15A fuse on circuit with 10A outlets

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blissfulhedgehog

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#273134 6-Aug-2020 11:59

Hi all,

I've just purchased a 70's house and am familiarising myself with the internals of it. We have an old style fusebox with a bunch of ceramic fuses in it. A few of the circuits (e.g. A/C and stove) have had the ceramic fuses replaced with plug-in circuit breaks.

All of the power outlets in the house are rated for 10A each, but the power fuse on those circuits is 15A. From some googling I have determined that it's not that unusual for a 15A fuse to be used. When I looked at getting some HPM plug-in circuit breakers, I saw that the 6A and 10A ones are recommended for lighting circuits with the 16A ones for power circuits.

I understand that with the outlets in serial >10A draw should be safe if the load is spread. However, doesn't a 15A fuse poorly protect against a single outlet drawing more than 10A?

I'm just worried that it's a fire risk.

sittingduckz

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#2535679 6-Aug-2020 12:06

Pretty normal I think. Mine are on 16A breakers. I had my entire fuse board changed and moved about 12 months ago so must be legit.

I'm not a complete idiot, I still have some parts missing.

timmmay

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#2535680 6-Aug-2020 12:06

My understanding is fuses are size to protect the wiring, rather than to limit what is plugged into the sockets. An 18 amp fuse is standard according to my electrician, but depends on various factors.

we have a fuse box similar to yours, mostly with circuit breakers now. Electrician advised us to upgrade it to modern standards at some point.

frankv

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#2535755 6-Aug-2020 13:07

blissfulhedgehog:

I understand that with the outlets in serial >10A draw should be safe if the load is spread. However, doesn't a 15A fuse poorly protect against a single outlet drawing more than 10A?

As someone else said, the fuse is to protect the wiring rather than the devices.

I think you mean outlets in parallel drawing more than 10A in total should be safe. But the fuse doesn't know where the load is... if the fuse is 15A then the wiring to *each* outlet should be good for more than 15A. If in doubt, replace the 15A fuse with a 10A fuse.

NB that fuses are relatively slow to blow so can allow currents above their nominal rating for a short period of time, whereas circuitbreakers react faster. So replacing a 10A fuse with a 10A circuitbreaker may result in the circuitbreaker tripping when a motor starts, even though the fuse didn't blow.

timmmay

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#2535764 6-Aug-2020 13:19

My understanding is that in most cases you shouldn't draw more than 10A per faceplate, so if it's a double plug you can take 5A + 5A, 9A + 1A, but not 10A + 10A.

tripper1000

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#2535765 6-Aug-2020 13:25

Matching a 10 amp CB on a 10 amp outlet can cause nuance tripping and would mean every outlet in the house would need its own wire back to the switchboard, greatly increasing the cost of wiring and the size of the switch board. So it is normal to install wiring & CB rated for 15 or 20 amps and feed multiple outlets. Yes, this means you can in theory draw 15 or 20 amps from one outlet continuously, but such an appliance would be illegal and/or faulty.

CB's are there to protect wiring and there is no universal, easy answer with circuit breaker sizes. Different ratings of C.B. are used depending on the thickness of the wiring, the length of the wiring run, the number of outlets connected to it, the thermal insulation that the wiring may or may not run through, and the type of appliances connected.

Don't get too hung up on the outlets ~~apparent~~ advertised 10 amp rating and it's relationship to the C.B - this isn't the self-destruct threshold for the outlet - it is the maximum permissible continuous draw for an appliance plugged into it. In reality sometimes more than this can be drawn (for a short period) and sometimes you can't even draw as much as 10 amps. Many appliances will draw more than 10 amps this for short periods of time (power tools & vacuums without soft start, compressors etc). Many circuits have multiple outlets on the one circuit breaker which means you can't even draw 10 amps from every socket simultaneously.

richms

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#2535767 6-Aug-2020 13:27

20A used to be common for power circuits till they had some guidelines to assume everything is insulated, so its all 16a now unless they get all spendy and use 4mm cables. Any appliance that takes more than 10A from the outlet is faulty by design, or is some muppet with a double adapter putting silly loads on it. Any powerstrip with more than 2 outlets needs to have an overcurrent protection, but there was a silly lapse in judgement when they made that rule and a double adapter was allowed to have no overcurrent in it. I think that is changing soon. Those deathtrap adapters and extension cables with piggyback plugs shouldnt be sold to the general ignorant population IMO.

Richard rich.ms

tripper1000

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#2535769 6-Aug-2020 13:33

richms:.... Those deathtrap adapters and extension cables with piggyback plugs shouldn't be sold to the general ignorant population IMO.

Awhh, I use those heaps because so few appliances have side entry plugs now days. It seems that no foreign fridge can be pushed up against the wall without mashing the wires exiting the back of the plug.

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frankv

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#2535827 6-Aug-2020 15:06

richms:.... Those deathtrap adapters and extension cables with piggyback plugs shouldn't be sold to the general ignorant population IMO.

Mighty useful for Xmas lighting though, where I'll have a dozen or more LED strings (at a few mA each) plugged into a single timer and power outlet.