75deg terminals, and 90deg wire.

[Matt Donovan]

Heres the situation...
- 4000A bus
- 3400A main fuse
- (8) sets of 500mcm CU THWN-2

Is the wire sufficiently sized? Or do we need (10) sets?

Here's my thinking...
- with (8) sets, the 75o temp is 3040A, and the 90o temp is 4300A.
- The wire may rise above 75o, but never above 90o. The wire is rated 90o, so the wire itself sufficiently protected.
- The bus is rated 4000A, so it will not be overloaded either.
- The terminals are 75o rated. But is that 75deg at 4000A, or 3040A? Do I consider the terminals part of the bus, or part of the wire? If part of the bus, you can say they will never be overloaded. But if part of the wire, they could rise above 75o and then be overloaded.

FWIW, the design load is 2400A, all continuous (2400A x 1.25 = 3000A).

 

[augie47]

Unless your fused disconnect is 90° rated, and very few are, the install would not be Code compliant. Some like the "weak link in the chain", if the device at either end is 75° rated, you can not exceed the 75° rating of the conductors.

 

[Carultch]

Heres the situation...
- 4000A bus
- 3400A main fuse
- (8) sets of 500mcm CU THWN-2

Is the wire sufficiently sized? Or do we need (10) sets?

Here's my thinking...
- with (8) sets, the 75o temp is 3040A, and the 90o temp is 4300A.
- The wire may rise above 75o, but never above 90o. The wire is rated 90o, so the wire itself sufficiently protected.
- The bus is rated 4000A, so it will not be overloaded either.
- The terminals are 75o rated. But is that 75deg at 4000A, or 3040A? Do I consider the terminals part of the bus, or part of the wire? If part of the bus, you can say they will never be overloaded. But if part of the wire, they could rise above 75o and then be overloaded.

FWIW, the design load is 2400A, all continuous (2400A x 1.25 = 3000A).

For circuits with an OCPD greater than 800A, you always need at least as much amps of wire and terminations as you have OCPD. If you use a 3400A main fuse, you need at least 3400A worth of wiring, both from the 75C column without derate factors (terminations), and from the 90C column (for 90C wire) with derate factors. It is OK to have more than 3400A worth of wiring and terminations.

 

[Matt Donovan]

yup

yup

You guys seem to agree, and thats what I thought as well. But since the 8 sets were already installed (and there's a change order for the add'l 2 sets), I was trying to force an argument to make it work. Thanks for the thoughts.

 

[tkb]

With the wire you stated, you can only have 3000 amp fuses.
You will need 9 sets to allow a 3400 amp fuse.

The only way to get to 3400 amps with 8 sets is to upsize to 700kcmil, 3680 amps.

 

[Matt Donovan]

New Question - Continuous rated fuses

New Question - Continuous rated fuses

Ok, heres a new question, on the same topic. In this scenario, if we use 3000A fuses that are continuous rated (such as A4BY Class L), is it still sufficient for the wire to be rated for only 3000A? It seems like the code assumes 80% rated OCPD, in which case a 3000A continuous fuse is effectively a 3600A standard fuse, which would require 3600A rated wire.

 

[Dennis Alwon]

Ok, heres a new question, on the same topic. In this scenario, if we use 3000A fuses that are continuous rated (such as A4BY Class L), is it still sufficient for the wire to be rated for only 3000A? It seems like the code assumes 80% rated OCPD, in which case a 3000A continuous fuse is effectively a 3600A standard fuse, which would require 3600A rated wire.

If the fuses are rated 3000 amps continuous then that would be sufficent for a 3000 amp load and wiring rated at 3000 amps under the 75C column

 

[Matt Donovan]

So the "continuous rated" fuses are treated no differently than non-continuous rated fuses, when it comes to wire sizing?

 

[Dennis Alwon]

So the "continuous rated" fuses are treated no differently than non-continuous rated fuses, when it comes to wire sizing?

I thought all fuses were continuous rating but don't quote me on that. If not then a continuous 3000 amp fuses I think should be referred to a 100% rated. A non continuous fuse would be 80% rated.

Say you have a continuous load of 2500 amps, then you would need a 3000 amp 100% rated fuse. If the load was 2400 or less than a standard 3000 amp fuse would work

 

[shortcircuit2]

Heres the situation...
- 4000A bus
- 3400A main fuse
- (8) sets of 500mcm CU THWN-2

Is the wire sufficiently sized? Or do we need (10) sets?

Here's my thinking...
- with (8) sets, the 75o temp is 3040A, and the 90o temp is 4300A.
- The wire may rise above 75o, but never above 90o. The wire is rated 90o, so the wire itself sufficiently protected.
- The bus is rated 4000A, so it will not be overloaded either.
- The terminals are 75o rated. But is that 75deg at 4000A, or 3040A? Do I consider the terminals part of the bus, or part of the wire? If part of the bus, you can say they will never be overloaded. But if part of the wire, they could rise above 75o and then be overloaded.

FWIW, the design load is 2400A, all continuous (2400A x 1.25 = 3000A).

Under the 2017 Code in 215.2(A)(1)(a) exception No. 2 allows a pull box on each end with 90 degree rated power distribution blocks or pressure connectors. Then you could use that wire at the 90 degree rating.

 

[Smart $]

Ok, heres a new question, on the same topic. In this scenario, if we use 3000A fuses that are continuous rated (such as A4BY Class L), is it still sufficient for the wire to be rated for only 3000A? It seems like the code assumes 80% rated OCPD, in which case a 3000A continuous fuse is effectively a 3600A standard fuse, which would require 3600A rated wire.

This article answers the question at least in part...

“Can I have a circuit draw 100% of my fuse’s ampere rating?”
The use of fuses at 100% of their marked ampere rating depends on their operating environment
and the associated components of the electrical system, such as switches and terminations.
Underwriters Laboratories performs tests according to fuse amperage per the following:
• 600A or less fuses that are UL Listed go through a 100% test (UL 248 section 8.2.3.1)
• 601 - 6000A fuses (Class L and T) that are UL Listed go through a 110% test (UL 248
section 8.2.3.2).
The limitations are the switch and terminations. A 100% rated switch needs to go through special
testing to verify that it can withstand the elevated temperatures that will exist when applied at
100% of its rating.
Therefore, if the switch is rated/listed for 100%, fuses can be applied at 100% of their ampere
rating, too.

Source: http://www.cooperindustries.com/con...se_Operation_at_100_Percent_Rated_Current.pdf

Note there is a difference between saying you are using a fuse at its continuous rating versus using a 100% rated fuse... and for the latter, the switch must be rated for 100% operation also. That may help you, but also note that you are required to use 90°C rated wire, but you cannot use it at an ampacity greater than the 75° column value.