400A service?

[PetrosA]

I should have been clearer... I am working on the ASSUMPTION that these are 400A services because the other building with the exact same layout as the one this disconnect is in, has single 400A fuses, but has two disconnects.

You know how you feel really stupid when you do one of those "genius" tests and realize you got it wrong because you missed something obvious? Well, that just happened to me now.

The second building has its service set up totally differently. It has two disconnects, each fused with single 400A fuses and fed with a set of 500 MCM AL USE feeders. That building has a service inspection from 1974. The building these photos came out of has a totally different setup, which may have been wrong from day one. I didn't check when I was there, but I'm wondering whether it even has an inspection sticker on it.

 

[FionaZuppa]

is there any rating on the disconnect?? i would fuse at or below that #.

 

[PetrosA]

Following up...

I called the POCO and they don't know what size the service is. I have to go through the application process for a new service to find out. I'm still concerned as to the legality of the installation in the first place for the following reasons:

- I have not been able to determine whether paralleled fuses were acceptable at the time of installation.
- Wiring and disconnects in the other building are for a 800A service with two 400A disconnects, while this building has wiring for a 500A service (transformer to disconnect to meters) and is fused at 800A. Other than the services, the buildings are identical.

What I'm pretty convinced of now is that the wiring is not adequately protected by the fuses as installed. It has obviously been in place a considerably long time. I could replace the 400A fuses with 250A fuses, but I'd risk putting 16 units into darkness if they blow. Not that I care about winning a popularity contest, but that would certainly put me high up on someone's list.

So at this point I need to decide how far to pursue the matter. These are the times when I miss working for somebody else

 

[wirebender]

What equipment do the outgoing 350's feed?

 

[iwire]

Following up...

I called the POCO and they don't know what size the service is. I have to go through the application process for a new service to find out. I'm still concerned as to the legality of the installation in the first place for the following reasons:

- I have not been able to determine whether paralleled fuses were acceptable at the time of installation.
- Wiring and disconnects in the other building are for a 800A service with two 400A disconnects, while this building has wiring for a 500A service (transformer to disconnect to meters) and is fused at 800A. Other than the services, the buildings are identical.

What I'm pretty convinced of now is that the wiring is not adequately protected by the fuses as installed. It has obviously been in place a considerably long time. Ih could replace the 400A fuses with 250A fuses, but I'd risk putting 16 units into darkness if they blow. Not that I care about winning a popularity contest, but that would certainly put me high up on someone's list.

So at this point I need to decide how far to pursue the matter. These are the times when I miss working for somebody else

Put your recording meter on it for a while, see where the load is.

 

[kwired]

The service is a 500 amp service. That may or may not be what was on a permit at one time, or in POCO records, but that is what the installed equipment is capable of. Paralleled fuses may have been acceptable at the time - and may have originally been 250 amp fuses, but have since been replaced with 400's because that is what fits and is probably easier to find as well. If you blow one fuse in a leg the other one is going to go shortly thereafter because it is now carrying all the load of that leg.

This switch about has to be rated for 800 amps. A smarter design decision would have been to use a 600 amp switch, but code probably would have allowed what is there at one time for a 500 amp circuit.

 

[PetrosA]

Put your recording meter on it for a while, see where the load is.

I will do that. What concerns me and what recording probably won't show is what happens in the summer when power goes out and comes back on with sixteen refrigerators, plus a whole boatload of air conditioners and water heaters all coming on at once

 

[iwire]

I will do that. What concerns me and what recording probably won't show is what happens in the summer when power goes out and comes back on with sixteen refrigerators, plus a whole boatload of air conditioners and water heaters all coming on at once

I hear you. Believe me I see that when I restart a large supermarket after an outage. Its often double its normal running load.

The other side of the coin would be that if 500 amps wont carry the load on restart the conductors are too small. (Obviously they look fine after years in place, but we are talking NEC rules and not reality)

 

[FionaZuppa]

Following up...
I could replace the 400A fuses with 250A fuses, but I'd risk putting 16 units into darkness if they blow. Not that I care about winning a popularity contest, but that would certainly put me high up on someone's list.

and if 800A fuse (2x 400) allows a wire to melt out and start a fire, then what?

you know with parallel fuses one will break 1st, and then there will be a surge on the other before that one pops.

 

[iwire]

you know with parallel fuses one will break 1st, and then there will be a surge on the other before that one pops.

How is that a problem?

 

[FionaZuppa]

How is that a problem?

if say 780A is a problem for anything in the feed and you have 1st fuse blow say at 390A (two fuses carrying 390A) but the 2nd one allows current to surge above 400A before it pops at say 425A. we would expect the fuses to do their thing but theres small probability a fuse acts funny, just as when i tested a AF breaker in my house panel, darn thing failed, made funny shorting noise and smoked until i turned off main breaker. in the current setup it seems like allowing ~800A on what is believed to be a 400A service just doesnt seem good, and having a 2nd fuse that perhaps can itself alone carry more than what the service is.

if the disconnect it rated ~400A then why are there two fuses?

so for OP, instead of swapping the 400A fuses for 250's, remove one 400A and see how the service works??

