Buy power electronics? Watch out for them wires... (1 Viewer)

[ntsqd]

I'm just a Mech playing at being a Elect. but if you're using NEC charts for vehicular low voltage DC you're being led astray. Those charts are intended for AC and generally a given size wire can carry more AC amps than DC amps because unlike DC the AC watts are never fixed and rarely at max. The wires get some chance to cool during the low voltage portion of the AC cycle when little to no current is flowing. Can have 10 Brazillion amps capacity, but if there's no voltage then the amps don't flow. There is a de-rating done in some (all?) of the NEC charts considering that most AC is in some sort of conduit that will inhibit wire cooling. Which is why the NEC specifies only so many of each wire size in a given size conduit. The whole thing is designed to make it so that the electrician in the field doesn't have to do math and can still assemble a safe electrical system that is Code Compliant regardless of the length of the circuit. That was how the whole NEC thing was explained to me ~20 years ago by a CA Licensed Electrician anyway. Memory has likely warped some of that.

I use the Anco Marine chart and formula because those are made for vehicular low voltage DC ("Low Voltage" being under 50 VDC), specifically for boats since that is Ancor's target market. Resources - https://ancorproducts.com/en/Resources I tend to only use the 3% Voltage Drop chart as a fair amount of what I'm working on is either voltage sensitive or performs better with higher voltage.

 

[e9999]

I'm just a Mech playing at being a Elect. but if you're using NEC charts for vehicular low voltage DC you're being led astray. Those charts are intended for AC and generally a given size wire can carry more AC amps than DC amps because unlike DC the AC watts are never fixed and rarely at max. The wires get some chance to cool during the low voltage portion of the AC cycle when little to no current is flowing. Can have 10 Brazillion amps capacity, but if there's no voltage then the amps don't flow. There is a de-rating done in some (all?) of the NEC charts considering that most AC is in some sort of conduit that will inhibit wire cooling. Which is why the NEC specifies only so many of each wire size in a given size conduit. The whole thing is designed to make it so that the electrician in the field doesn't have to do math and can still assemble a safe electrical system that is Code Compliant regardless of the length of the circuit. That was how the whole NEC thing was explained to me ~20 years ago by a CA Licensed Electrician anyway. Memory has likely warped some of that.

I use the Anco Marine chart and formula because those are made for vehicular low voltage DC ("Low Voltage" being under 50 VDC), specifically for boats since that is Ancor's target market. Resources - https://ancorproducts.com/en/Resources I tend to only use the 3% Voltage Drop chart as a fair amount of what I'm working on is either voltage sensitive or performs better with higher voltage.

^ I have not looked at exactly what assumptions are built in the NEC code as far as AC vs DC, but I still have to disagree respectfully with some of the above. For one thing, I can't imagine that in practice the cooling during low instantaneous AC voltage bit is significant given the thermal inertia of the wire as compared to the frequency, and the RMS value definition of the amps and voltage already averages for the AC fluctuations anyway. Yes, there has to be derating for various ambient temperatures, more wires in the cable etc. The NEC provides correction factors for that. In fact, that is also the case for some of the tables I saw in your Ancor catalog where I saw that they use different values of ampacity for the engine compartment and outside, for different numbers of conductors etc. Looking at the values given for the DC (I assume) ampacity in your reference, it seems to me that the numbers are in fact similar to those in the NEC ampacity tables I usually look at, with very similar or identical derating factors (where do you think they may have gotten those?), which would seem to be contradicting your argument that AC amps somehow are better cooled. Some of the first glance difference where the DC ampacity seem higher (not lower) may be because some of the wires from Ancor are rated at 105C which is a bit higher than the 90C in the NEC ampacity chart, and because Ancor starts at 2 conductors, not 3 like the NEC (IIANM). Note that you can only use those Ancor tables for those specific wires then, they don't seem to differentiate based on wire construction. If you are using these Ancor tables for other wires, that may not be correct then. Further, I don't believe that there is anything somehow physically so different about below 50V vs a bit above 50V . And in the big picture, I have to assume that the numbers provided by a standards regulatory organization at the National level with hundreds of millions of people potentially affected are probably sound (and likely more so than those from a private company with a much more limited audience). And I would not be surprised at all if Ancor had simply taken standards numbers (NEC or some boat manufacturer association maybe) and tweaked them a bit for their specific uses, I kinda doubt they have done much in-house lab research. But anyway, overall, I would say that the numbers are very similar in any case. I am happy to be corrected if I'm wrong on that.

