Wiring size charts, applicable to AC and DC?

[Dzl]

Dzl: thanks for the very reasoned response. Its interesting that the West Marine Table you reference pointedly states that its for 12 V systems only. That leads one to think that the same table for 24 V or 48 V systems would be considerably different. So again, voltage matters?

Yeah, I was confused/bothered by this as well. One possible implication is that voltage matters.

How often is 24V-48V DC used in a marine setup? IDK. They may say 12V because their source has not tested it in a 24V-48V system.
Just a SWAG on my part.

Pretty often if we are talking about 24v, I believe we can largely thank the marine industry for the abundance of components available at that voltage compared with 48v.

The actual ABYC source material that West Marine is basing their chart on has tables for 12 and 24 (not 48). I was planning to put together a post on it, but that might take me a while so for now I'll link to the PDF and point to the relevant page numbers (note this is the 2008 edition, 2018 edition may be different but it looked like it would cost me $195 to find out).

Ampacity table for DC and AC circuits (single wire/not bundled) - Pg 60 of PDF
3% Voltage drop tables for 12v and 24v - Pg 67 of PDF
Relevant explanation of wire sizing - Pg 28-30 of PDF

Spoiler: Here is the section relevant to DC conductors from pg 28-29

11.14.2.5 Conductors used for panelboard or switchboard main feeders shall have ampacities as determined in E-11.8.1.1. Conductors used for branch circuits or in electrical systems that do not use a panelboard or switchboard shall have their ampacities determined by their loads (See 2TABLE II).

11.14.2.6 Voltage Drop - Conductors used for panelboard or switchboard main feeders, bilge blowers, electronic equipment, navigation lights, and other circuits where voltage drop must be kept to a minimum, shall be sized for a voltage drop not to exceed three percent. Conductors used for lighting, other than navigation lights, and other circuits where voltage drop is not critical, shall be sized for a voltage drop not to exceed 10 percent.

11.14.2.7 Conductor Size - To determine conductor size and insulation temperature rating, use the ampacity as specified in E-11.14.2.5 in conjunction with TABLE VI. Then use TABLE IX or TABLE X to check the conductor size for compliance with the maximum allowable voltage drop specified in E-11.14.2.6. In the event of a conflict between the ampacity table and the voltage drop tables, the larger conductor size shall be used.

11.14.2.7.1 To use TABLE IX and TABLE X , measure the length of the conductor from the positive power source connection to the electrical device and back to the negative power source connection. Use the conductor length, the system voltage, and the ampacity as specified in E-11.14.2.5, in conjunction with the appropriate volt drop table, i.e., three percent or 10 percent – TABLE IX or TABLE X, to determine conductor size

NOTES:
1. The power source connection may be the battery, or a panelboard or switchboard, if used.
2. If the ampacity as specified in E- 11.14.2.5 exceeds the ampacities in TABLE IX and TABLE X, the conductor size necessary to keep voltage drop below the maximum permitted level may be calculated by means of the following formula:
CM = ( K x I x L ) / E

Where:
CM = Circular mil area of conductor.
K = 10.75 (constant representing the resistivity of copper)
I = Load current in amperes
L = Length of conductor from the positive power source connection to the electrical device and back to the negative power source connection, measured in feet.
E = Maximum allowable voltage drop at load in volts (e.g., for a three percent voltage drop at nominal 12V, E= 0.03 x 12 = 0.36; for a 10 percent voltage drop at nominal 12V, E = 1.2). 3.

UseTABLE XI or TABLE XIIto convert circular mils (cm) to conductor gauge. If the cm area falls between two gauge sizes, the larger conductor shall be used

