Wire Ampacity - NEC or ABYC?

[dave948]

We definitely got side tracked from the original question but I think some of the wrong information posted needed to be addressed.

Myself I would wire for ABYC on the DC side and NEC on the AC side. The NEC tables are very conservative and will build in a nice safety factor.

A few other points, over current protection and conductor ampacity are two different things. Size the wire to the load and size the over current to protect the complete circuit. Also the 105*C wire is great but a well designed system factors in weak links. You will be hard pressed to find devices and terminations rated at 105*C so this will affect your overall ability to load that circuit to the 105 table.

[F51]

Originally Posted by redsky49

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!

Yes, you would think that documents that purport to enhance safety of life and property would be freely available to all. If the small independent marine electrical technician is heavily burdened by having to purchase these price-inflated codes, won't he be tempted to skimp on something else? Thus safety is not enhanced, it is degraded.

[um saudade]

The NEC will make more sense if you consider the higher current and ambient temp wires (105 C for example) are primarily for derating purposes.

The temperature rating of the lowest rated piece of hardware in the circuit determines the maximum ampacity of the circuit. For example: A breaker rated at 60C would be the weak link in a circuit and at that point the circuit must not carry more than the rated ampacity of 60C wire, no matter what the ampacity rating of the wire.

You still derate starting with the higher ampacity of the wire. So let’s say the NEC ampacity for 60C #12 is 20 amps and you use a 60C breaker but higher ampacity wire. You deduct the ambient temp derating, number of conductors in the conduit derating and so on until all the derating is done. If the ampacity after derating is over 20 amps you can use #12, protected by the 20 amp breaker(60C). If the ampacity after derating is under 20 amps you have to use #10 for the run to be correct.

This allows engineers and electricians to use smaller wire than the code would allow if all wire was derated from the lower temperature ampacities. It’s all about being able to take heat without degradation to the insulation on the wire or the devices to which the wire is terminated.

[Wotname]

Originally Posted by dave948

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.

We are most certainly on the same page and my post wasn't intended to be critical of you. Rather it was a small correction of fact.

Some readers don't care to know the fine detail and that is OK
Some readers already know the fine detail and would happily pass over small inaccuracies, also quite OK

However some readers use these threads to further develope their limited knowledge of the subject matter and will (IMO) benefit from getting the fine detail correct.

So if one measures the peak to peak values over a full cycle of either current or voltage, one can determine the average and the RMS values by using the following.
RMS = P to P times 0.35035
Average = P to P times 0.25.

Conventional AC measurements are RMS values as this gives us a better idea of the amount of work or heat that any particular AC cicuit will deliver and is thus similar to the same DC value.

Thus the power avaiable in say 100 volts and 10 amps of DC is the same as 100 volts (RMS) and 10 amps (RMS) of AC.

Note, all this only applies to sine wave AC and not other waveforms.

Again, not directed to Dave, he already knows this stuff.

[transmitterdan]

Technically the average value of a sinusoidal wave is identically zero. If it were not so then transformers would not work.

Best to talk only about peak, peak to peak and RMS which are all linearly related for a sinusoid. Don't worry about the average voltage as it is not useful for any computations.

Major thread drift ahead:

The concept of average vs. RMS trips up even good engineers. A DC system where the current varies over time has an RMS value that is not the same as the average or DC current. I have seen several engineers not realize this especially when selecting cables for inverters. The RMS current can be higher than the average DC current by a significant amount.

[svlamorocha]

Originally Posted by dave948

Myself I would wire for ABYC on the DC side and NEC on the AC side. The NEC tables are very conservative and will build in a nice safety factor.

You are confusing a safety factor with difference in assumptions.

It is unsafe to wire a a boat´s AC system following NEC. NEC does not take in to account many factors that apply to boats, such as them being on water, being subject to vibration, etc. People get killed as a result of those differences. I bet that there is a good reason for every difference between ABYC E-11´s part on AC and and NEC, at least I have found it every time I have looked into one of those differences.

C

[dave948]

Originally Posted by svlamorocha

You are confusing a safety factor with difference in assumptions.

It is unsafe to wire a a boat´s AC system following NEC. NEC does not take in to account many factors that apply to boats, such as them being on water, being subject to vibration, etc. People get killed as a result of those differences. I bet that there is a good reason for every difference between ABYC E-11´s part on AC and and NEC, at least I have found it every time I have looked into one of those differences.

C

I never meant it to read that I would use the NEC to wire a boat. I simply meant I would use the table 310.15(B) for ampacity of A/C conductors.

[Saltyhog]

Originally Posted by transmitterdan

Technically the average value of a sinusoidal wave is identically zero. If it were not so then transformers would not work.

I was going to reply to say the same thing, but then I decided to happily pass over this inaccuracy.

[Wotname]

Gentlemen, I stand corrected. I have had to resort to opening my dust covered reference books.
The avearage voltage or current in AC (sinewave) circuit is given by:
V(average) = V(peak) times 2 divided by 3.1417
Thus is .637 of peak voltage rather than .5 of peak voltage. My only defense is too many years of good living

EDIT: and here is an on-line reference for those who can't see my dust covered reference books http://www.electronics-tutorials.ws/...e-voltage.html

Originally Posted by transmitterdan

Technically the average value of a sinusoidal wave is identically zero. If it were not so then transformers would not work.

Best to talk only about peak, peak to peak and RMS which are all linearly related for a sinusoid. Don't worry about the average voltage as it is not useful for any computations.

Major thread drift ahead:

The concept of average vs. RMS trips up even good engineers. A DC system where the current varies over time has an RMS value that is not the same as the average or DC current. I have seen several engineers not realize this especially when selecting cables for inverters. The RMS current can be higher than the average DC current by a significant amount.

Originally Posted by Saltyhog

I was going to reply to say the same thing, but then I decided to happily pass over this inaccuracy.

[jwcolby54]

Originally Posted by Dave Lochner

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.

I don't think that is true. AC is used because you can run it through a winding, generate a changing magnetic field, and cut that magnetic field with another winding and get a different voltage out. I.e. a transformer.

DC is not changing rhythmically and thus no ability to use a transformer.

The "heating" of wires is about one thing and ONLY one thing, the resistance of the wire, and the current through the wire.

Power is I^2 X R. Thus for any given resistance R, as you up the current, the power consumed IN THE WIRE goes up. That power consumption in the wire is HEAT.

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)

This is true (kinda), doubling the voltage (for a given power consumption) halves the current I which radically affects heat generated in the wiring.

The current isn't "lost" (we know where it went) it is just converted into heat.