3 phase furnace

mjbarcelona · Sep 6, 2019

Hi guys, we have a 3 phase 415V 36kw furnace. Just wanted to know the fuse needed for each phase. we put a 40A fuse for each phase and 1 phase just blew when started firing it up. I tried to calculate the right fuse and I come up with 65A per phase? Is that right?

Megawatt · Sep 6, 2019

mjbarcelona said:
Hi guys, we have a 3 phase 415V 36kw furnace. Just wanted to know the fuse needed for each phase. we put a 40A fuse for each phase and 1 phase just blew when started firing it up. I tried to calculate the right fuse and I come up with 65A per phase? Is that right?

MJ if I’m right and help me out 3 phase calculations is 415x1.73=719.9 36,000 divided by 719=50 amps since you are using a fuse 50x 1.75 = 87 is this the way y’all do your calculations?
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Megawatt said:
MJ if I’m right and help me out 3 phase calculations is 415x1.73=719.9 36,000 divided by 719=50 amps since you are using a fuse 50x 1.75 = 87 is this the way y’all do your calculations?

That’s the USA calculations

mjbarcelona · Sep 6, 2019

Megawatt said:
MJ if I’m right and help me out 3 phase calculations is 415x1.73=719.9 36,000 divided by 719=50 amps since you are using a fuse 50x 1.75 = 87 is this the way y’all do your calculations?

Hi megawatt, What I did was 36000/3 = 12000VA per phase, 12000/240 = 50 x 1.25 = 62.5A per phase. Where 240 is line to neutral 415/1.732=240. does that make sense?
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Megawatt said:
MJ if I’m right and help me out 3 phase calculations is 415x1.73=719.9 36,000 divided by 719=50 amps since you are using a fuse 50x 1.75 = 87 is this the way y’all do your calculations?
[automerge]1567730867[/automerge]

That’s the USA calculations

I see, interesting...
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Megawatt said:
MJ if I’m right and help me out 3 phase calculations is 415x1.73=719.9 36,000 divided by 719=50 amps since you are using a fuse 50x 1.75 = 87 is this the way y’all do your calculations?
[automerge]1567730867[/automerge]

That’s the USA calculations

Where'd you get that 1.75? 175% of rated current since i'm using a fuse?

Megawatt · Sep 6, 2019

mjbarcelona said:
Hi megawatt, What I did was 36000/3 = 12000VA per phase, 12000/240 = 50 x 1.25 = 62.5A per phase. Where 240 is line to neutral 415/1.732=240. does that make sense?
[automerge]1567733017[/automerge]

I see, interesting...
[automerge]1567733131[/automerge]

Where'd you get that 1.75? 175% of rated current since i'm using a fuse?

In the USA we use a chart for your overcurrent protection fuse are 1.75% if you are using inverse time breakers you can use 2.25%
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Megawatt said:
In the USA we use a chart for your overcurrent protection fuse are 1.75% if you are using inverse time breakers you can use 2.25%

If it started for you I would leave it there it’s just inrush current you have to get through
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Megawatt said:
In the USA we use a chart for your overcurrent protection fuse are 1.75% if you are using inverse time breakers you can use 2.25%
[automerge]1567734155[/automerge]

If it started for you I would leave it there it’s just inrush current you have to get through

I guess different ways of doing things but as long as we get it running

telectrix · Sep 6, 2019

Q.1. what size cable is feeding this furnace? this will determine the size of fuse, as long as it's ccc is > the load current.
Q.2. have you dead tested the furnace to ensure that there are no faults, esp on the phase that blew the fuse. ?

for that load i would expect a 10mm cable.

Lenny · Sep 6, 2019

36,000 / (400x1.732) = 51.96A. This is your design current (Ib)

Select a fuse rating (In) that suits your (Ib) and make sure the tabulated ccc (It) of your supply cable is greater than (In) after any correction factors have been applied.

Alex-SL · Sep 6, 2019

I would advise you to use suitable calculators. You can find many free ones on the net. For example:

[ElectriciansForums.net] 3 phase furnace

Power factor = 1.0

Design current = 50A,

Protection = fuse gG 63A, or MCB C63A

Note that the furnace has a short starting current while the heating elements are still cold.