Next Size Up Rule for Terminals?

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SunFish

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Sr. PV Systems Design Engineer
I have a circuit with continuous current @ 216.9 A. 216.9 A x 1.25 = 271.125 A so fusing at 300 A. 90° C conductors, three phase 4 wire circuit so using 80% adjustment per Table 310.15(B)(3)(a). Looking at 300 MCM @ 320 A under 90° C rating. 320 x 0.8 = 256 A. So technically ok to use 300 A fuses per next size up rule 240.4(B).

The question is can one use the next size up rule for the 75° C terminals that are rated 285 A with 300 MCM? 285 A is still greater than 125% of the continuous current (217.125 A). 240.4(B) speaks more to conductors and doesn't mention terminals... wondering what others are doing.

Thanks!
 

augie47

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Tennessee
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State Electrical Inspector (Retired)
Depending on the nature of the load you may not have to apply the 80% rating (majority of load being non-linear is rare). In any event, 210.19 would require the adjusted amapcity of the conductor be at above the calculated load,
 

SunFish

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Sr. PV Systems Design Engineer
Ok, sounds good. But 80% derating aside, does 240.4 allow for the next size up rule to be used for 285 A rated 75°C terminals to be protected by 300 A fuses?
 

paulengr

Senior Member
Please don’t do this. Standard cable sizes are 250, 350, 500 MCM. 300 and 400 exist but are not very standard. That’s like using 575 MCM DLO...sure you CAN but everything is special order.
 

Carultch

Senior Member
Location
Massachusetts
Please don’t do this. Standard cable sizes are 250, 350, 500 MCM. 300 and 400 exist but are not very standard. That’s like using 575 MCM DLO...sure you CAN but everything is special order.

I think you are misunderstanding the question. This isn't about the use of non-standard products. This is about using 240.4(B), with the termination rating.

The answer is yes. If 285A is sufficient for the load with any factors that apply (such as 125%), then you can protect it with a 300A OCPD. Both the conductor ampacity and termination ampacities are considered "protected" they can "round up" to the OCPD you are using, as 240.4(B) allows.
 

bwat

EE
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NC
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EE
I could be wrong, but I believe Paul's post was a side note/suggestion to not use 300 MCM if possible; regardless of what the OP question was. I have been guilty several times of specifying something that certainly exists and is plainly right there in the table, but I didn't realize the contractor would have much rather preferred the next size up, for example, which could be more common and readily available. So since I've personally screwed that up, and it looks like the OP is wanting to use 300 MCM, I can appreciate the note.
 

paulengr

Senior Member
I guess it depends where you are, we use 300 kcmil and 400 kcmil all of the time.

It’s sort of like using 600, 750 and so on. I’ve heard of a utility using 1200 MCM THHN. Above 500 MCM copper lugs are made to order. Even at 350 and 500 MCM it’s usually a bit of a hunt and we stock it in house. Similarly many cable sizes are special order such as 3/0 (though this is more common) or 150 MCM. So sure I have dies and I can certainly use 300 MCM. But when the plant is down and it often takes weeks to get wire or lugs, that’s a big problem.

I have a ton of personal experience with the very annoying 300 MCM cable. It just barely works for mid size dragline mining excavators at 7200 V. But it is all special order. When you stock out of materials at the mine it’s a really, really bad day. But never an issue at 350 MCM.

And yes lots of blanking MEPs around here spec it. And fly by night construction contractors that don’t give a rip once they put in the first one don’t care either. They have weeks to order it ahead of time. It’s the maintenance and service folks that suffer.
 
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