Quick Question For Trainees Only

[Marvo]

I have a heater tape measuring 1.2 meters that's wrapped around a pipe to stop it freezing up. The tape is nothing fancy, it comes on a roll and consists of two insulated heating wires set in silicone, you just cut the length you need and fit two end caps, one to allow the supply wiring to be connected at one end and the other end cap to connect the two heater wires directly together at the far end to complete the circuit.

The 1.2 meter heater tape I made draws 4.4 Amps and it's on a dedicated 2.5mm radial circuit from the CU that's protected by a 5A MCB.

Problem is it's too short so I removed it and made a new heater tape that's 2 meters long which I'm about to install in its place. My question is will I need to upgrade the MCB in the CU to a higher value to accommodate the longer heater tape?

 

[Jay 1987]

I have a heater tape measuring 1.2 meters that's wrapped around a pipe to stop it freezing up. The tape is nothing fancy, it comes on a roll and consists of two insulated heating wires set in silicone, you just cut the length you need and fit two end caps, one to allow the supply wiring to be connected at one end and the other end cap to connect the two heater wires directly together at the far end to complete the circuit.

The 1.2 meter heater tape I made draws 4.4 Amps and it's on a dedicated 2.5mm radial circuit from the CU that's protected by a 5A MCB.

Problem is it's too short so I removed it and made a new heater tape that's 2 meters long which I'm about to install in its place. My question is will I need to upgrade the MCB in the CU to a higher value to accommodate the longer heater tape?

View attachment 55079

Hi Marvo

Thanks for the thought provoking question. here’s my attempt.....

The 1.2 meter heating tape is being increased to 2.0 meters.

I believe that would mean a 40% increase in the current it would draw.

Therefore, If 1.2 meters draws 4.4 amps

and it’s now been increased by 40%?

An 0.8 meter increase would draw an extra 1.76 amps


this brings the combined total demand of a 2.0 meter run of heater tape to 6.16 amps.

so a 5 amp MCB would not suffice.


Jay

 

[Marvo]

Hi Jay,

Thanks for having a stab at the answer.

Using your theory, if the 1.2m heater draws 4.4 Amps that works out at 3.666 Amps per meter so a 2 meter heater would draw 7.333Amps.

In this case I'm afraid that's not the right answer though...when I installed the 2 meter heater tape it worked fine with the 5 Amp MCB.

The heater tape is simply a wire with a resistance or more accurately it's 2 wires with a resistance joined at the far end and we can work out the overall resistance of the 1.2 meter tape using ohms law because we know the voltage and the current it draws.

Using this thinking, see if you can work out the current draw of the 2 meter tape.

 

[Jay 1987]

Hi Marvo,

thanks for the additional information. Here’s my second attempt.

1.2 meter of heating tape draws 4.4 amps

using ohms law to calculate resistance would give us V 230
—— — — R = 52.72 ohms
I 4.4
The heating tape is then increased to 2.0 meters, which is a 40% increase.

So 40% Of 52.72 ohms = 20.90 ohms

Now add them together 52.72 + 20.90 = 73.17 ohms.

with the new resistance value for our 2.0 meter run, we can use ohms law to determine the new current demand.

230. V
——
73.17 ohms. I = 3.14 amps.

So a 5 amp breaker will be sufficient.

Jay.

 

[davesparks]

It took me a minute to get it, I don't think I've ever seen a productthat works the way that one does in the UK.
I've seen a lot of heat tapes that work a bit differently to that.

 

[Marvo]

Hi Marvo,

thanks for the additional information. Here’s my second attempt.

1.2 meter of heating tape draws 4.4 amps

using ohms law to calculate resistance would give us V 230
—— — — R = 52.72 ohms
I 4.4
The heating tape is then increased to 2.0 meters, which is a 40% increase.

So 40% Of 52.72 ohms = 20.90 ohms

Now add them together 52.72 + 20.90 = 73.17 ohms.

with the new resistance value for our 2.0 meter run, we can use ohms law to determine the new current demand.

230. V
——
73.17 ohms. I = 3.14 amps.

So a 5 amp breaker will be sufficient.

Jay.

Winner winner chicken dinner. Thanks for persevering.

 

[Jay 1987]

Winner winner chicken dinner. Thanks for persevering.

Thanks Marvo,

I have a few questions for you, I hope you don’t mind.....

1, “ Using your theory, if the 1.2m heater draws 4.4 Amps that works out at 3.666 Amps per meter so a 2 meter heater would draw 7.333Amps”.

how did you get to this ?

2, what is the relationship between current and resistance ? In the respect that even though we lengthened the heating tape the resistance went up but the current draw went down ?

 

[Marvo]

It took me a minute to get it, I don't think I've ever seen a productthat works the way that one does in the UK.
I've seen a lot of heat tapes that work a bit differently to that.

We do get heater tapes like this but they only work between certain lengths for obvious reasons. They have a minimum safe length which prevents the tape getting too hot and a maximum length where the resistance becomes so high it no longer provides sufficient heat. We also get just a single insulated heater wire which works the same way and is handy for trace heating around doors etc to stop them freezing closed or an application where the item needing heat is in a loop shape.

Finally we get the type of heater tape that has two conductors in it that aren't connected at the far end, instead there's a semi-conducting material between them so the longer the tape the higher the load gets. Again there would be a length limitation if you were custom making these. I'm guessing this is the one you are referring to as commonly use in the UK.

Which type we use for a particular application is one of those radial/ring circuit type arguments and often there's more than one way to skin the cat. I would suggest to anyone working with these types of heater wires and tapes that you establish exactly which type it is before messing with it.
[automerge]1578511256[/automerge]

Thanks Marvo,

I have a few questions for you, I hope you don’t mind.....

1, “ Using your theory, if the 1.2m heater draws 4.4 Amps that works out at 3.666 Amps per meter so a 2 meter heater would draw 7.333Amps”.

how did you get to this ?

You stated that the length of the tape had increased by 40% so the current would increase by 40%. Your maths was a bit skew it's actually a 67% increase so I worked it out a different way using your theory.

2, what is the relationship between current and resistance ? In the respect that even though we lengthened the heating tape the resistance went up but the current draw went down ?

The wires inside the tape have a resistance of several ohms per meter, the longer the tape gets the longer the wires get therefore the higher the resistance gets. Longer tapes draw less current and produce less heat.

 

[Jay 1987]

Thanks for the information.

the way I worked out the 40% was

0.6 x 2.0 = 1.2

So I thought 0.8 was the additional 40%

What calculations did you use to get 67% ?

 

[Marvo]

Thanks for the information.

the way I worked out the 40% was

0.6 x 2.0 = 1.2

So I thought 0.8 was the additional 40%

What calculations did you use to get 67% ?

1.2m plus 67% equals 2.004m which is close enough to 2 meters.

if the 1.2m tape was 40% longer it would be 1.68m