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Breaker Box Tolerances (1 reply and 2 comments)

5 months ago
Alex2II2 5 months ago

Hi There,

I'm wondering if anyone can tell me the max load for a;

 - 125amp breaker box

 - 63amp breaker box

 - 32amp breaker box

And how you have arrived at that number



5 months ago
gabj3 5 months ago

I'm not sure which country you're from.

In Australia we have 240v circuits. So theoretically a 10A breaker could sustain 2400w (watt = amps*voltage) without fault. Typically our electrical appliances/cabling are designed to take a heavier load than what they're rated for (for safety reasons - if you have a 10A cable and it's copper wiring is only rated at 10A if it drops x amount of amps due to increased resistance over long runs etc it'll become hazardous if the maximum for said appliances/cabling is 10A, as an increase in resistance a drop in amperage requires more voltage).

In terms of your question, it requires more information. A 63a single phase breaker on a 240v circuit could theoretically sustain 15000w however, most 63A breakers I've encountered are 3phase - 3 phases of frequency so it'd be 3*63*240 = 45360w. 

Typically that's the rating for the 'main switch' and we tie in from there (we add temp circuits to). 

Unless you have a licensed spark, don't do the above. If you have the distribution readily available to you say 32a 3 phase out and a tested 3 phase distro sure you could pull 23000w (at that level we typically go lower at 20000w).

Keep in mind, if you're safety switch for your power circuit is 32A but you only have a 15a plug... don't pull 32A... we occasionally push the 15a (3600w) to around 3600-4000 but don't like doing so. 

Like all things electrical, if you don't know what you're doing... don't do it.



5 months ago

Hey There, Thanks for you reply - I just wanted someone to re-explain it to me as I was in a studio yesterday and noticed their distro boxes scattered around and I couldn't remember what I learned in film school haha. (Don't' worry I wasn't sparking).
So would you agree what i've written below would be correct? I'm in England btw, so 240 volts also.

125amp - 90kw max - 30 per phase - 27kw for safety
63amp - 45kw max - 15kw per phase - 12kw safety
32amp - 23kw max - 7kw per phase - 5kw safety

Also, side bar - When you leave the world of C form plugs and boxes and get into the world of powerlock distro boxes where a 400 AMP ISU 3 PHASE box exists. Would I be correct in thinking that, that box would be

400amp - 288Kw max - 96kw per phase - 93kw for saftey


The Byre
5 months ago

Your safety margin must be much greater because of the load of the resistive properties of the wire and various other things such as the plugs. (Jules law)

You must also bear in mind what sort of load is being used - inductive (e.g. motors and transformers) or resistive (incandescent lights). Inductive loads can be close to a short-circuit for a few milliseconds until the magnetic fields in their coils have been established. (Laws of induction)

The basic formula is what I call the poo-ee law (P = U x I) or in the UK Power (Watts) equals Tension (Volts) times Current (Amps).

So Watts = V x A.

3-phase supplies bring new complications and for 240V per phase, you are working with c.a. 400V and for 220V supply, it's 380V. There are excellent Wikipedia pages on this subject, should you wish to learn more.

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