Spoiler: Here is the (sparse) section relevant to AC conductors from Pg 29-30

11.14.3.1 Conductors shall have a minimum rating of 600 volts.
11.14.3.2 Flexible cords shall have a minimum rating of 300 volts.
11.14.3.3 The temperature rating of conductors and flexible cords shall be at least 140F (60C) dry. NOTE: Conductor rating temperatures refer to the insulation maximum operating temperature of the conductors.
11.14.3.4 All conductors and flexible cords shall meet the flame retardant and moisture resistant requirements of UL 83, Thermoplastic-Insulated Wires and Cables.
11.14.3.5 All conductors and flexible cords shall meet the requirements of the applicable standards of Underwriters Laboratories Inc.
11.14.3.6 Conductors and flexible cords shall be stranded copper according to TABLE XI.
NOTE: Some currently available wire types that meet all of the above requirements are listed in Table VIII.
11.14.3.7 Conductors and flexible cords shall be of size according to TABLE VI and TABLE XII.
14.3.7.1 Where single conductors or multi-conductor cables are bundled for a distance greater than 24 inches (610 mm), the allowable ampacity of each conductor shall be reduced as shown in TABLE VI and TABLE XII .
NOTE: When determining the allowable amperage of bundled conductors using TABLE VI and TABLE XIII, the AC grounding conductor and a neutral conductor that carries only the unbalanced current from other conductors are not considered to be current carrying conductors.
11.14.3.7.2 The AC grounding conductor shall be permitted to be one size smaller than the current carrying conductors on circuits rated greater than 30 amperes.

Spoiler: Ampacity Table for single conductors (single wire, not bundled)

Spoiler: 3% Voltage Drop Table (12v and 24v)

 

[Sgt Raven]

Yeah, I was confused/bothered by this as well. One possible implication is that voltage matters.



Pretty often if we are talking about 24v, I believe we can largely thank the marine industry for the abundance of components available at that voltage compared with 48v.

The actual ABYC source material that West Marine is basing their chart on has tables for 12 and 24 (not 48). I was planning to put together a post on it, but that might take me a while so for now I'll link to the PDF and point to the relevant page numbers (note this is the 2008 edition, 2018 edition may be different but it looked like it would cost me $195 to find out).

Ampacity table for DC and AC circuits (single wire/not bundled) - Pg 60 of PDF
3% Voltage drop tables for 12v and 24v - Pg 67 of PDF
Relevant explanation of wire sizing - Pg 28-30 of PDF

Come to think about it. Lots of US Military is 24V too. Specially USN vehicles and stuff.
Some of this stuff I haven't thought about for years and years. Reading threads here is bringing back those long buried memories.
I just remembered having 4 6V batteries in series on a Army 2.5 ton truck. Using some clamps to grab 12V off of them for a 12V radio.
I haven't thought about that in 40+ years.

I have seen big rig trucks, that used 24V for their starter but 12V for the rest of their systems.
They had two sets of 6V batteries in a 2S2P configuration, with a relay that put them together for 24V to the starter.

 

[Dzl]

Come to think about it. Lots of US Military is 24V too. Specially USN vehicles and stuff.
Some of this stuff I haven't thought about for years and years. Reading threads here is bringing back those long buried memories.

I have seen big rig trucks, that used 24V for their starter but 12V for the rest of their systems.
They had two sets of 6V batteries in a 2S2P configuration, with a relay that put them together for 24V to the starter.

Yeah, Boats, big rigs, military vehicles, and some buses are what comes to midn when I think 24v. Somebody on here, I cant remember who, is refurbing/building out a cool military light transport 4x4 (LMTV) that is native 24v I believe.

Also, check my last post again, I edited it to add the actual ampacity and voltage drop tables from the ABYC code.

 

[Sgt Raven]

Yeah, Boats, big rigs, military vehicles, and some buses are what comes to midn when I think 24v. Somebody on here, I cant remember who, is refurbing/building out a cool military light transport 4x4 (LMTV) that is native 24v I believe.

Also, check my last post again, I edited it to add the actual ampacity and voltage drop tables from the ABYC code.

Yeah, I edited my post too. Since they tend to use 6V-12V batteries in series to get their 24V. You can grab 12V off of them for a radio or such.
I've done that to run a CB radio in a Army truck driving down freeways in the US.
You guys are getting brain cells working, that I haven't used in years.

 

[upnorthandpersonal]

Its interesting that the West Marine Table you reference pointedly states that its for 12 V systems only. That leads one to think that the same table for 24 V or 48 V systems would be considerably different. So again, voltage matters?

No, power matters. If your device draws 100A at 12V or 100A at 24V that is a difference of 1.2kW, so to make a comparison you'd have to use your 12V@100A and 24V@50A. That's why that table is for 12V only - it's because they assume a current draw on the load. With a higher voltage, you can use less current to send the same amount of power but your wire can be thinner.

 

[offgriddle]

I think I get what you are saying in theory (though I would still like you to define how you are using the term power).

But am I correct in understanding that 120V nominal AC and 120V nominal DC will be comparable (that AC in practice has the RMS calculation baked into it)?

