Wire Ampacity - NEC or ABYC?

[redsky49]

Explain this one to me please.

Per widely published data, ABYC states that 12/3 wire, for example, has an ampacity of 45 amps outside of an engine room, 38.3 amp inside an engine room.

NEC however, to the best of my knowledge, allows a maximum of 20 amp breaker for this size wire.

Am I to believe that boats are less demanding than commercial and residential construction? What gives?

Personally, I follow the NEC for all AC circuits on board, but use the 3% tables for all 12V wiring.

I find this perplexing and potentially dangerous. For our foreign friends, the NEC is the National Electric Code and is the mandatory governing standard for all US electrical construction (other than some military and other applications). The ABYC is the American Boat and Yacht Council and is a voluntary non-governmental organization for boat builders.

Any answers?

[smac999]

are you comparing DC to AC values? the DC vaule is 45/38 single conductor, no bundle. which never happens. a 12/3 carrying DC drops to 32/27 vs single open strand. but I'm pretty sure AC has it's own table.

I don't think I've looked at an ABYC AC table for years. I just use 15/20/30 for 14awg / 12awg / 10awg.


also most house stuff is 75degree rated and boat wire is 105 which adds a lot to the tables.



in DC once you start using the voltage drop tables the max current one becomes pointless anyways.

[transmitterdan]

The ABYC ratings depend on the temperature rating of the insulation, number of conductors and ambient temperature. In the case you mention the current ampacity is based on 105C insulation rated wire for a single conductor in free air of not more than 30C ambient temperature (table VI-A). Therefore it is definitely not the correct table for 12-3 wire.

You have to be sure each installation meets the specified conditions. For higher ambient you have to derate and for multiple wires in a bundle further derating is required. So for your example of 12-3 the ABYC table VI-B specifies allowed ampacity for 105C insulation wire in 30C maximum ambient would be 32A outside the engine room and 27A inside. If you have more than one such group of wires there is further derating.

Using the correct table and following all the derating factors for a specific situation I think you will find the ABYC ratings are fairly similar or even more conservative than NEC.

[redsky49]

Taken directly from one major US source of marine wire:

"12/3 AWG marine grade wire is a flat triplex 3 conductor wire features flexible, tinned copper stranding which is ideal for your boat wiring needs. Our boat wire is made in the USA and meets ABYC and US Coast Guard specifications while UL listed as UL1426 Boat Cable. The maximum allowable amperage of 12/3 AWG marine grade primary wire is 45 amps outside of engine spaces and 38.3 amps inside the engine space."

How would you interpret this?

[transmitterdan]

Originally Posted by redsky49

Taken directly from one major US source of marine wire:

"12/3 AWG marine grade wire is a flat triplex 3 conductor wire features flexible, tinned copper stranding which is ideal for your boat wiring needs. Our boat wire is made in the USA and meets ABYC and US Coast Guard specifications while UL listed as UL1426 Boat Cable. The maximum allowable amperage of 12/3 AWG marine grade primary wire is 45 amps outside of engine spaces and 38.3 amps inside the engine space."

How would you interpret this?

Based on my reading of the ABYC tables they are wrong. The tables specifically says those ampacities are for a single conductor in free air. And last time I checked 12-3 was not a single conductor cable.

[smac999]

http://assets.bluesea.com/files/reso...ence/21731.pdf


2nd table
"Up to three conductors in a sheath, conduit or bundle)"


12/3 would be sheathed cable. I would only count it as 2 conducter in AC but same table as 3 anyways


but I still think there is an AC table. but I don't have my E11 handy

[redsky49]

Per the 2007 draft requirements from ABYC committee, under alternating current:

Sec. E-11, Table VII A, For No More Than Two Circuits Bundled:

Chart shows again 45/38A for 12/3 105C cabling.

I have a secured document and haven't found a way yet to copy and post.

It's on page 84 of my copy.

[transmitterdan]

Originally Posted by redsky49

Per the 2007 draft requirements from ABYC committee, under alternating current:

Sec. E-11, Table VII A, For No More Than Two Circuits Bundled:

Chart shows again 45/38A for 12/3 105C cabling.

I have a secured document and haven't found a way yet to copy and post.

It's on page 84 of my copy.

Does your copy differ in any significant way to this?

http://www.paneltronics.com/atimo_s/...11Excerpts.pdf

[redsky49]

I have found a later edition of the standard here:

http://www.paneltronics.com/atimo_s/...11Excerpts.pdf

In this version, conductors are grouped together DC and AC. It shows 12/3 105C being good for 31.5A outside engine room, 26.8A inside. This is Table VI-B.

?????

This later version is different than the draft form I have.

[svlamorocha]

Originally Posted by redsky49

I have found a later edition of the standard here:

http://www.paneltronics.com/atimo_s/...11Excerpts.pdf

In this version, conductors are grouped together DC and AC. It shows 12/3 105C being good for 31.5A outside engine room, 26.8A inside. This is Table VI-B.

?????

This later version is different than the draft form I have.

The data posted by redsky49 (31.5 and 26.8A for 12/3 with 105°C insulation matches exactly my copy of the current ABYC E-11 standard (table VI-B).

[Dave Lochner]

Originally Posted by redsky49

Explain this one to me please.

Per widely published data, ABYC states that 12/3 wire, for example, has an ampacity of 45 amps outside of an engine room, 38.3 amp inside an engine room.

NEC however, to the best of my knowledge, allows a maximum of 20 amp breaker for this size wire.

Am I to believe that boats are less demanding than commercial and residential construction? What gives?

Personally, I follow the NEC for all AC circuits on board, but use the 3% tables for all 12V wiring.

I find this perplexing and potentially dangerous. For our foreign friends, the NEC is the National Electric Code and is the mandatory governing standard for all US electrical construction (other than some military and other applications). The ABYC is the American Boat and Yacht Council and is a voluntary non-governmental organization for boat builders.

Any answers?

All amps are not the same. 40 amps at 12 volts is equal to about 4 amps at 120 volts. (see Volts/Amps/Watts Converter)

Voltage and amperage are inversely related. For the same amount of electrical power the higher the voltage the lower the amperage. Also, AC and DC are not entirely equivalent in these calculations. AC travels through wires much more efficiently than DC which tends to heat up wires more quickly than AC causing more resistance in the wire and subsequently more heat and power loss.

Without getting into all the nitty gritty, this is why larger boats are moving towards 24 volt DC systems (fewer amps and less current loss) and Power grids use extremely high voltage (Hundreds of Kilovolts) to deliver AC to your home.

Since shore power is essentially an extension of the power grid to your boat, using NEC sizes for the AC system is OK. But, do not use household wire or automotive wire on the boat. Stick with marine grade wire.

For DC wiring use the ABYC guidelines and marine grade wire.

[Colin A]

I believe they did change the tables sometime in the last 10 years. As noted the E11 I have seems to be the same as the last post from redsky. The insulation is important the 105c rated does change things quite a bit.

[redsky49]

Dave Lochner - I don't dispute this. My concern is for alternating current and what maximum ampacity is allowed under ABYC versus NEC.

From all I can see, ABYC permits much higher current than NEC. This applies to all wire gauges and insulation types. 31.5A per ABYC is way higher than 20A per NEC, for 12/3 for example.

[Colin A]

On the ABYC table 60C wire bundled up to 3 conductors is rated at 17.5 amps. I believe NEC ratings are for 60c or 75c wire insulation. This would put them inline with NEC 20 amp rating (actually a bit more conservative).

[Colin A]

I found a link to an NEC table here which seems to put the ABYC as more conservative.
Houston Wire & Cable Company - NEC Table 310.16