Is cable size dependent on watts or amps?

[pupenhausen]

I'm trying to buy 3 core flex for to run from my battery to my electric bike wheel. I was thinking of using 2.5mm cable, but its only rated for 20 amps. My bike runs at 87.6v x 45amps= 3942 watts, would it still be ok to use? What I don't understand is why all cable is rated in amps and not watts, does this mean if I were powering a 300w mini amp at 3volts I'd need 100amp rated cable? I don't get it.

 

[EFLImpudence]

I don't get it.

Your post indicates that you, indeed, do get it.

Ohm's law is unchangeable I = V / R.

Everything is related.

 

[EFLImpudence]

What I don't understand is why all cable is rated in amps and not watts,

I missed that bit.

The cable can carry a certain amperage before overheating.

It cannot be rated in watts because this will vary depending on the voltage.

My bike runs at 87.6v x 45amps= 3942 watts, would it still be ok to use?

If you have that correct, you will need 6mm² conductors.

 

[pupenhausen]

I don't get it.

Your post indicates that you, indeed, do get it.

Ohm's law is unchangeable I = V / R.

Everything is related.

So basically you're saying that I'd need cable like a hose pipe to power a measly 300w amp at 3v? Yes, I understand Ohm's law but this doesn't answer my original question. Say I have a household cooker running at 3800 watts at 230v= 16 amp cable needed. My bike runs at the same power 3800w but at 87v= 43 amp cable. What I don't get is why would I need cable more than twice as thick to power my bike wheel than my cooker, even though they consume the same amount of power. Or is this the case? I've seen on this site that 2.5mm flex is ok for 4800w but at 20 amps.

Disclaimer: Helpful answers only please.

 

[EFLImpudence]

So basically you're saying that I'd need cable like a hose pipe to power a measly 300w amp at 3v?

Well, you would.
Are you saying you have such a thing?

Say I have a household cooker running at 3800 watts at 230v= 16 amp cable needed. My bike runs at the same power 3800w but at 87v= 43 amp cable. What I don't get is why would I need cable more than twice as thick to power my bike wheel than my cooker, even though they consume the same amount of power.

The true answer is - "because that's how it works" (electricity, that is).

It depends on the voltage.

Or is this the case? I've seen on this site that 2.5mm flex is ok for 4800w but at 20 amps.

2.5mm² cable is not ok for 4800W per se. The voltage must be stated as well.

4800W @ 230V = 20.87A which 2.5mm² can safely carry.

Disclaimer: Helpful answers only please.

Does that mean you thought my previous posts unhelpful?

 

[JohnW2]

So basically you're saying that I'd need cable like a hose pipe to power a measly 300w amp at 3v?

Indeed. Have you ever wondered why power is moved around the national grid at over 100,000 volts, rather than 230V, or even 3V? It's because what matters in terms of the cable is the current flowing through it, regardless of the amount of power being delivered to the load at the end - so the higher the voltage, the lower the current for a given amount of power delivered to the load, hence the small the cable that can be used ...

...Your 300W amp needs 100A at 3V and therefore needs a hosepipe. If it used 300V it would need only 1A, and could use very small cable, and if it used 300,000V it would use only 0.001A and could be fed with a cable you probably couldn't even see!

What I don't get is why would I need cable more than twice as thick to power my bike wheel than my cooker, even though they consume the same amount of power.

Because the cable has no clue as to how much power is being delivered to the load - all it knows is how much current is flowing through it. If it knows that 1A is going through it, it could be delivering 3W to the load if its in a 3V circuit, or 230W to the load if its in a 230V circuit.

The current-carrying capacity of a cable is limited by heating within the cable, which is a functon of the amount of power disipated in the cable (not in the load). That power dissipated in the cable is equal to I²R (where I is the current and R the resistance of the cable). The other issue is that the amount of voltage 'lost' due to the cable is equal to current times the resistance of the cable (and resistance of cable obvioulsy gets smaller as it gets bigger) - and you can accept a larger voltage loss ('drop') in higher voltage cicuits than in low voltage ones. A 'loss' of 2.5V would obvioulsy be disasterous to your 3V amp ('cos the amp would only get the remaining 0.5V), but 2.5V loss in a 230V circuit would hardly have any noticeable effect, since 227.5V would still be 'left' for the load.

Does that help at all?

Kind Regards, John

 

[securespark]

Disclaimer: Helpful answers only please.

You have had comprehensive answers to your questions. No one has been rude to you.

You don't understand why you need the thicker cable, and seem frustrated, but that's no reason to be rude to the people who have helped you.

 

[ban-all-sheds]

Yes, I understand Ohm's law.

That my be true.

If it is though it means that you're refusing to accept what is says and want the laws of physics changed to suit your prejudices. Not going to happen.

