" ? A circuit board is the board full of electronics that is in a component, such as the circuit board in your VHF or television. Usually brown, green, or blue fiberglass material.

If you mean the circuit breaker distribution panel in your electrcial system? Your only concern there should be that all the wiring rom the battery to THIS circuit, is able to carry this load. Other circuits are not involved, and if your VHF or anchor light is on a 5A circuit wired up with 14AWG wire, that's just not relevant.

As long as the wiring on THIS circuit, and the master hot/ground wires to the distribution panel, can carry this load, all you need to be concerned with is this load and this circuit.

As to wire sizes--that chart is for total wire length, which is the distance both TO and FROM the load. So if your macerator is 15' from the panel, you need to look at the loads for 30' of wire.

30' of wire carrying a 15A load would require 8AWG wire. (And AWG and SAE wire sizes are different. If you use "automotive" SAE wire, you need about a 6-SAE to match an 8-AWG). Since a macerator normally runs for short times, 8AWG probably is good enough although stepping up one more size to 6-AWG would not be unrasonable. Macerators work MUCH better with higher voltage.

And of course, on a boat you want fully tinned wire.

[David M]

Originally Posted by CarinaPDX

I disagree. Basically there are two issues with selecting a wire gauge: voltage drop and maximum current. The tables referred to determine wire gauge for a specified voltage drop (usually 1-3%). Since 12V devices take about ten times the current as 120V devices of the same power consumption, 12V systems need large cables to keep the voltage drops to a minimum. The other issue is related to the heating of the wire created by the current, and whether the insulation can withstand that heating. This is why the thermal rating of the insulation is a variable in the table, as is whether the cable is routed through the (hot) engine compartment. In this (15A) case, for a very short run, the voltage drop is negligible if you used 12ga or even 14ga wire. And as far as heating, consider that the 20A circuits in your house are 14ga - it is not a problem.

Greg

20 amp circuits in household circuits require 12 AWG.

[hellosailor]

OK, using an actual voltage loss calculator.
For 9 meters (~30 feet) of 8 AWG wire
with a 15A load
and a battery voltage at 12.4, nearly fully charged (12.6)
the actual voltage drop would be 0.9 volts, nearly a 7% voltage drop.

The macerator would be running on 11.49 volts.

That might be acceptable, might not. It certainly shows that wire size will have a significant effect on performance here though. Might ask the macerator company how they think it will work at 11.5.

Can thinner wire be used without burning down the boat? Sure. Will it deliver twelve volts to the macerator? No. Heck, maybe 11 volts is all it needs.

[Paul L]

The Bluesea circuit calculator is my favorite for calculating wire size. It allows for a numbers issues, like being in an engine room or conduit, plus how much loss you want to accept.
Blue Sea Systems

[Jbaffoh]

Originally Posted by hellosailor

OK, using an actual voltage loss calculator.
For 9 meters (~30 feet) of 8 AWG wire
with a 15A load
and a battery voltage at 12.4, nearly fully charged (12.6)
the actual voltage drop would be 0.9 volts, nearly a 7% voltage drop.

The macerator would be running on 11.49 volts.

That might be acceptable, might not. It certainly shows that wire size will have a significant effect on performance here though. Might ask the macerator company how they think it will work at 11.5.

Can thinner wire be used without burning down the boat? Sure. Will it deliver twelve volts to the macerator? No. Heck, maybe 11 volts is all it needs.

Not sure where you get your information.

First, 12.4v is NOT "nearly fully charged"--it's about 70% capacity. 12.73 v is fully charged according to Trojan.

Second, a 9 meter run of 8 AWG, at 13 volts and 15 amps, gives a voltage drop of 3%. 10 AWG would be a 4% voltage drop.

I get the same results using 12v instead of 13v.

So the Macerator pump will receive 12.35v at 15A from a fully charged battery.

More importantly, my macerator pump uses only 4A continuously, and I doubt any use 15A for more than a nanosecond. We don't want beakers to open for transient spikes, hence the 15A requirement. But his pump will NOT be a constant 15A load, and he will experience nowhere near the 3% voltage drop calculated for that bogus constant load.

[CarinaPDX]

It's only been a quarter of a century or so since I looked at the NEC (National Electrical Code) so no surprise. Perhaps it was for a 15A circuit? I am remembering now that it was a 15A circuit that I put in with 14ga.

At any rate the OP's question was two parts: the wiring size to the macerator, and the wire size behind the panel. The new run out to the macerator will need to be as calculated - absolutely no discussion about that. But a <slightly> smaller wire for a foot or so behind the panel will not be a problem. Still, I am a bit of a perfectionist and I would be looking at the entire wiring for the panel, and the main panel fuse size, while I was at it. But it will not be a hazard to have up to 14ga for 15A for a foot or so, if the larger issue of re-evaluating the panel is kicked down the road a bit. I should add that I am in the process of shuffling most of my panel wiring, as the move to LEDs and CFLs has reduced the need for power for lighting, while the addition of refrigeration and other goodies means those larger breakers are needed elsewhere. A cruising boat is always a work in progress.

BTW the reason for the 15A circuit breaker is so that it won't trip during the startup current.

Greg