 

[Ponchik]

As suggested earlier I would put my recording meter on and see what is going on. I am sure in the last 40 years they have had power outages and the whole building has come back on line at once and the existing fuses seem to be original or are very OLD, so they have been changed in some years.

Changing the existing set up with smaller fuses may create a headache for you. IMO, I would not touch the existing fuses with smaller one since it has been working that way for number of years and I am sure it will be OK for a while. However, I would mail a certified letter to the owner and the management company stating your concern and the solution.

 

[kwired]

if say 780A is a problem for anything in the feed and you have 1st fuse blow say at 390A (two fuses carrying 390A) but the 2nd one allows current to surge above 400A before it pops at say 425A. we would expect the fuses to do their thing but theres small probability a fuse acts funny, just as when i tested a AF breaker in my house panel, darn thing failed, made funny shorting noise and smoked until i turned off main breaker. in the current setup it seems like allowing ~800A on what is believed to be a 400A service just doesnt seem good, and having a 2nd fuse that perhaps can itself alone carry more than what the service is.

if the disconnect it rated ~400A then why are there two fuses?

so for OP, instead of swapping the 400A fuses for 250's, remove one 400A and see how the service works??

I would expect the 400 amp fuse to hold at 425 amps for quite a long time, exactly how long will depend on trip curve of the fuse used, and if things in the disconnect are in good condition then both fuses are carrying approximately same amps, if not in good condition it is time for repairs or replacement, as problems will only continue to get worse.

 

[FionaZuppa]

I would expect the 400 amp fuse to hold at 425 amps for quite a long time, exactly how long will depend on trip curve of the fuse used, and if things in the disconnect are in good condition then both fuses are carrying approximately same amps, if not in good condition it is time for repairs or replacement, as problems will only continue to get worse.

they will indeed carry same amps until one of them pops, but one will pop before the other leaving the 2nd to perhaps allow way more than what would be expected.

if the wires are rated 500A total then having 800A of fuse doesnt seem good. a wire can allow huge currents for short times as long as they dont heat too much causing damage. question is, can the fuses (if they were 250A each) support the transient currents that were mentioned? i suspect the wire can. probably a case of poor design and to allow the big transient loads two 400A fuses were used. but having a short that allows say 700A to flow indefinitely will likely cause something bad to happen. in this scenario at 700A the fuses stay ok, one of the wires will melt and then break causing the shorting load to move to one fuse which will then pop (and in this case its the side with the good fuse that had the issue, and, the good fuse still allows voltage on the wire up to point where it broke = not good). i suspect it takes much more then the rated wire Amps to have the wire melt, but you start to see the bad scenarios.

 

[FionaZuppa]

the fusing current of 350 size AL wire will be right around 2500A (w/o insulation in free air).
i am certain though the insulation will burn off way before that # and then it will cause a short.
aside from NEC and ratings of items, strictly physics shows having 800A worth of fuse is probably just fine (its really 400A per wire).

however, fusing current is just a # and doesnt offer any good info on wire heating in real world applications. i think this is where the ampacity #'s are derived. 350 AL is rated ~250A (w/ insulator).

in current state, the wire is fused 1.6x more than wire ampacity rating. i suspect NEC doesnt want to see more than 1x ??

not sure i can offer any more useful or non-useful info on this specific issue other than perhaps doing a full test of trying to record a peak transient current during a full power-on and then select fuses from there.

 

[FionaZuppa]

NEC 240.4 Protection of Conductors
Conductors, other than flexible cords, flexible cables, and
fixture wires, shall be protected against overcurrent in
accordance with their ampacities specified in 310.15, unless
otherwise permitted or required in 240.4(A) through (G).


http://www.mjobee.com/projects&news/NEC Art 310.15.pdf

 

[iwire]

if the disconnect it rated ~400A then why are there two fuses?

I don't believe anyone has said that the disconnect is a 400 amp one.

 

[iwire]

I would put a pair of 250 amp fuses in and move on.

 

[FionaZuppa]

I don't believe anyone has said that the disconnect is a 400 amp one.

if the other disco has about same physical dimensions on contacts and uses what looks like same materials, and its marked "400A", then i might say the one with two fuses has same rating. not 100% but a good hypothesis. ??

 

[iwire]

if the other disco has about same physical dimensions on contacts and uses what looks like same materials, and its marked "400A", then i might say the one with two fuses has same rating. not 100% but a good hypothesis. ??

I am a pretty practical guy and it seems very unlikely that any manufacturer produced a 400 amp rated fused switch that could accept 800 amps of overcurrent protection. I won't say it is not possible, just unlikely.


It should go without saying that if this switch is actually labeled as 400 amp that it should not be used beyond that.