I can't believe your patience you all

I'm talking about the PV cables from the panels to the MPPT. The delivery to batteries after that is irrelevant to my question which is, how the hell is a 10mm2 PV cable OK to conduct 15A@60V but not for 15A@30V.

@BobTPH you are getting very close to making me a happy boy, but if the voltage drop is the same, why is the calculator offering a smaller gauge for the higher voltage setup? Is it a matter of not wasting power rather than not burning the cable? Like would the 10mm2 be OK at 30V safety wise despite the voltage drop representing a bigger portion of 30V than it would of a higher voltage?

10mm2 pv cable is fine for 15A irrispective of voltage
its the insulation type thats relevant for voltage .

 

I can't believe your patience you all

I'm talking about the PV cables from the panels to the MPPT. The delivery to batteries after that is irrelevant to my question which is, how the hell is a 10mm2 PV cable OK to conduct 15A@60V but not for 15A@30V.

@BobTPH you are getting very close to making me a happy boy, but if the voltage drop is the same, why is the calculator offering a smaller gauge for the higher voltage setup? Is it a matter of not wasting power rather than not burning the cable? Like would the 10mm2 be OK at 30V safety wise despite the voltage drop representing a bigger portion of 30V than it would of a higher voltage?

That app's recommendation is based on its criteria of wanting to drop no more than 3% of the voltage across the cable. Whether or not that criteria is relevant in your situation is something you have to determine. It is based on a very generic assumption that a load rated for a certain voltage may not function properly if that voltage is more than a certain percentage less that what it is rated for.

At 60 V, you can drop twice as much voltage in the cable compared to a 30 V system at the same percentage, meaning that you can use a smaller cable because you can tolerate a larger voltage drop across it (IF the criteria that you can drop the same percentage of the voltage is a relevant factor). In doing so, you will dissipate more power as heat in the cable, but the fraction of power wasted will be the same.

 

That app's recommendation is based on its criteria of wanting to drop no more than 3% of the voltage across the cable.

OK, that's definitely key to me understanding this whole thing. The app is not necessarily recommending for fire safety but compliance with nominal voltage values.

The cable doesn't really have any awareness of the power that is passing around it

Can we say that a cable is a resistive load and replace the pipes in your analogy with a resistor? If yes, why is a resistor rated for power?

 

Can we say that a cable is a resistive load and replace the pipes in your analogy with a resistor? If yes, why is a resistor rated for power?

Power is heat
Any given electrical device that has resistance , and passes a current, dissipates power
.w = I *I * r
Resistors of different types and sizes have different power dissipations before the get to hot .

 

Power is heat
Any given electrical device that has resistance , and passes a current, dissipates power
.w = I *I * r
Resistors of different types and sizes have different power dissipations before the get to hot .

Which was my original reason for asking why it's OK to run 15A@120V=1800W in a 10mm2 cable but not 15@30V=450W. I guess I'll find solace in @WBahn response:

That app's recommendation is based on its criteria of wanting to drop no more than 3% of the voltage across the cable.

The app is not recommending for heat safety but nominal voltage tolerance.

 

Which was my original reason for asking why it's OK to run 15A@120V=1800W in a 10mm2 cable but not 15@30V=450W. I guess I'll find solace in @WBahn response:

The app is not recommending for heat safety but nominal voltage tolerance.

Why do you think the voltage changing was a problem ?

 

Because the calculator doesn't know that I'm sourcing the amps from solar panels that will never give me more that 15A anyway so it worries that I may try and pull more than 15A through that cable if I go above the recommended 3% voltage drop, so it gives me a bigger cable gauge?

 

Because the calculator doesn't know that I'm sourcing the amps from solar panels that will never give me more that 15A anyway so it worries that I may try and pull more than 15A through that cable?

Sounds like the answer is then your using the wrong calculations !

 

So given the panel specs:
- 15Ampp
- 30Vmpp
- Total cable run 20m (10m each way)

What is the right way to figure out the required cable gauge?

 

So given the panel specs:
- 15Ampp
- 30Vmpp
- Total cable run 20m (10m each way)

What is the right way to figure out the required cable gauge?

Cable gauge is only about voltage drop and power dissipstion..
.assuming copper cable .
1mm cable at 15A , drops 0.54v per m
4mm cable at 15A ,drops 0.14 v per m
10mm cable at 15A , drops 0.05 v per m

What voltage drop do you want to accept ?
Over 20 m round trip , the 20mm cable will drop 1.1 v at 15A
Which sounds very good to me.

 

Can we say that a cable is a resistive load and replace the pipes in your analogy with a resistor? If yes, why is a resistor rated for power?

Because any current flowing in it will cause it to dissipate heat. That heat will cause it's temperature to rise. There is a maximum allowable temperature rise and that happens at a particular level of power dissipation.

The same is the case for a cable. Current flowing in it causes it to dissipate heat. That heat causes the temperature to rise. There is a maximum allowable temperature rise.

The difference is practical. In a power cable we have two considerations -- keeping the temperature below a max limit, and also keeping the voltage drop across the cable below some maximum acceptable value.

Since the power cable is spread out, the temperature isn't related to the total power dissipated by the cable, but rather the power dissipated per unit length of the cable. A cable that is ten times as long can dissipate about ten times the heat for the same temperature rise because it has ten times the cooling available to it. The power dissipation per unit length is related to the current, which is why we primarily work with a current rating on a cable and not a power rating. For a resistor, the heat is all concentrated in one place, so the temperature is related to the total power dissipated.