But all of that is moot. If I'm going to err, I'll try to do that on the conservative side when it comes to this sort of things. I'd rather not start a fire in my house, let alone in my boat if I'm on the middle of the ocean (wish I could do that), so if I see 2 charts with different values, I'll just use the lower ampacity values for critical tasks. Yes, that may be a bit overkilling it, but that's only a small bit of money for bigger wires in the grand scheme of things and I'd rather sleep soundly at night (especially if I'm sleeping right above the batteries wiring...). But to each their own.

 

[ntsqd]

Consider just how much of any given time period the wire spends at, let's say, within 10% of max AC voltage. The remaining 80% of the time is at a lower voltage. Are we to understand that the current in this portion of time is still whatever it is at max voltage? Like I said, I'm a Mech, not E., but it doesn't seem logical that you would get the same current flow at a lower potential. Isn't that what "RMS" is all about? Maybe RMS brings an AC measurement closer to a DC measurement?
About the cooling comment, that was the Licensed Electrician's statement, not mine. Yeah, his schooling doesn't make him an EE, but it does cover all of this in a practical manner. I can see where R might fall because the wire cools, resulting in a higher I than at Vmax and it all becoming mutually offsetting, but I don't have the education or the testing experience to say just how offsetting this might be.

As to 50VDC and lower being "Low Voltage", that is the line of definition that I've encountered in multiple places and from multiple people who work with it daily. It's where the label fits, not anything technically significant. In AC generation stations "Low Voltage" is 120VAC or less. Working around anything higher requires Arc Flash Protective gear, even just to plug something in. It's all just a label.

I suspect that Ancor's foundational formula used to generate those tables and is published on one of those pages is based on or taken from ABYC standards.

Early-on when I tried to ref NEC stds I found that the numbers didn't align very well. That was when I had the conversation with the Electrician and was too long ago to recall what the differences were. I've since just used stds created for boats and Ancor's page is easy to find.

By definition Engineers are "conservative." I've no issue with increasing the Factor of Safety unless we're launching or racing the vehicle. Neither of which I plan to sleep in. I consider the Ancor charts and formula to be the minimum required, not the maximum needed.

 

[e9999]

No, of course, the current varies like the voltage, and yes the current varies between max and zero if you look at it with high enough time resolution, but the RMS value given for the AC amps or voltage is by definition the appropriate average over the periodic cycle for power calculations. Importantly, it is not just 1/2 the peak value or even the arithmetic cumulative average of a half-cycle, though, it is in fact specifically the RMS in the mathematical sense because it is the square of the values that is important for the power calculations, so the average is based on the square of the values (an actual RMS calculation), and that average is 1 / square root of 2 times the peak value (for a true sinusoid). At the risk of repeating myself, if you look at it with a scope, there is nothing 120V about our US household utility power besides that average "label" we slap on it. 120V is NOT the peak value, it is a DC power-based equivalent average. Yes, an RMS meter will directly display 120V but that is because it's convenient (for power calculations!).

Mathematical gobbledygook aside, in plain English, the RMS values we all use for AC are calculated in such a way that they reflect the equivalent DC power you would have in a resistor. So they did the work for you. "5" (RMS) Amps at "50" (RMS) Volts AC gives the same power in a resistor as 5 (real) A at 50 (real) V DC, because that's how the AC values are *defined* in the first place.

 

[e9999]

On a related note, I looked at the equivalent regulatory standards ampacity charts used in the UK. Those show very similar numbers to the NEC, unsurprisingly, but those charts also explicitly state for the 2 conductors cables that the ampacity numbers are for both AC and DC. (Of course, with RMS values for AC, as discussed above.) Not a surprise, but there you go if you're worried about that.

 

[trainbufftony]

So, the wire on the charger is in your opinion undersized for the stated current output of the charger of 60 ampres?
Does this fabulous Asian miracle charger deluxe even make 10amps?

It could very well be that the safety margin is built in due to the sticker rating being entirely fabricated. What's the actual amperage being produced?
Maybe hook it to a bolt, see what it will deliver to a dead short. My guess is that the circuit protection kicks, or the whole thing goes up in smoke, long before it even thinks about delivering 60 amps to a battery.

 

[e9999]

yup, can't trust many of these chinese devices "ratings". And mind-boggling what low prices they can achieve with electronics. The 2 being correlated at times, of course.

Interestingly, though, I have to say that I have begun to see some higher-end chinese companies that seem to be doing a good job. So the market may be slowly progressing for the better.