 

[pupenhausen]

So basically you're saying that I'd need cable like a hose pipe to power a measly 300w amp at 3v?

Well, you would.
Are you saying you have such a thing?

No, I only understand electricity/electronics using examples.

Or is this the case? I've seen on this site that 2.5mm flex is ok for 4800w but at 20 amps.

2.5mm² cable is not ok for 4800W per se. The voltage must be stated as well.

4800W @ 230V = 20.87A which 2.5mm² can safely carry.

Disclaimer: Helpful answers only please.

Does that mean you thought my previous posts unhelpful?

The first one was, as you already gathered I knew Ohm's law but still told me "I do get it" and put up a picture of Ohm's law. I just didn't know how to apply it to cable thickness. I typed my response to this as you were writing your follow up answer, which I found slightly helpful so thanks for that.

 

[pupenhausen]

So basically you're saying that I'd need cable like a hose pipe to power a measly 300w amp at 3v?

Indeed. Have you ever wondered why power is moved around the national grid at over 100,000 volts, rather than 230V, or even 3V? It's because what matters in terms of the cable is the current flowing through it, regardless of the amount of power being delivered to the load at the end - so the higher the voltage, the lower the current for a given amount of power delivered to the load, hence the small the cable that can be used ...

...Your 300W amp needs 100A at 3V and therefore needs a hosepipe. If it used 300V it would need only 1A, and could use very small cable, and if it used 300,000V it would use only 0.001A and could be fed with a cable you probably couldn't even see!

What I don't get is why would I need cable more than twice as thick to power my bike wheel than my cooker, even though they consume the same amount of power.

Because the cable has no clue as to how much power is being delivered to the load - all it knows is how much current is flowing through it. If it knows that 1A is going through it, it could be delivering 3W to the load if its in a 3V circuit, or 230W to the load if its in a 230V circuit.

The current-carrying capacity of a cable is limited by heating within the cable, which is a functon of the amount of power disipated in the cable (not in the load). That power dissipated in the cable is equal to I²R (where I is the current and R the resistance of the cable). The other issue is that the amount of voltage 'lost' due to the cable is equal to current times the resistance of the cable (and resistance of cable obvioulsy gets smaller as it gets bigger) - and you can accept a larger voltage loss ('drop') in higher voltage cicuits than in low voltage ones. A 'loss' of 2.5V would obvioulsy be disasterous to your 3V amp ('cos the amp would only get the remaining 0.5V), but 2.5V loss in a 230V circuit would hardly have any noticeable effect, since 227.5V would still be 'left' for the load.

Does that help at all?

Kind Regards, John

Thanks very much John, you understood and answered my question perfectly. I find it hard to understand electrical theory by just looking at formulae but it "sinks in" when people explain it to me using analogies.

 

[pupenhausen]

Yes, I understand Ohm's law.

That my be true.

If it is though it means that you're refusing to accept what is says and want the laws of physics changed to suit your prejudices. Not going to happen.

Read, and understand the title of my question before you type drivel.

 

[pupenhausen]

Disclaimer: Helpful answers only please.

You have had comprehensive answers to your questions. No one has been rude to you.

You don't understand why you need the thicker cable, and seem frustrated, but that's no reason to be rude to the people who have helped you.

My disclaimer still stands. This is not an answer either. Do you think putting up an Ohm's law table (when he already gathers that I understand it) and telling me "I do get it" is a helpful answer? If I did "get it" I wouldn't have joined this forum to ask the question. Teachers don't tell their students that they know the answers without teaching them, and I'm not a Chinese Zen master who has the answers "within me". I know it now as John understood and answered my question very well.

 

[ban-all-sheds]

Read, and understand the title of my question before you type drivel.

Drivel?

Don't be ridiculous.

I wrote what I did because I assumed that when you wrote

Yes, I understand Ohm's law.

you were telling the truth.

But I guess you weren't.

 

[JohnW2]

Thanks very much John, you understood and answered my question perfectly. I find it hard to understand electrical theory by just looking at formulae but it "sinks in" when people explain it to me using analogies.

You're welcome. I hope things are now clearer to you.

Kind Regards, John.

 

[pupenhausen]

Read, and understand the title of my question before you type drivel.

Drivel?

Don't be ridiculous.

I wrote what I did because I assumed that when you wrote

Yes, I understand Ohm's law.

you were telling the truth.

But I guess you weren't.

Oh, I got you wrong. You are just plain stupid thinking that I have "prejudices" about electrical theory and that I want the laws changed to suit me just by not understanding how to apply Ohm's law to cable thickness. God help anyone who asked you a question directly, sarcasm is the lowest form of wit. To come on a question thread, not provide an answer, and basically just "troll" is pathetic. The Youtube comments section would be a better vent for your trolling frustrations.