Calculating DC Wire Gauge: Double Length?

[MCI-89]

I am converting a 1989 MCI, and have a 50A shore power connection running mostly AC. There are a few DC items I am going to run, so I have an AC to DC power converter that connects to a DC fuse block.

My issue is I have two diesel heaters that are 20' and 30' from the fuse block. I tried using a calculator tool, and it said I needed 4 gauge wire, so I hope I'm doing something wrong.

What I'm finding conflicting information on is whether I need to double the length of the wire run (ex: 30' would be input in a calculator as 60').

I don't want to mess this step up, and also need to wire a long run of lights, so any help would be appreciated!

 

[Kentucky Dreaming]

Yes, DC circuit wire size is based on voltage drop and heat from resistance for the total length of the circuit. The electrons have to make the trip from source to load, and then back to source.

Don't hold me to this, but you may be able to downsize a bit if you ground the circuit to the chassis near the load, instead of running a wire all the way back to the source.

Mixing AC and DC circuits on a steel tube tent is something I'm not sure about when it comes to chassis grounding.

Smarter folks will chime in soon...

 

[Rucker]

12 volt DC amp carrying capacity is very tricky over distance. The voltage drop is precipitous when you're talking significant amps.

My diesel heater clocks in with about an 8 or 9 amp load initially, then once running it drops to 3 or 4 amps. Let's assume your peak load is 8 amps.

The very thinnest wire you can use is 14 AWG (believe it or not it can carry those amps, just not over any distance). You'd be hard pressed to go more than a few feet with that wire size.

For sizing based on load over distance I use this link: Advanced Wire Ampacity Calculator

My opinion based on the chart: if your heaters have similar load characteristics, for your 20 foot run you need #8 AWG both directions, but that's marginal. #6 would be better. For your 30 foot run you need a #6 no question, both ways. Anything smaller risks the wire heating up because the copper has its own resistance, which converts to heat (with a subsequent voltage drop)

I don't recommend using the chassis as a return path, even though in theory you can do so. Based on my understanding, you might be able to drop to the next smaller wire size, but hardly worth it for the troubleshooting issues it will create over time, even at the current cost of copper.

 

[MCI-89]

Thanks for these replies. Aside from the hassle and $ of using lower gauge wire, I'm also unsure about what gauge my fuse block can accept. There are 30a fuses on it, but I'm just not sure if the wire can physically fit on the block.

I have more to work with now, thanks for your comments!

 

[kidharris]

12 volt DC amp carrying capacity is very tricky over distance. The voltage drop is precipitous when you're talking significant amps.

My diesel heater clocks in with about an 8 or 9 amp load initially, then once running it drops to 3 or 4 amps. Let's assume your peak load is 8 amps.

The very thinnest wire you can use is 14 AWG (believe it or not it can carry those amps, just not over any distance). You'd be hard pressed to go more than a few feet with that wire size.

For sizing based on load over distance I use this link: Advanced Wire Ampacity Calculator

My opinion based on the chart: if your heaters have similar load characteristics, for your 20 foot run you need #8 AWG both directions, but that's marginal. #6 would be better. For your 30 foot run you need a #6 no question, both ways. Anything smaller risks the wire heating up because the copper has its own resistance, which converts to heat (with a subsequent voltage drop)

Are you saying that the wire is sized for the one way distance? then used as 2 separate runs to & fro?

 

[MCI-89]

I have a related question, but I'm not sure if I should start a new post.

I'm using this fuse block, and the instructions say 12gauge wire is as low as it can accept.

Looking around, I haven't found a fuse block that can accept lower gauge wires, so I'm unsure how I'm supposed to run a 6 or 8 gauge wire to the diesel heaters.

Any ideas? It's a 100A fuse block, with slots to power multiple devices. Would it be a safety issue to use a lower gauge wire, or is it just a matter of getting it to fit well?

Thanks.

 

[kidharris]

I have a related question, but I'm not sure if I should start a new post.

I'm using this fuse block, and the instructions say 12gauge wire is as low as it can accept.

Looking around, I haven't found a fuse block that can accept lower gauge wires, so I'm unsure how I'm supposed to run a 6 or 8 gauge wire to the diesel heaters.

Any ideas? It's a 100A fuse block, with slots to power multiple devices. Would it be a safety issue to use a lower gauge wire, or is it just a matter of getting it to fit well?

Thanks.

I would just run the bigger wire to the battery and use an inline fuse. Don't know if this is "Correct" way.

 

[MCI-89]

Or is there a small and reliable 'wall wart' style AC to DC converter someone could recommend that I could use exclusively to power the diesel heater? Seems like it would be simple to just run an AC outlet next to the heater and plug a converter in there. Not sure if this is common practice or not.

