400A service?

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PetrosA

PetrosA

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Lancaster, Pennsylvania
I recently worked on a couple of 400A services that needed sealing the conduits entering the buildings. What I saw puzzled me, maybe because I don't normally work on services this size. Service #1 has a 400A main disconnect with single fuses in it. Service #2 has a different brand disconnect with paralleled 400A fuses in it. From what I can find, this is not allowed, but it was installed and inspected back in the 1970s. Doesn't paralleling 400A fuses effectively increase the fusing of the service to almost 800A? There are pairs of 350MCM AL USE coming in from the POCO transformer and going out to meter banks, which would indicate a 400A service (I was not able to read the switch rating anymore).
 

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are they same exact fuse? and yes, two fuses in parallel basically allows fuse x2 to flow. perhaps remove one and keep 2nd as a spare. one is upside down from the other, almost made me think that perhaps the fuse has a huge diode in it, but i dont think so.

which btw, the ecn-400 is an obsolete part.
 
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I agree, fuses in parallel add up.

It is no longer allowed by the NEC but you may still see listed equipment such as HVAC units have parallel fuse.

The real question is if one 400 amp fuse will carry the connected load. :eek:hmy:
 
Yes, they appear to be the exact same fuse. Here is another shot of the setup with both legs shown. One fuse is different out of the four and appears older than the other three. Maybe something blew at some point? Without doing some serious calculations, I would hesitate to pull fuses since this is a main disconnect serving 16 units in a residential setting.
 

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are the fuses isolated in their mountings??
whats the ampacity of the wire on the bottom side of fuse?? if its less than 800A (400A if the wires are used in parallel feeder?) then you see the issue.
 
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PetrosA said:
Like I said, there are two 350 MCM AL coming in, and two 350 MCM AL going out per leg. That's why I'm thinking this is a serious issue.
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it would be odd that this was a serious issue 25yrs ago and it still went into service. maybe someone swapped the fuses out for wrong size?
350 AL is right around 250 ampacity.
 
I'm wondering the same thing. I'm not real familiar with fuse sizes, but I guess if there's no 500A fuse, maybe this was considered a 600A service using the next size up rule? It would still mean the wrong size fuses are installed.
 
A 250 amp fuse is a standard size so a pair of them would be correct for your wire size assuming no odd situations of temperature or bundling etc.

240.6 Standard Ampere Ratings.
(A) Fuses and Fixed-Trip Circuit Breakers. The standard
ampere ratings for fuses and inverse time circuit
breakers shall be considered 15, 20, 25, 30, 35, 40, 45, 50,
60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300,
350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000,
2500, 3000, 4000, 5000, and 6000 amperes. Additional
standard ampere ratings for fuses shall be 1, 3, 6, 10, and
601. The use of fuses and inverse time circuit breakers with
nonstandard ampere ratings shall be permitted.
Click to expand...
 
iwire said:
A 250 amp fuse is a standard size so a pair of them would be correct for your wire size assuming no odd situations of temperature or bundling etc.
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if 350 AL is 250 ampacity, then i would think you would fuse using 225 x2 (max), or possibly 200 x2 to match the service?
 
iwire said:
250 amp conductor, 250 amp fuse is fine.

Why do you think is should be 200 or 225?
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OP said it was a 400A service.

200 x2 fuses = 400A
225 x2 fuses = 450A, 50A above service rating

the 350 AL is rated 250ampacity, x2 because there are two = 500A. wire can handle the service, fuses should also protect the service and disconnect rating, no?
 
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FionaZuppa said:
OP said it was a 400A service.

200 x2 fuses = 400A
225 x2 fuses = 450A, 50A above service rating

the 350 AL is rated 250ampacity, x2 because there are two = 500A. wire can handle the service, fuses should also protect the service and disconnect rating, no?
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I would say it is not a 400 amp service.
 
What size are the wires leaving the DISCO.
It rarely matters what size the wire is the POCO installs. POCO's in CA rarely install wire that size of the service rating but mostly based on the Calculated load that was provided at the time of planning.
For example:
For a 200 amp Resi. We install 4/0 AL but the POCO often installs #1 in the underground conduit we provide for them.
In fact I have done service upgrades from 100 to 200 at the request of the homeowner and the POCO used the same wires.
 
It's been wayyyy back, but I have seen that previously. Do you think that is the reason for the wording of 404.17 & 240.8 ?
In a 1993 Handbook it mentions fuses in parallel being legal for switches over 600 amp, stating that "at one time it was legal"
 
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augie47 said:
It's been wayyyy back, but I have seen that previously. Do you think that is the reason for the wording of 404.17 & 240.8 ?
In a 1993 Handbook it mentions fuses in parallel being legal for switches over 600 amp, stating that "at one time it was legal"
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Why would you think it was illegal at the time of install in the first place. The problem seems to be the rating.. at 800. amps
 
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