If I switch a 100w light on and off over the course of an hour, it'll use some amount less than 100wh over the course of an hour. But a 100w light bulb on either an AC or DC circuit would have a power draw of 100W and consume 100Wh per hour, correct?

DC voltage will pass more electrons in a given time period than an equivalent peak to peak AC voltage.

 

[upnorthandpersonal]

DC voltage will pass more electrons in a given time period than an equivalent peak to peak AC voltage.

There is no equivalent between a DC voltage and a peak-to-peak AC voltage. DC can only be compared with the RMS of the AC voltage. Anything else is apples and oranges.

 

[Dzl]

Its interesting that the West Marine Table you reference pointedly states that its for 12 V systems only. That leads one to think that the same table for 24 V or 48 V systems would be considerably different. So again, voltage matters?

No, power matters. If your device draws 100A at 12V or 100A at 24V that is a difference of 1.2kW, so to make a comparison you'd have to use your 12V@100A and 24V@50A. That's why that table is for 12V only - it's because they assume a current draw on the load. With a higher voltage, you can use less current to send the same amount of power but your wire can be thinner.

My evolving understanding is that voltage and/or power, are not relevant when it comes to calculating voltage drop.
Edit: My evolving understanding isn't clear enough to state, here are the formulas I've found:

The aforementioned West Marine article gives the formula for calculating voltage drop:

Voltage Drop = Current x Length x Ohms per foot

And the ABYC code gives the formula for calculating wire size for a given maximum voltage drop:

CM = (K x I x L) / E

CM = 'circular mil' (area of conductor)
K  = 10.75 (fixed resistivity of copper)
I  = Current/amperage
L  = Round trip length
E  = Target Voltage Drop % x Voltage

 

[upnorthandpersonal]

@Dzl exactly correct. They only made that table '12V specific' because they state a power (current) draw by the load (at said 12 V). The formula is correct.

 

[upnorthandpersonal]

pass more electrons

Also, nitpick, electrons aren't moving much at all, about 1mm/second (drift velocity). In AC they just oscillate in place even. Charge however is moved.

 

[offgriddle]

Also, nitpick, electrons aren't moving much at all, about 1mm/second (drift velocity). In AC they just oscillate in place even. Charge however is moved.

Current = exchange of a free valence electron from the valence shell as they move from atom to atom.

Anyhow, DC voltage will pass more electrons in a given time period than the same peak to peak AC voltage.

Increasing an AC peak voltage in a calculation above a DC peak voltage untill it passes more free valence electrons in a given time period than the DC does is avacadoes for pomegranates.

 

[upnorthandpersonal]

Agree to disagree?

 

[offgriddle]

Agree to disagree?

Of course, otherwise how would we ever get through life! These discussions are important by the way, thank you for sharing your information.

 

[BiduleOhm]

Anyhow, DC voltage will pass more electrons in a given time period than the same peak to peak AC voltage.

That's not because you repeat it 10 times and write it in bold that it makes it true.

But I'll stop trying here, no point in wasting each other's time.

 

[Supervstech]

That's not because you repeat it 10 times and write it in bold that it makes it true.

But I'll stop trying here, no point in wasting each other's time.

Well... he is correct... it’s pointless... but he is right.

 

[offgriddle]

That's not because you repeat it 10 times and write it in bold that it makes it true.

But I'll stop trying here, no point in wasting each other's time.

It's okay, I had the same trouble about a month ago trying to convince a mob of angry, slide rule weilding, chest beating technoweenies that two 115 vac signals must be in the correct phase relationship with one another in order to create the maximum, peak to peak, 230 VAC output.
In the phase case just like this one, every possible argument against this was launched but I just kept returning to the simple statement, "it's all about that phase, that phase that phase ...".
I also learn from all of you and I am not angry with nor do I dislike anybody and I kinda dream of us all meeting somewhere sometime, (in the presence of Will and/or a neutral third party as referee to moderate techno-spats), at something like an exposition that features solar power and off-grid/sustainable living, affordable housing, boondocking, prepping yada.
Regards, OG

 

[BiduleOhm]

I also learn from all of you and I am not angry with nor do I dislike anybody and I kinda dream of us all meeting somewhere sometime

Yep, no worries, I'm not angry too

 

[upnorthandpersonal]

This is such a friendly forum. So un-internet like

 

[ghostwriter66]

 

[upnorthandpersonal]

But but... we haven't gotten to the math yet! There is at least a couple of differential equations that will explain everything and I think we should discuss these in-depth