 

[MCI-89]

Update: I ended up finding a fuse block that can accept 6AWG wire, Blue Sea Systems 7748 SafetyHub 150 Fuse Block.

 

[Rucker]

Are you saying that the wire is sized for the one way distance? then used as 2 separate runs to & fro?

Missed this question earlier. The recommendation for the 20 foot run is to use #8 (positive supply and ground back to panel), and for the 30 foot run use #6, a separate set of wires(one positive supply and one ground back to panel).

Make sense?

 

[Rucker]

Update: I ended up finding a fuse block that can accept 6AWG wire, Blue Sea Systems 7748 SafetyHub 150 Fuse Block.

Kind of two things here.

Note that the fuse protects the wire, so a fuse matching the wire ampacity needs to be installed very close to the source of power (as well).

You might be in a similar situation to mine with the DCDC charger--the terminals on the device are too small for the gauge of wire I need (because of the length of run). I'm using a bus bar mounted close to the device so I can connect the heavy wire to the bus bar, then run a smaller wire a foot or less to the device (which can also have a fuse matching the device's amperage).

If you have a relatively thin wire coming from the heater, connect it to a bus bar that has terminals big enough to accommodate the heavier feeder wire.

But include a fuse at the battery as mentioned above.

 

[MCI-89]

Kind of two things here.

Note that the fuse protects the wire, so a fuse matching the wire ampacity needs to be installed very close to the source of power (as well).

You might be in a similar situation to mine with the DCDC charger--the terminals on the device are too small for the gauge of wire I need (because of the length of run). I'm using a bus bar mounted close to the device so I can connect the heavy wire to the bus bar, then run a smaller wire a foot or less to the device (which can also have a fuse matching the device's amperage).

If you have a relatively thin wire coming from the heater, connect it to a bus bar that has terminals big enough to accommodate the heavier feeder wire.

But include a fuse at the battery as mentioned above.

Thanks! I'm operating exclusively on shore power, no battery. The plan is to have a utility closet with a 120 outlet, and plug in a Powermax AC to DC converter. Then I connect a fuse block to that, and run my DC from there.

One question that came up when I read your reply... I do have relatively thin wires running out from the diesel heaters and other DC appliances. Could I connect a bus bar downstream of a fuse block, is that safe?

 

[Jono14]

Wait - what are the amps?

Hi all,
A little late to the discussion but I want to go back to what the amp draw for the heater is. Rucker was estimating 8 amps peak and 3-4 amps continuous - does that sound right MCI-89?

Rucker is brilliant and I've learned a lot from him, but by my calculations and by ABYC standards, a much smaller wire gauge would be acceptable for 8 amps. They use 2 levels of acceptable voltage drop: ≤ 3% for 'critical' loads (communication, navigation) and ≤ 10% for non-critical loads (heaters, sound systems). The wire calculator in Rucker's post only allows 1-5% drop and my guess is the calculator tool that showed 4 AWG was set to 1% voltage drop which drove the calculations. BTW in this case, heating up the wire is a non-issue if under 10 amps for 14AWG wire and larger.

By my voltage drop calculations: running 8 amps over 60 feet of round trip wire on a 12 volt system, 8 AWG wire would give you 2.5% drop, but 14 AWG wire could also be used and be just under a 10% voltage drop. (pure copper, not copper coated aluminum - CCA wire).
If the round trip distance is 40 ft, a 14 AWG wire would have a 6.6% drop and 10 AWG wire would be under 3% at 2.6%.

I would guess that the heater control board would be OK with a 1.2 volt drop (~10% for 14 AWG at 60 ft) but you could check the heater manual and see what the lower voltage limits are.

So, I think you can safely use 12-14 AWG wire for both heater circuits - maybe a 15 Amp fuse for each heater would be fine - the red/black 2 conductor wire can be nice to work with (just make sure pure copper as CCA is similar to going up 2 gauges).

 

[Rucker]

Thanks! I'm operating exclusively on shore power, no battery. The plan is to have a utility closet with a 120 outlet, and plug in a Powermax AC to DC converter. Then I connect a fuse block to that, and run my DC from there.

One question that came up when I read your reply... I do have relatively thin wires running out from the diesel heaters and other DC appliances. Could I connect a bus bar downstream of a fuse block, is that safe?

Yes-that's exactly what I was trying to describe. Basically, those manufacturers use the thinnest, shortest wire they can get away with.

I have a heavy (and properly fused) supply and ground wire going from my distribution panel near my battery over to a sub-panel, and those heavy wires connect to terminal blocks. From there, I run thinner supply wire through a secondary fuse block to fuse the smaller wire runs to the various devices.

 

[Rucker]

Hi all,
A little late to the discussion but I want to go back to what the amp draw for the heater is. Rucker was estimating 8 amps peak and 3-4 amps continuous - does that sound right MCI-89?

