OCPD in parallel?

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Tony S

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An extract from NFPA-NEC 2014

240.8 Fuses or Circuit Breakers in Parallel.

Fuses and circuit breakers shall be permitted to be connected in parallel where they are factory assembled in parallel and listed as a unit. Individual fuses, circuit breakers, or combinations thereof shall not otherwise be connected in parallel.

I was surprised when I asked if there was a minimum value parallel OCPD’s could be used I was told “no”.

This is a 200A breaker consisting of two 100A units used in a service disconnect until recently.

G-E-200ampmain_zps8zgqu1rr.jpg.2a4fc9fe321e9f9f75dc1eed50f7c0a3.jpg


The only time I’ve seen this used in the UK has been for specialist applications such as semiconductor fuses, not for bog standard protection. If you need a 3200A fuse you buy a 3200A fuse not two 1600’s.

semicon-fuse_zpshicremjd.jpg.d6473214ef4042b43dbba4d457f71333.jpg


I know RoB will step in with his 8000A ACB he had made out of two 4000A units in parallel and I can counter with 4000A units in series due to voltage and fault levels. Both are exceptions.

It just seems a weird idea to me and wondered if anyone else knew of other instances.

 
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The only time I've used parallel fuses in a panel by design was to accommodate an extraordinarily high PFC. Some manufacturers will supply pre-matched fuses specifically for this and we actually ended up with 4x fuses in parallel to meet the potential fault current requirements. In some cases it can't be done because a voltage derating value must be applied to paralleled OCPD's. It's not permitted in our local domestic installation regs but even though it's not encouraged it's not forbidden in our machine regs.

 
I've used semicon fuses in parallel, as an assembly, but nowt else.

Personally I don't like the idea.

Why?

I don't know, but, it's just my gut feeling.

 
It’s one of those “if it doesn’t look right, it can’t be right”.

It just seems so wrong. I can understand it for exceptionally large breakers but then I’d ask the question “why not use a higher voltage”, it would lower the current and get it in to a standard frame.

My series breaker was for 4000A @660V. With the MPFC at that voltage and I would be pushing my luck. A four pole breaker with two poles in series got around that but not without the go ahead from the manufacturers.

The septics just seem to think it normal.

 
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Only ever used it for semiconductor fuses, but they are pre welded together from the manufacturer. 

Aside from the Frankenstein job of an ACB i had designed as a bespoke MCC job I've not had the need. 

I've paralleled contactors up numerous times, and pushed higher than the rated voltage to replace shorting bars on liquid starters. The manufactures of the contactors wouldn't advise if it would work, but were sure interested in me ringing and informing them if they did or not. I never did get back to them. 

 
I've paralleled contactors up numerous times, and pushed higher than the rated voltage to replace shorting bars on liquid starters. The manufactures of the contactors wouldn't advise if it would work, but were sure interested in me ringing and informing them if they did or not. I never did get back to them. 


I’d forgotten about that one. We both came up with the same idea about the same time, different companies and miles apart.

1700V on bog standard contactors had my sphincter twitching for the first run.

PS, I also forgot to call Telemecanique back, “try it and could you let us know if it works?”

 
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