Oversized Fuses for service entrance

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roninblayd

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We are preparing to upgrade a service at a 6 unit condo building located in Chicago and of course are trying to minimize costs. We calculated a demand load of 410A, so unfortunately need a 600 amp rated disconnect. My question is: if we use a fusible disconnect, can we size the fuse for let's say 500amps (if 500 amp fuses are available) to protect two sets of parallel 3/0 service entrance conductors? This is my reasoning for not installing a full sized 600amp service with parallel 250 or 350kcmil conductors. The chicago inspectors do not like the smaller conductor allowance per table 310.15(B)(6) and even though the calculated load is only 410 amps, say that we should install a full 600amp service.

Reasoning: 3/0 conductors are rated at 225 amps for residential services in both chicago and nec. parallel 3/0 would provide 450amps. next size standard fuse is 500 amps and 600 amp rated disconnect is the only size after 400 amps. Using parallel 3/0 also allows us to use #2awg grounding electrode conductor in 3/4" conduit and considering that the water meter is very far away and inconveniently located, saves considerably over the 1/0 or 2/0 if we had a fully sized 600 amp service.

Are 500amp fuses readily available? Is what I propose acceptable per code? What is normally done? I can always fudge the AC loads which are not even currently exisiting so the calculated load is less than 400amps but prefer not to do that. We just planned for the near future where owners are going to install AC's.
 
Ron,

Ok. ready for the good news?? The allowance for 3/0 on res. services to

equal 225a are for single phase only 120/240vac and the list stops at 400a.

#2. You still need to change the GEC to 1/0 if you use 2 x 3/0 service

conductors, you have to add them together before using table 250.66.

#3 310.16 table. 3/0 copper thwn-xhwn 75deg 200a, that would get you a

400a service.

#4 If you run a 35a circuit for an air cond. and use a 60a disconnect to

fuse it, do you have to run 60a conductors per the Inspector?

Ron, believe me, I'm trying to help you out here, spend a little more time

on this or even ask someone in the trade you trust. If you try to save the

customer to much money, it will cost you a fortune.
 
Ron check out 110.14(C) and (C)(1) You can only use the ampacity column in 310.16 that is equal to the lowest rated device or terminal in the circuit. Most terminals above 100 amps are 75 degree rated.
 
Although you're correct about parallel #3/0 requiring only a #2 CU GEC you can not use 2 sets of #3/0 for this service. Since your calculated load is 410 amps you'll need to use conductors rated for at least 410 amps and can go up to the next higher standard size OCPD which according to 240.6 is 450 amps. Looks like you'll need a minimum of parallel #4/0's with a #1/0 Cu GEC.
 
My question is: if we use a fusible disconnect, can we size the fuse for let's say 500amps (if 500 amp fuses are available) to protect two sets of parallel 3/0 service entrance conductors?
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Yes you can, and place a sticker within the panel stating "Replace with 500 amp only, due to your wire sizing . Because you said condos.... In Columbus ohio we have problems with more than 10KA Available fault currents (Typically 25K-50K) feeding 10KAIC breaker panels in Condos.
A 100KVA Transformer at 2% can deliver 42,000 amps of fault current on the secondary side.
Just make sure if this happends you use the series rated tables by UL, to protect the downstream breaker boxes within the Condo.
500 amp Class J or RK-1 while available might not be in stock in smaller Electrical Distributors, try the larger Distributors.
Just my $.02
 
Another point is that 310.15(B)(6) can be applied only to conductors of an individual dwelling unit. The service conductors are supplying six dwelling units.


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