Rucker is brilliant and I've learned a lot from him, but by my calculations and by ABYC standards, a much smaller wire gauge would be acceptable for 8 amps. They use 2 levels of acceptable voltage drop: ≤ 3% for 'critical' loads (communication, navigation) and ≤ 10% for non-critical loads (heaters, sound systems). The wire calculator in Rucker's post only allows 1-5% drop and my guess is the calculator tool that showed 4 AWG was set to 1% voltage drop which drove the calculations. BTW in this case, heating up the wire is a non-issue if under 10 amps for 14AWG wire and larger.

By my voltage drop calculations: running 8 amps over 60 feet of round trip wire on a 12 volt system, 8 AWG wire would give you 2.5% drop, but 14 AWG wire could also be used and be just under a 10% voltage drop. (pure copper, not copper coated aluminum - CCA wire).
If the round trip distance is 40 ft, a 14 AWG wire would have a 6.6% drop and 10 AWG wire would be under 3% at 2.6%.

I would guess that the heater control board would be OK with a 1.2 volt drop (~10% for 14 AWG at 60 ft) but you could check the heater manual and see what the lower voltage limits are.

So, I think you can safely use 12-14 AWG wire for both heater circuits - maybe a 15 Amp fuse for each heater would be fine - the red/black 2 conductor wire can be nice to work with (just make sure pure copper as CCA is similar to going up 2 gauges).

Thanks for the compliment! I stick to 1-2% voltage sag because a) sometimes the electronics malfunction and b) I'm not an expert so I try to avoid doing things that might cause intermittent behavior that is difficult to troubleshoot.

It's probably also worth noting that you lose functionality sooner on those longer, thinner runs if your battery and system voltage is low due to discharged battery.

 

[MCI-89]

Hi all,
A little late to the discussion but I want to go back to what the amp draw for the heater is. Rucker was estimating 8 amps peak and 3-4 amps continuous - does that sound right MCI-89?

Rucker is brilliant and I've learned a lot from him, but by my calculations and by ABYC standards, a much smaller wire gauge would be acceptable for 8 amps. They use 2 levels of acceptable voltage drop: ≤ 3% for 'critical' loads (communication, navigation) and ≤ 10% for non-critical loads (heaters, sound systems). The wire calculator in Rucker's post only allows 1-5% drop and my guess is the calculator tool that showed 4 AWG was set to 1% voltage drop which drove the calculations. BTW in this case, heating up the wire is a non-issue if under 10 amps for 14AWG wire and larger.

By my voltage drop calculations: running 8 amps over 60 feet of round trip wire on a 12 volt system, 8 AWG wire would give you 2.5% drop, but 14 AWG wire could also be used and be just under a 10% voltage drop. (pure copper, not copper coated aluminum - CCA wire).
If the round trip distance is 40 ft, a 14 AWG wire would have a 6.6% drop and 10 AWG wire would be under 3% at 2.6%.

I would guess that the heater control board would be OK with a 1.2 volt drop (~10% for 14 AWG at 60 ft) but you could check the heater manual and see what the lower voltage limits are.

So, I think you can safely use 12-14 AWG wire for both heater circuits - maybe a 15 Amp fuse for each heater would be fine - the red/black 2 conductor wire can be nice to work with (just make sure pure copper as CCA is similar to going up 2 gauges).

I'm using a Lavaner 5KW diesel heater, and here's the specs I got from them: 5KW unit: 3.33amp, 40W when running at the highest level after the very first 90 seconds; 10.83amp 130W when starting (the very first 90 seconds)

 

[Jono14]

I'm using a Lavaner 5KW diesel heater, and here's the specs I got from them: 5KW unit: 3.33amp, 40W when running at the highest level after the very first 90 seconds; 10.83amp 130W when starting (the very first 90 seconds)

Numbers are close to Rucker's estimate, and Rucker's point about lower voltage on a run-down battery is an appropriate general design principle. But because you are using a Powermax AC to DC converter that is not an issue in your case.

Your only consideration is voltage drop (not heat) during the 90 seconds of startup when pulling 10.8 Amps. If you used 12 AWG wire over 60 ft round trip your voltage drop is about 1 volt (and 14 AWG = 1.6V). I expect that the Powermax AC to DC converter will output something higher than 12 volts and may be adjustable based on model. (For example, to charge LiFePO4 batteries, they might be set to ~14.2 volts.) So you should have plenty of voltage room to lose 1 or even 1.6 volts in the wire and still provide > 12 Volts at the heater.

So the smaller (12-14 AWG) wires should work and you can just run each heater off separate fuse in your fuse block.

 

[MCI-89]

Thanks to you both for all the great information. I had already ordered some 6/2 wire, so I'm just going to use that for the heaters, but this information will help me make good choices for the remaining DC items I have to wire.