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Old 12-08-2015, 10:16   #16
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Re: Wire Ampacity - NEC or ABYC?

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Originally Posted by Dave Lochner View Post
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.
This is not correct. The standard practice in AC systems is to describe AC currents by the "root mean square" average of the waveform, which is a long way of saying that one amp of DC creates the same heat as one amp of AC, provided the length and cross section of the wire are the same.

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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.
.
I disagree. NEC ampacity tables are based on an ambient temperature (30C) that is appropriate for houses, not engine rooms. ABYC assumes that engine room temperatures are higher and there is good reason for that.
30C. In addition, the NEC ampacity tables do not include a column for insulation rated for 105C, which is common on boats.

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For DC wiring use the ABYC guidelines and marine grade wire.
I will not argue with that.
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Old 12-08-2015, 10:51   #17
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Re: Wire Ampacity - NEC or ABYC?

Attached is a page from Charlie Wing's Book on Marine Electrical Systems

The chart shows AC wire sizes to be roughly the same as the NEC standards. The over current protection should be lower than the wire's capacity in order to protect the wire.

"This is not correct. The standard practice in AC systems is to describe AC currents by the "root mean square" average of the waveform, which is a long way of saying that one amp of DC creates the same heat as one amp of AC, provided the length and cross section of the wire are the same. "

Yes, provided that the voltage is the same. 1 amp at 120 Volts has more energy than 1 amp at 12 V. You would need to have 10 amps at 12v to have the same energy as 1 amp at 120. (assuming resistive loads and not inductive loads) Thus we have very high voltage lines on the power grid.
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Old 12-08-2015, 10:59   #18
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Re: Wire Ampacity - NEC or ABYC?

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Originally Posted by Dave Lochner View Post
1 amp at 120 Volts has more energy than 1 amp at 12 V. You would need to have 10 amps at 12v to have the same energy as 1 amp at 120. (assuming resistive loads and not inductive loads) Thus we have very high voltage lines on the power grid.
All that is great stuff but has nothing do with with ampacity-based wire sizing.

If you have one wire carrying 1A of AC and another wire carrying 1A of DC in the same environment then both will have the same temperature. Therefore the ampacity-based gauge is the same.

Another story is that AC usually uses higher voltages, and higher voltages let you carry more power with the same current (say 1A). In other words, at usual AC voltages (higher than usual DC voltages) you need smaller currents for the same appliance.

Still, the formulas to determine ampacity-base gauge from current at a point in a circuit are the same for AC and DC and have nothing to do with voltage between ends of a circuit. It is all about heat generation and dissipation!
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Old 12-08-2015, 12:00   #19
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Re: Wire Ampacity - NEC or ABYC?

So is the consensus that ABYC ampacity tables are not substantially more liberal than NEC for like cable and environmental conditions? I would be surprised if ABYC allowed way higher current than NEC. Both bodies give a lot of deference to NFPA so it would not make sense for them to arrive at completely different recommendations.
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Old 12-08-2015, 13:45   #20
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Re: Wire Ampacity - NEC or ABYC?

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So is the consensus that ABYC ampacity tables are not substantially more liberal than NEC for like cable and environmental conditions? I would be surprised if ABYC allowed way higher current than NEC. Both bodies give a lot of deference to NFPA so it would not make sense for them to arrive at completely different recommendations.

I am not yet convinced of that.

First, despite efforts in the past by NFPA to form (or threaten to form) a model building code, the NEC, NFPA and I assume the ABYC, give no deference to anyone, and that includes especially when they feel someone is stepping on their turf. The battle amongst the various model building codes (ICBO, BOCA, SBC, etc.) in the past have been downright silly.

Second, and particularly for NFPA and ABYC, the fact that they seem to think their code documents need to be treated with secrecy and stealth to all but their members is asinine. This info is public and should be made so without subscription or burdensome costs. In the age of the internet this is stone age thinking. Their current business model sucks!

Third, with all the various data sources and conflicting information, posted primarily by third party commercial interests, spurious data is presented as fact, without context or clarification. At the very least we have should have a global source, readily accessible, for clear and concise factual data. Not a Wiki of data (I don't support crowd sourcing for electrical data), but a vetted, authoritative source.

Comparing NEC and ABYC is not apples to apples. THWM 90C rating cable is not the same as BC5-W2 cabling in an engine compartment. And SAE wiring, which you might encounter on propulsive machinery, is different entirely!!
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Old 12-08-2015, 14:25   #21
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Re: Wire Ampacity - NEC or ABYC?

Boat wire is very fine strand with multiple smaller wires wrapped together to form I #12 wire. House wire has 1 solid wire. Electricity travels around the outside of each strand. Thus boat wire, like welders cable has a higher amp rating than solid #12 house wire.
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Old 12-08-2015, 14:49   #22
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Re: Wire Ampacity - NEC or ABYC?

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Originally Posted by Dave Lochner View Post
Attached is a page from Charlie Wing's Book on Marine Electrical Systems

Yes, provided that the voltage is the same. 1 amp at 120 Volts has more energy than 1 amp at 12 V. You would need to have 10 amps at 12v to have the same energy as 1 amp at 120. (assuming resistive loads and not inductive loads) Thus we have very high voltage lines on the power grid.
Dave you are confused, "wattage", the sum of volts times amps on DC system,has nothing to do with wire heating, the resistance of the wire impeding the flow of current is what causes heat.