 

Since the power cable is spread out, the temperature isn't related to the total power dissipated by the cable, but rather the power dissipated per unit length of the cable. A cable that is ten times as long can dissipate about ten times the heat for the same temperature rise because it has ten times the cooling available to it. The power dissipation per unit length is related to the current, which is why we primarily work with a current rating on a cable and not a power rating. For a resistor, the heat is all concentrated in one place, so the temperature is related to the total power dissipated.

THIS is exactly what I was after
Sorry to all it took so long and thanks to everyone who chimed in!

 

In the case of the solar setup an increase in voltage due to series panel doesn't decrease the amps, otherwise no matter how many panels are wired in series the output would always be the same. And I still can't reconcile how a cable conducting just as many amps and more power overall will run cooler...

That would be correct. I have a solar panel that delivers 12 Volts at 10 amps. I add 3 more solar panels in series. I now have 48 volts at 10 amps. I have increased the voltage but not the current I have a solar panel delivering 12 volts at 10 amps and now I add 3 more panels but this time in parallel. I now have 12 volts but 40 amps available. It's a matter of a source in series or parallel.

They seem to like to represent solar panels in watts.

I see more post as of the moment.

So given the panel specs:
- 15Ampp
- 30Vmpp
- Total cable run 20m (10m each way)

What is the right way to figure out the required cable gauge?

OK max current would be 15 amps @ 30 volts. Your distance is 20 meters total. I check and AWG12 has an ampacity rating of 20 amps. I look at the resistivity of AWG 12 and see where it has a resistance of 1.59 Ohms / 1,000 feet. Your 20 meters round trip is about 65.5 feet. Looking at your distance the wire resistance becomes about negligible. Figure 1 foot is about 0.3048 meters. Sorry about the meters vs. feet but my data comes up for my side of the pond. Your total tun is about 65.6168 feet. AWG 12 wire has a diameter of 2.05mm. Personally considering solid copper wire cost I would likely go with AWG 10 (2.59mm) wire but AWG 12 for the short run would be fine. This is all based on solid copper wire, not stranded.

If I have any of this screwed up, please another forum member correct me.

Just My Take
Ron

 

Sorry about the meters vs. feet but my data comes up for my side of the pond.

I should apologise! I'll even apologiZe to convert the spelling as well Thanks for the effort Ron! I know now to look for ampacity ratings rather than use calculators or tables.

 

There's not much evidence for that claim in what you are saying.



Yes, they do. Almost all electrical cables are very ohmic in their behavior.

You can ignore the inductance and capacitance, but you can't ignore the temperature coefficient

 

You can ignore the inductance and capacitance, but you can't ignore the temperature coefficient

Oh, I don't know that it needs to be taken into account in most situations. IIRC, most wire used for common building wiring is rated for a max temperature of about 90°C, or an increase over room temp of about 65°C. The tempco of copper is about 0.4%/°C, so it goes up by about 26%. Not insignificant, but if you are sizing things such that a 26% increase is a cause for concern, you probably should be incorporating more safety margin to begin with (or, you need to be doing a more detailed analysis and design, probably of some other factors as well). If you are wiring something to most electrical codes, this safety margin is already built in. For example, IIRC, the NEC requires a minimum 14 AWG wire on a 15 A circuit, even though the ampacity of 14 AWG is 20 A. It also targets a max temperature of 60°C, which would be an increase in resistance of about 15%. But, if nothing else, it's always good to have in the back of your mind the awareness that the resistivity is going to go up and, hence, the power dissipation and voltage drop.

 

@Robyn
Are you happy now with the answer for your panels ?

 

Oh, I don't know that it needs to be taken into account in most situations. IIRC, most wire used for common building wiring is rated for a max temperature of about 90°C, or an increase over room temp of about 65°C. The tempco of copper is about 0.4%/°C, so it goes up by about 26%. Not insignificant, but if you are sizing things such that a 26% increase is a cause for concern, you probably should be incorporating more safety margin to begin with (or, you need to be doing a more detailed analysis and design, probably of some other factors as well). If you are wiring something to most electrical codes, this safety margin is already built in. For example, IIRC, the NEC requires a minimum 14 AWG wire on a 15 A circuit, even though the ampacity of 14 AWG is 20 A. It also targets a max temperature of 60°C, which would be an increase in resistance of about 15%. But, if nothing else, it's always good to have in the back of your mind the awareness that the resistivity is going to go up and, hence, the power dissipation and voltage drop.

Depends on your country’s regulations. Calculations for change in resistance due to temperature are all over the place in BS7671.

 

@Robyn
Are you happy now with the answer for your panels ?

Yes, still reading the comments about temperature but my conundrum is fixed! We've meandered many ways but my initial question was about the fact that cables and resistors didn't seem to get spec'd the same way despite being both resistive loads. I understand now that power is critical for a resistor due to it's high and fixed resistance, whereas a cable is spec'd mainly to current due its lower resistance, variable length and way bigger heat dissipation surface.

 

You have two things to consider. Firstly, you need a cable that is big enough to carry the current you need. That sets a minimum cross sectional area.
Then you have to consider how much power you lose in the cable. If you lose too much at the minimum cross sectional area required for safety, a thicker cable might be appropriate.
As a percentage of total power, the power lost in the cable depends on the voltage.

As the power lost in the cable depends on the square of the current, the percentage lost varies with the current. for applications when the peak power occurs only intermittently, such as solar power, extra losses at peak power may not be a problem.