Volt drop and fusing

[Oracle]

All,

If you have lets say a 16mm cable, which can hypothetically carry 100+ amps, but its length is 60m.

To comply with volt drop, the cable is limited to 50A (hypothetically), do you fuse this cable to 50A or to 100A? ?:|

O. ; \

 

[Andy™]

Depends on R1Rn. if its too high due to length, then a 100A may not open incase of fault

 

[Oracle]

You mean Zs? (There is no requirement for just R1Rn)

Assume that Zs is ok, then do you need to "fuse down" to comply with VD?

O.

 

[daryl631]

Can someone correct me if i am wrong just calculated volt drop as 16.8v so current carrying capacity is about 83A so 100A mcb would be too much need to downsize

 

[Robojin]

Can someone correct me if i am wrong just calculated volt drop as 16.8v so current carrying capacity is about 83A so 100A mcb would be too much need to downsize

Show your full calcs and I'm sure we can help

 

[Robojin]

All,If you have lets say a 16mm cable, which can hypothetically carry 100+ amps, but its length is 60m.

O. ; \

16mm of what type, e.g. T&E is only rated at 85A in the OSG

 

[Andy™]

You mean Zs? (There is no requirement for just R1Rn)Assume that Zs is ok, then do you need to "fuse down" to comply with VD?

O.

If loop between live and neutral is too high then a 100A wont operate and you will need to downgrade.

 

[Robojin]

You mean Zs? (There is no requirement for just R1Rn)Assume that Zs is ok, then do you need to "fuse down" to comply with VD?

O.

as VD is calculated from design load x length etc, assuming you can't reduce the load or shorten the run, then look at increasing the csa

 

[daryl631]

Well was working on original question hyporthetical 100A

16mm cable 70 deg pvc clipped direct volt drop 2.8mv per mtr

2.8x100x60

-----------

1000 = 16.8v so 100-16.8=83.2 so 83A 100A circuit breaker wont opperate in time hope i am right on this like the original question said hypotheticalPray

 

[Oracle]

forget the sums and calcs.

If a cable is capable of carrying 100a, the current load is lets say 40a, but taking into account volt drop - this allows a max load of just under 60amps for it to comply. My question is do you fit a 100a fuse (max rating of cable) or a 60a fuse (to limit the current so that vd isn't a problem)?

O.

 

[Andy™]

Fit the fuse to design current. you cannot forget the sums and calcs.

 

[Robojin]

Well was working on original question hyporthetical 100A16mm cable 70 deg pvc clipped direct volt drop 2.8mv per mtr

2.8x100x60

-----------

1000 = 16.8v so 100-16.8=83.2 so 83A 100A circuit breaker wont opperate in time hope i am right on this like the original question said hypotheticalPray

Without knowing what you are trying to hypothetically design here, and unless I've lost the proverbial plot, if domestic the only point where you might see loads approaching 60A-80A is up to the CU incomer so calcs for 80A would apply to the tails, typically short runs

 

[The Godfather]

The key element here, is disconnection time surely?

(Under a fault condition (while meeting the protection requirements of the cable))

The Godfather

 

[kme]

forget the sums and calcs.If a cable is capable of carrying 100a, the current load is lets say 40a, but taking into account volt drop - this allows a max load of just under 60amps for it to comply. My question is do you fit a 100a fuse (max rating of cable) or a 60a fuse (to limit the current so that vd isn't a problem)?

O.

The key element here, is disconnection time surely?(Under a fault condition (while meeting the protection requirements of the cable))

Would disagree, don......

As Mr. Oracle`s post above, if design current is 40A, then protective device is 45/50/63A, dependant on type. As long as cable meets disconnection times, and V. drop, what`s the issue?

Why are we talking about 100A fuses, for a 40A load?

Thread makes no sense. It is illogical, Captain. Nanu Nanu.

KME

 

[daryl631]

The key element here, is disconnection time surely?(Under a fault condition (while meeting the protection requirements of the cable))

Would disagree, don......

As Mr. Oracle`s post above, if design current is 40A, then protective device is 45/50/63A, dependant on type. As long as cable meets disconnection times, and V. drop, what`s the issue?

Why are we talking about 100A fuses, for a 40A load?

Thread makes no sense. It is illogical, Captain. Nanu Nanu.

KME

nice one that will teach me to read through things carefullyApplaud Smiley

 

[Lillpete]

Ignoring all the why's and wherefore's my take on it is that if you need a 60 meter run you have to increase the size of the cable to take into account voltage drop eg increasing from 10mm to 16mm you would then however still fuse it at the lower level you where originally working towards as if you increased the possible load (obviously this would not be the case inthe real world as you will be working towards a design current) this would in turn increase the voltage drop so you would have to increase the cable size again ; -)