Same is true for AC but it's volts times amps times power factor. On a true resistive load power factor would be 1.
Also listen to svlamorocha, he has it right. In AC it's the RMS current, the average current draw across the complete cycle of the sine wave. Peak to peak tends to be about 2.8 times the average or RMS.
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Old 12-08-2015, 15:05   #23
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Re: Wire Ampacity - NEC or ABYC?

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Originally Posted by Monepit View Post
Boat wire is very fine strand with multiple smaller wires wrapped together to form I #12 wire. House wire has 1 solid wire. Electricity travels around the outside of each strand. Thus boat wire, like welders cable has a higher amp rating than solid #12 house wire.
Not the case for DC. You are describing "skin effect", and this only applies for alternating current. It is actually quite minor at 50-60 Hz (8.5 mm at 60Hz, so for the sizes we are talking about effectively the entire wire is being used.) For DC current, skin effect is non-existent.

Also, to get around skin effect by using stranded wire, each individual strand must be insulated. This type of wire is called "Litz wire" and is sometimes done for wire used in high-frequency transformers and inductors.
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Old 13-08-2015, 04:53   #24
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Re: Wire Ampacity - NEC or ABYC?

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... the NEC, NFPA and I assume the ABYC, give no deference to anyone ...
FWIW: The National Electrical Code (NEC) IS NFPA 70
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Old 13-08-2015, 05:43   #25
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Re: Wire Ampacity - NEC or ABYC?

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Originally Posted by Dave Lochner View Post
Attached is a page from Charlie Wing's Book on Marine Electrical Systems

The chart shows AC wire sizes to be roughly the same as the NEC standards. The over current protection should be lower than the wire's capacity in order to protect the wire.

"This is not correct. The standard practice in AC systems is to describe AC currents by the "root mean square" average of the waveform, which is a long way of saying that one amp of DC creates the same heat as one amp of AC, provided the length and cross section of the wire are the same. "

Yes, provided that the voltage is the same. 1 amp at 120 Volts has more energy than 1 amp at 12 V. You would need to have 10 amps at 12v to have the same energy as 1 amp at 120. (assuming resistive loads and not inductive loads) Thus we have very high voltage lines on the power grid.
This is wrong. For power distribution applications, the required size of the wire is determined only by the amps to be carried by the wire. Voltage carrying capacity of the wire is determined by the insulation between it's source and return path. Hence, a 12 awg wire carrying 20 amps @ 1 volt delivers 20 watts of power. The same 12 awg wire carrying the same 20 amps @ 600 volts will deliver 12,000 watts. Both are acceptable for 12 awg (given the insulation material is rated @ 600 volts, typical in household romex type wire).
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Old 13-08-2015, 06:27   #26
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Re: Wire Ampacity - NEC or ABYC?

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Second, and particularly for NFPA and ABYC, the fact that they seem to think their code documents need to be treated with secrecy and stealth to all but their members is asinine. This info is public and should be made so without subscription or burdensome costs. In the age of the internet this is stone age thinking. Their current business model sucks!

Third, with all the various data sources and conflicting information, posted primarily by third party commercial interests, spurious data is presented as fact, without context or clarification. At the very least we have should have a global source, readily accessible, for clear and concise factual data. Not a Wiki of data (I don't support crowd sourcing for electrical data), but a vetted, authoritative source.

I agree. Great post!
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Old 13-08-2015, 07:10   #27
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Re: Wire Ampacity - NEC or ABYC?

Tesla vs Edison. Tesla's idea eventually won out mostly because for the same gauge wire and the same current, AC can transfer power a greater distance.

https://en.wikipedia.org/wiki/War_of_Currents
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Old 13-08-2015, 07:23   #28
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Re: Wire Ampacity - NEC or ABYC?

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Dave you are confused, "wattage", the sum of volts times amps on DC system,has nothing to do with wire heating, the resistance of the wire impeding the flow of current is what causes heat.
Exactly

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.
Also listen to svlamorocha, he has it right. In AC it's the RMS current, the average current draw across the complete cycle of the sine wave. Peak to peak tends to be about 2.8 times the average or RMS.
Not quite but close
The relationship of AC currents in pure resistive loads are
RMS current = 0.707 times Peak current
Average current = 0.5 times Peak current

The RMS current is for all intends and purposes the "real" current as far as power and heating aspects are concerned.
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Old 13-08-2015, 13:45   #29
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Re: Wire Ampacity - NEC or ABYC?

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Exactly


Not quite but close
The relationship of AC currents in pure resistive loads are
RMS current = 0.707 times Peak current
Average current = 0.5 times Peak current

The RMS current is for all intends and purposes the "real" current as far as power and heating aspects are concerned.
You may be right, I'm not sure but I think we are on the same page but thinking of different values. Your right about peak back to zero, a quarter of a sine wave. I was referencing a full cycle, zero to peak back to zero then zero to peak back to zero in the opposite direction. 2.8 peak to peak.
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Old 13-08-2015, 17:30   #30
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Re: Wire Ampacity - NEC or ABYC?

Dave,

You are right, the peak to peak voltage is 2.8 times the RMS voltage for a sinusoid.

It is important to consider the voltage drop because for 12V DC the percentage drop is much higher than for AC circuits for the same current and wire size. The current that causes a 1% voltage drop for 120V AC will cause a 10% drop on a 12V DC circuit even though the current and wire size are the same. The allowed ampacity is only one part of the system design. Voltage drop also has to factor into the decision about wire size. Similarly the size of the wire creates an upper limit on circuit breaker size but there is nothing wrong with using a smaller breaker/fuse.
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